A Population Genetics-Based and Phylogenetic Approach to Understanding the Evolution of Virulence in the Genus Listeria

The genus Listeria includes (i) the opportunistic pathogens L. monocytogenes and L. ivanovii, (ii) the saprotrophs L. innocua, L. marthii, and L. welshimeri, and (iii) L. seeligeri, an apparent saprotroph that nevertheless typically contains the prfA virulence gene cluster. A novel 10-loci multilocus sequence typing scheme was developed and used to characterize 67 isolates representing six Listeria spp. (excluding L. grayi) in order to (i) provide an improved understanding of the phylogeny and evolution of the genus Listeria and (ii) use Listeria as a model to study the evolution of pathogenicity in opportunistic environmental pathogens. Phylogenetic analyses identified six well-supported Listeria species that group into two main subdivisions, with each subdivision containing strains with and without the prfA virulence gene cluster. Stochastic character mapping and phylogenetic analysis of hly, a gene in the prfA cluster, suggest that the common ancestor of the genus Listeria contained the prfA virulence gene cluster and that this cluster was lost at least five times during the evolution of Listeria, yielding multiple distinct saprotrophic clades. L. welshimeri, which appears to represent the most ancient clade that arose from an ancestor with a prfA cluster deletion, shows a considerably lower average sequence divergence than other Listeria species, suggesting a population bottleneck and a putatively different ecology than other saprotrophic Listeria species. Overall, our data suggest that, for some pathogens, loss of virulence genes may represent a selective advantage, possibly by facilitating adaptation to a specific ecological niche.Population genetics-based and phylogenetic studies have greatly contributed to the understanding of the evolutionary history and ecology of bacterial pathogens. In particular, multilocus sequence analyses (MLSA) and single-nucleotide polymorphism (SNP)-based population genetics research have revealed the microevolutionary patterns of species complexes like the Bacillus cereus complex (12) or the microevolution of well-known pathogens like Yersinia pestis (2), Salmonella enterica serovar Typhi (57), and Mycobacterium tuberculosis (18). One of the common findings of these studies is that obligate pathogens generally have a genetically clonal population structure as inferred by MLSA (1), while the population structure of free-living facultative pathogenic bacteria is characterized by relatively high genetic variability (12, 70). It has been hypothesized that these differences in population structure are related to the fact that some obligate pathogens represent epidemic clones (38), i.e., clonal lineages whose members have an epidemiological advantage compared to other lineages and are therefore able to quickly spread within the population. Because this dispersal of the members of an epidemic clone occurs rapidly, there is not enough time to accumulate mutations.In this paper we present a phylogenetic and population genetics study of the genus Listeria. This genus consists of six closely related pathogenic (L. monocytogenes and L. ivanovii) and nonpathogenic (L. innocua, L. welshimeri, L. seeligeri, and a newly described species, L. marthii) species as well as a distantly related species, L. grayi (22). Another new species, L. rocourtiae, has been recently reported (33), but isolates were not available for inclusion in the study reported here. Because of the distant phylogenetic relatedness of L. grayi to the other Listeria species, it has been suggested that this species should be put in a separate genus, Murraya (63); L. grayi was thus not included in our study reported here. L. monocytogenes and L. ivanovii are facultative pathogens of warm-blooded animals and are the causative agents of a severe infectious disease, listeriosis (67). While L. monocytogenes has a wide host range, including humans, the host range of L. ivanovii seems to be largely restricted to ruminants, in particular sheep (13), even though some human listeriosis cases caused by L. ivanovii have been reported (34).Key virulence genes in Listeria include (i) six genes (prfA, plcA, hly, mpl, actA, and plcB) clustered in a genomic element, designated the prfA virulence cluster or the Listeria pathogenicity island (LiPI), and (ii) members of the internalin family (61). Genes in the prfA cluster encode functions that that are necessary for inter- and intracellular motility and intracellular survival in the host cell. While some internalin genes encode proteins essential for host cell invasion (e.g., inlA and inlB) (3), inlC has recently been shown to encode a protein critical for cell-to-cell spread (52), and the functions of a number of other internalin proteins still remain to be elucidated (40). A number of internalin genes are also organized in clusters, including the inlAB operon, the inlGHE operon (which can also be present as an inlGC2DE or as an inlC2DE operon), which is found in L. monocytogenes and an L. ivanovii species-specific pathogenicity island encoding sphingomyelinase and numerous internalins (13). Importantly, the presence or absence of the prfA cluster and virulence characteristics can also be used to classify Listeria species and clades into three groups, including (i) species that do contain the prfA virulence cluster and are known pathogens, like L. monocytogenes and L. ivanovii, (ii) species that lack the prfA virulence cluster and are nonpathogenic (L. marthii and L. welshimeri), and (iii) species in which the presence of the prfA virulence cluster varies by strain. The last group contains L. seeligeri, which is nonpathogenic, although the majority of strains in the population contain the prfA virulence cluster (69), and L. innocua, which is also nonpathogenic, and although most strains lack the prfA virulence cluster, a small proportion of strains do carry this cluster (31, 68). The facts that the genus Listeria contains closely related nonpathogenic and pathogenic species and that strains with and without the prfA cluster within the same species make this genus an interesting model system for studies on the evolution of pathogenicity in opportunistic environmental pathogens. In addition, an improved understanding of the phylogeny and evolution of pathogenic and nonpathogenic Listeria spp. will also help in the development of appropriate assays for the specific detection and identification of human and animal pathogenic Listeria strains as well as regulations and intervention strategies that specifically target pathogenic species and strains.     

Molecular Ecology of Listeria monocytogenes: Evidence for a Reservoir in Milking Equipment on a Dairy Farm

A longitudinal study aimed to detect Listeria monocytogenes on a New York State dairy farm was conducted between February 2004 and July 2007. Fecal samples were collected every 6 months from all lactating cows. Approximately 20 environmental samples were obtained every 3 months. Bulk tank milk samples and in-line milk filter samples were obtained weekly. Samples from milking equipment and the milking parlor environment were obtained in May 2007. Fifty-one of 715 fecal samples (7.1%) and 22 of 303 environmental samples (7.3%) were positive for L. monocytogenes. A total of 73 of 108 in-line milk filter samples (67.6%) and 34 of 172 bulk tank milk samples (19.7%) were positive for L. monocytogenes. Listeria monocytogenes was isolated from 6 of 40 (15%) sampling sites in the milking parlor and milking equipment. In-line milk filter samples had a greater proportion of L. monocytogenes than did bulk tank milk samples (P < 0.05) and samples from other sources (P < 0.05). The proportion of L. monocytogenes-positive samples was greater among bulk tank milk samples than among fecal or environmental samples (P < 0.05). Analysis of 60 isolates by pulsed-field gel electrophoresis (PFGE) yielded 23 PFGE types after digestion with AscI and ApaI endonucleases. Three PFGE types of L. monocytogenes were repeatedly found in longitudinally collected samples from bulk tank milk and in-line milk filters.Listeria monocytogenes can cause listeriosis in humans. This illness, despite being underreported, is an important public health concern in the United States (23) and worldwide. According to provisional incidence data provided by the Centers for Disease Control and Prevention (CDC), 762 cases of listeriosis were reported in the United States in 2007. In previous years (2003 to 2006), the number of reported annual listeriosis cases in the United States ranged between 696 and 896 cases per year (5).Exposure to food-borne L. monocytogenes may cause fever, muscle aches, and gastroenteritis (30), but does not usually cause septicemic illness in healthy nonpregnant individuals (7, 30). Elderly and immunocompromised people, however, are susceptible to listeriosis (22, 10), and they may develop more-severe symptoms (10). Listeriosis in pregnant women may cause abortion (22, 30) or neonatal death (22).Dairy products have been identified as the source of several human listeriosis outbreaks (4, 7, 10, 22). Listeria is ubiquitous on dairy farms (26), and it has been isolated from cows'' feces, feed (3, 26), and milk (21, 35). In ruminants, L. monocytogenes infections may be asymptomatic or clinical. Clinical cases typically present with encephalitis and uterine infections, often resulting in abortion (26, 39). Both clinically infected and healthy animals have been reported to excrete L. monocytogenes in their feces (20), which could eventually cause contamination of the bulk tank milk or milk-processing premises (39).On-farm epidemiologic research provides science-based information to improve farming and management practices. The Regional Dairy Quality Management Alliance (RDQMA) launched a combined United States Department of Agriculture (USDA)-RDQMA pilot project in January 2004 to scientifically validate intervention strategies in support of recommended best management practices among northeast dairy farms. The primary goal of the project was to track dynamics of infectious microorganisms on well-characterized dairy farms. Target species included Salmonella spp. (6, 36, 37), Mycobacterium avium subsp. paratuberculosis (13, 24), and L. monocytogenes.The objectives of this study were to describe the presence of L. monocytogenes on a dairy farm over time and to perform molecular subtyping by pulsed-field gel electrophoresis (PFGE) on L. monocytogenes isolates obtained from bulk tank milk, milk filters, milking equipment, feces, and the environmental samples to identify diversity among L. monocytogenes strains, persistence, and potential sources of bulk tank milk contamination.     

Genetic Features of Resident Biofilms Determine Attachment of Listeria monocytogenes

Planktonic Listeria monocytogenes cells in food-processing environments tend most frequently to adhere to solid surfaces. Under these conditions, they are likely to encounter resident biofilms rather than a raw solid surface. Although metabolic interactions between L. monocytogenes and resident microflora have been widely studied, little is known about the biofilm properties that influence the initial fixation of L. monocytogenes to the biofilm interface. To study these properties, we created a set of model resident Lactococcus lactis biofilms with various architectures, types of matrices, and individual cell surface properties. This was achieved using cell wall mutants that affect bacterial chain formation, exopolysaccharide (EPS) synthesis and surface hydrophobicity. The dynamics of the formation of these biofilm structures were analyzed in flow cell chambers using in situ time course confocal laser scanning microscopy imaging. All the L. lactis biofilms tested reduced the initial immobilization of L. monocytogenes compared to the glass substratum of the flow cell. Significant differences were seen in L. monocytogenes settlement as a function of the genetic background of resident lactococcal biofilm cells. In particular, biofilms of the L. lactis chain-forming mutant resulted in a marked increase in L. monocytogenes settlement, while biofilms of the EPS-secreting mutant efficiently prevented pathogen fixation. These results offer new insights into the role of resident biofilms in governing the settlement of pathogens on food chain surfaces and could be of relevance in the field of food safety controls.Listeria monocytogenes is a food pathogen that has been implicated in numerous food-borne disease outbreaks (5, 58). This organism is found not only in food products but also on surfaces in food-processing plants (18). It is well documented that L. monocytogenes is able to adhere and form persistent biofilms on a variety of solid materials, such as stainless steel, glass, or polymers (18, 48, 51, 52). However, in food-manufacturing plants (and particularly in fermented-food-processing environments), it is most likely that the first contact between a pathogen and a surface will concern a resident microbial biofilm covering the solid surface (10, 35, 46). In this context, such a resident biofilm may be regarded as a “conditioning film” that modifies the topographic and physicochemical characteristics of the surface and hence the adhesion capability of planktonic microorganisms coming into contact with this substratum (6).Once the pathogens are immobilized on the surface, interactions between the pathogens and their environment (physiological interactions with resident flora, nutrient availability, pH, water activity, temperature, and cleaning and disinfection procedures) govern the long-term settlement and persistence of the pathogens on the surface. Various studies have demonstrated the inhibition of L. monocytogenes development by natural “protective” biofilms (10, 66). Competition for nutrients has been demonstrated as a major mechanism underlying the inhibition of pathogen development (25, 27). The production of antimicrobial agents (bacteriocins, acids, and hydrogen peroxide) has also been reported as being of importance to such interactions (13, 20, 36). For example, Lactococcus lactis has been described as being exceptionally efficient in controlling the development of L. monocytogenes on food-processing surfaces by means of competitive exclusion (66) or bacteriocin production (35). It has been reported that treating a surface with a bacterial polysaccharide prevented the adhesion of different nosocomial pathogens (60). Furthermore, alginate-overexpressing Pseudomonas aeruginosa biofilms reduced the retention of Cryptosporidium parvum oocysts (54). Other recent studies have shown that the composition and quantity of specific exopolysaccharides (EPS) in Pseudomonas biofilms can inhibit the fixation of Escherichia coli or Erwinia chrysanthemi planktonic cells in porous media (37, 38).The present study investigated those properties of resident biofilms that could affect the settlement of L. monocytogenes. L. lactis was used as a model resident biofilm strain, as this is widely used in dairy fermentations and its cell wall properties have been the subject of considerable study (22, 23). Cell wall mutants of L. lactis MG1363 were used to create a set of model biofilms that differed in terms of their architecture, EPS synthesis, and cell surface hydrophobicity. These biofilms were used to evaluate the attachment of fluorescent inert polystyrene microbeads and of two reference strains of L. monocytogenes (LO28 and EGDe) using in situ confocal fluorescence imaging.     

Galleria mellonella as a Model System for Studying Listeria Pathogenesis

Essential aspects of the innate immune response to microbial infection are conserved between insects and mammals. This has generated interest in using insects as model organisms to study host-microbe interactions. We used the greater wax moth Galleria mellonella, which can be reared at 37°C, as a model host for examining the virulence potential of Listeria spp. Here we report that Galleria is an excellent surrogate model of listerial septic infection, capable of clearly distinguishing between pathogenic and nonpathogenic Listeria strains and even between virulent and attenuated Listeria monocytogenes strains. Virulence required listerial genes hitherto implicated in the mouse infection model and was linked to strong antimicrobial activities in both hemolymph and hemocytes of infected larvae. Following Listeria infection, the expression of immune defense genes such as those for lysozyme, galiomycin, gallerimycin, and insect metalloproteinase inhibitor (IMPI) was sequentially induced. Preinduction of antimicrobial activity by treatment of larvae with lipopolysaccharide (LPS) significantly improved survival against subsequent L. monocytogenes challenge and strong antilisterial activity was detected in the hemolymph of LPS pretreated larvae. We conclude that the severity of septic infection with L. monocytogenes is modulated primarily by innate immune responses, and we suggest the use of Galleria as a relatively simple, nonmammalian model system that can be used to assess the virulence of strains of Listeria spp. isolated from a wide variety of settings from both the clinic and the environment.Listeriae are rod-shaped, motile, facultative, anaerobic Gram-positive bacteria that are ubiquitously distributed in the environment (28). Of the six species that comprise the genus Listeria, only L. monocytogenes and L. ivanovii are pathogenic and cause disease, while strains of the species L. innocua, L. welshimeri, L. seeligeri, and L. grayi are generally considered to be nonpathogenic (26). L. monocytogenes is a major food-borne pathogen, and listeriosis is an invasive disease that in its severest form can lead to meningitis, meningoencephalitis, septicemia, and abortions (38). Listeriosis occurs primarily in pregnant women, newborn infants, and the elderly as well as in immunocompromised patients, with a mortality rate of about 30% (22, 36). The virulence of L. monocytogenes has been linked to a 9.6-kb pathogenicity island designated vgc (virulence gene cluster) that comprises six genes encoding its major virulence determinants. These are (i) prfA, a master regulator of many known listerial virulence genes; (ii) hly, encoding listeriolysin, a hemolysin required for bacterial escape from the host primary vacuole to the host cytoplasm; (iii) two phospholipase genes denoted plcA and plcB, for facilitating lysis of host cell membranes; (iv) actA, encoding a surface bound protein that directs polymerization of host cell actin and is required for intracellular motility; and (v) mpl, encoding a metalloproteinase which is thought to work together with the plcB product to facilitate cell-to-cell spread (28). Presently, identification and characterization of novel virulence factors rely on assessing mutant bacteria for growth in the organs of infected mice. Nevertheless, the dependence on mouse infection models limits large-scale screening for additional mutants defective in their ability to grow in the host intracellularly or for those required to overcome host innate defenses (33).The possibility of addressing many aspects of mammalian innate immunity in invertebrates has opened a new arena for developing invertebrate models to study human infections. Recently the use of invertebrate models, in particular the fruit fly Drosophila melanogaster, has been introduced for the study of septic listerial infections (37). Listeria mutants attenuated for virulence in a mouse model exhibited lowered virulence in this model. The Drosophila model system has powerful genetic tools available and has thus provided deeper insights into molecular mechanisms of the interactions between Listeria and the insect innate immune system (1, 8-10, 18, 24). However, a recent study has shown that even nonpathogenic L. innocua strains cause lethal infections of Drosophila, limiting it use as a discerning model for the study of virulence potential among pathogenic L. monocytogenes isolates (32).We have a longstanding interest in host-pathogen interactions of the greater wax moth, Galleria mellonella, in particular with entomopathogenic microbes (55). Recently, Galleria has also emerged as a reliable model host to study the pathogenesis of many human pathogens (7, 11, 12, 17, 21, 30, 31, 39-42, 44, 46, 48-51). Among the advantages provided by the Galleria model host (e.g., low rearing costs, convenient injection feasibility, and status as an ethically acceptable animal model), it is of particular importance that Galleria has a growth optimum at 37°C, to which human pathogens are adapted and which is essential for synthesis of many virulence/pathogenicity factors. Significantly, a correlation between the virulence of a pathogen in G. mellonella and that in mammalian models has been established (16, 25).The innate immunity of Galleria is a complex, multicomponent response involving hemolymph coagulation, cellular phagocytosis, and phenol oxidase-based melanization. Importantly, killing of pathogens is achieved similarly to that in mammals, i.e., by enzymes (e.g., lysozymes), reactive oxygen species, and antimicrobial peptides (e.g., defensins). Galleria employs recognition of nonself microbe-associated molecular patterns by germ line-encoded receptors (e.g., Toll and peptidoglycan recognition proteins) (52). Recently, we have found that Galleria also senses pathogens by danger signaling, by detecting either nucleic acids released from damaged cells or peptides resulting from proteolytic cleavage of self proteins by matrix metalloproteinases (3-6).In this work we examined the Galleria model of septic infection for its ability to differentially distinguish between infections caused by strains with different virulence potentials in the mouse infection model, as well as in avirulent strains of Listeria. We found that the Galleria model is highly discriminatory in assessing the pathogenic potential of Listeria spp., and we observed a strong correlation with the virulence previously determined in the mouse model of infection. Here, we present data indicating that the Galleria model also replicates many aspects of innate immune function, such as the constitutive expressions of potential antimicrobial factors following infection. Also, prior induction of immunity in Galleria can protect larvae from septic infection with highly pathogenic L. monocytogenes.     

Competition of Listeria monocytogenes Serotype 1/2a and 4b Strains in Mixed-Culture Biofilms

The majority of Listeria monocytogenes isolates recovered from foods and the environment are strains of serogroup 1/2, especially serotypes 1/2a and 1/2b. However, serotype 4b strains cause the majority of human listeriosis outbreaks. Our investigation of L. monocytogenes biofilms used a simulated food-processing system that consisted of repeated cycles of growth, sanitation treatment, and starvation to determine the competitive fitness of strains of serotypes 1/2a and 4b in pure and mixed-culture biofilms. Selective enumeration of strains of a certain serotype in mixed-culture biofilms on stainless steel coupons was accomplished by using serotype-specific quantitative PCR and propidium monoazide treatment to prevent amplification of extracellular DNA or DNA from dead cells. The results showed that the serotype 1/2a strains tested were generally more efficient at forming biofilms and predominated in the mixed-culture biofilms. The growth and survival of strains of one serotype were not inhibited by strains of the other serotype in mixed-culture biofilms. However, we found that a cocktail of serotype 4b strains survived and grew significantly better in mixed-culture biofilms containing a specific strain of serotype 1/2a (strain SK1387), with final cell densities averaging 0.5 log₁₀ CFU/cm² higher than without the serotype 1/2a strain. The methodology used in this study contributed to our understanding of how environmental stresses and microbial competition influence the survival and growth of L. monocytogenes in pure and mixed-culture biofilms.A prominent food-borne pathogen, Listeria monocytogenes can cause severe infections in humans, primarily in high-risk populations, though the disease (listeriosis) is relatively rare (11, 30, 43). Outbreaks of listeriosis have resulted from the contamination of a variety of foods by L. monocytogenes, especially meat and dairy products (27). L. monocytogenes is ubiquitous in the environment, able to grow at refrigeration temperature, and tolerant of the low pHs (3 to 4) typical of acidified foods (28, 32, 44). The capacity to produce biofilms confers protection against stresses common in the food-processing environment (13, 33).Biofilms are characterized by dense clusters of bacterial cells embedded in extracellular polymeric substances which are secreted by cells to aid in adhesion to surfaces and to other cells (4, 5). Strains of L. monocytogenes have been known to persist for years in food-processing environments, presumably in biofilms. Of the 13 known serotypes of L. monocytogenes, three (1/2a, 1/2b, and 4b) account for >95% of the isolates from human illness (21). Serotype 1/2a accounts for >50% of the L. monocytogenes isolates recovered from foods and the environment, while most major outbreaks of human listeriosis have been caused by serotype 4b strains (1, 3, 14, 15, 17, 22, 29, 31, 41, 47, 49,). No correlation between L. monocytogenes strain fitness and serotype has been identified (16, 19). Some studies have reported that strains repeatedly isolated from food and environmental samples (defined as persistent strains) had a higher adherence capacity than strains that were sporadically isolated (2, 36), while this phenomenon was not observed by others (7). Serotype 4b strains exhibited a higher capacity for biofilm formation than did serotype 1/2a strains (36), whereas this was not observed by Di Bonaventura and colleagues (6). It has been suggested that serotype 1/2a strains could be more robust than serotype 4b strains in biofilm formation under a variety of environmental conditions. Furthermore, strains of these serotypes differ in terms of the medium that promotes biofilm formation. Biofilm formation by serotype 4b strains was higher in full-strength tryptic soy broth than in diluted medium, whereas the opposite was observed with serotype 1/2a strains, which produced more biofilm in diluted medium (12).There is limited information on microbial competition between strains of different serotypes in biofilms or on how the environmental stresses present in food-processing environments may affect the biofilm formation and survival of L. monocytogenes of different serotypes. In food-processing plants, the environmental stresses encountered by bacteria are more complex and variable than most laboratory systems used for microbial ecology and biofilm studies. A simulated food-processing (SFP) system has been developed to address this issue (38). The SFP system incorporates several stresses that may affect bacteria in biofilms in the food-processing environment, including exposure to sanitizing agents, dehydration, and starvation. When biofilms were subjected to the SFP regimen over a period of several weeks, the cell numbers of L. monocytogenes strains in the biofilms initially were reduced and then increased as the culture adapted (38). The development of resistance to sanitizing agents was specific to the biofilm-associated cells and was not apparent in the detached cells (38). This suggested that extracellular polymeric substances present in the biofilm matrix were responsible for the resistance to sanitizing agents. It was subsequently found that real-time PCR, in combination with propidium monoazide (PMA) treatment of samples prior to DNA isolation, was an effective method for enumerating viable cells in biofilms (37).The objective of this study was to determine if strains of serotype 1/2a or 4b have a selective advantage under stress conditions. We investigated and compared the initial attachment and biofilm formation capabilities of L. monocytogenes strains of these two serotypes and analyzed the survival and growth of bacteria of each serotype in mixed-serotype biofilms in the SFP system by using PMA with quantitative PCR.     

Temperature-Dependent Phage Resistance of Listeria monocytogenes Epidemic Clone II

Listeria monocytogenes epidemic clone II (ECII) has been responsible for two multistate outbreaks in the United States in 1998-1999 and in 2002, in which contaminated ready-to-eat meat products (hot dogs and turkey deli meats, respectively) were implicated. However, ecological adaptations of ECII strains in the food-processing plant environment remain unidentified. In this study, we found that broad-host-range phages, including phages isolated from the processing plant environment, produced plaques on ECII strains grown at 37°C but not when the bacteria were grown at lower temperatures (30°C or below). ECII strains grown at lower temperatures were resistant to phage regardless of the temperature during infection and subsequent incubation. In contrast, the phage susceptibility of all other tested strains of serotype 4b (including epidemic clone I) and of strains of other serotypes and Listeria species was independent of the growth temperature of the bacteria. This temperature-dependent phage susceptibility of ECII bacteria was consistently observed with all surveyed ECII strains from outbreaks or from processing plants, regardless of the presence or absence of cadmium resistance plasmids. Phages adsorbed similarly on ECII bacteria grown at 25°C and at 37°C, suggesting that resistance of ECII strains grown at 25°C was not due to failure of the phage to adsorb. Even though the underlying mechanisms remain to be elucidated, temperature-dependent phage resistance may represent an important ecological adaptation of L. monocytogenes ECII in processed, cold-stored foods and in the processing plant environment, where relatively low temperatures prevail.Listeria monocytogenes is responsible for an estimated 2,500 cases of serious food-borne illness (listeriosis) and 500 deaths annually in the United States. It affects primarily pregnant women, newborns, the elderly, and adults with weakened immune systems. L. monocytogenes is frequently found in the environment and can grow at low temperatures, thus representing a serious hazard for cold-stored, ready-to-eat foods (18, 31).Two multistate outbreaks of listeriosis in the United States, in 1998-1999 and in 2002, respectively, were caused by contaminated ready-to-eat meats (hot dogs and turkey deli meats, respectively) contaminated by serotype 4b strains that represented a novel clonal group, designated epidemic clone II (ECII) (3, 4). ECII strains have distinct genotypes as determined by pulsed-field gel electrophoresis and various other subtyping tools, and harbor unique genetic markers (6, 8, 11, 19, 34). The genome sequencing of one of the isolates (L. monocytogenes H7858) from the 1998-1999 outbreak revealed the presence of a plasmid of ca. 80 kb (pLM80), which harbored genes mediating resistance to the heavy metal cadmium as well as genes conferring resistance to the quaternary ammonium disinfectant benzalkonium chloride (10, 29).Listeria phages (listeriaphage) have long been used for subtyping purposes (33), and extensive research has focused on the genomic characterization (2, 24, 26, 35), transducing potential (14), and biotechnological applications of selected phages (25). In addition, applications of listeriaphage as biocontrol agents in foods and the processing plant environment have been investigated (12, 15, 22). However, limited information exists on phages from processing plant environments and on the impact of environmental conditions on susceptibility of L. monocytogenes strains representing the major epidemic-associated clonal groups to such phages. We have found that strains harboring ECII-specific genetic markers can indeed be recovered from the environment of turkey-processing plants (9). Furthermore, environmental samples from such processing plants yielded phages with broad host range, which were able to infect L. monocytogenes strains of various serotypes, and different Listeria species (20). In this study, we describe the impact of growth temperature on susceptibility of L. monocytogenes ECII strains to phages, including phages isolated from turkey-processing plant environmental samples.     

Effect of Octenidine Hydrochloride on Planktonic Cells and Biofilms of Listeria monocytogenes

Listeria monocytogenes is a food-borne pathogen capable of forming biofilms and persisting in food processing environments for extended periods of time, thereby potentially contaminating foods. The efficacy of octenidine hydrochloride (OH) for inactivating planktonic cells and preformed biofilms of L. monocytogenes was investigated at 37, 21, 8, and 4°C in the presence and absence of organic matter (rehydrated nonfat dry milk). OH rapidly killed planktonic cells and biofilms of L. monocytogenes at all four temperatures. Moreover, OH was equally effective in killing L. monocytogenes biofilms on polystyrene and stainless steel matrices in the presence and absence of organic matter. The results underscore OH''s ability to prevent establishment of L. monocytogenes biofilms by rapidly killing planktonic cells and to eliminate preformed biofilms, thus suggesting that it could be used as a disinfectant to prevent L. monocytogenes from persisting in food processing environments.Listeria monocytogenes is a major bacterial pathogen (2), accounting for approximately 28% of the deaths resulting from food-borne illnesses in the United States (22). It is widespread in nature and occurs in soil, vegetation, fecal matter, sewage, water, and animal feed (14). Because it is ubiquitous, L. monocytogenes is frequently isolated from foods and food processing environments (13, 23), thereby presenting a significant challenge to the food industry. Several studies have shown that L. monocytogenes is capable of adhering to food contact surfaces, such as glass, stainless steel, rubber, and polystyrene (6, 11, 28). Upon attachment to such surfaces, L. monocytogenes establishes biofilms and persists for long periods of time in the food processing environment (18, 30). This potentially poses a food safety hazard since biofilms are an important source of contamination of food products that come into contact with them. In addition, biofilms also protect the underlying bacteria from desiccation, antimicrobials, and sanitizing agents (7, 16). Thus, eradication of L. monocytogenes biofilms in processing plants is critical for improving food safety.When problems with persistent L. monocytogenes are encountered in food processing facilities, plant hygiene and sanitation are emphasized (31). This involves preventing the establishment of L. monocytogenes biofilms in the food processing environment and reducing contamination of product contact surfaces. A variety of cleaners and disinfectants, including quaternary ammonium compounds and hypochlorite, have been evaluated for this purpose (20). Although these compounds are approved by the Food and Drug Administration for use as disinfectants in processing plants, they are not effective in killing L. monocytogenes (24, 25), especially in the presence of soil or organic matter and at low temperatures. Therefore, there is a need for an effective disinfectant that can eliminate listerial biofilms in the presence of organic matter at a wide range of temperatures. Octenidine hydrochloride (OH) is a positively charged bispyridinamine that exhibits antimicrobial activity against plaque-producing organisms, such as Streptococcus mutans and Streptococcus sanguis (5). Toxicity studies with a variety of species have shown that OH is not absorbed through mucous membranes and the gastrointestinal tract, and there have been no reports of carcinogenicity, genotoxicity, or mutagenicity of this compound (17, 19, 29).The objective of this study was to investigate the efficacy of OH for inactivating planktonic cells and preformed biofilms of L. monocytogenes at 37, 21, 8, and 4°C in the presence and absence of organic matter on two matrices, polystyrene and stainless steel.     

Temperature-Dependent Requirement for Catalase in Aerobic Growth of Listeria monocytogenes F2365

Listeria monocytogenes is a Gram-positive, psychrotrophic, facultative intracellular food-borne pathogen responsible for severe illness (listeriosis). The bacteria can grow in a wide range of temperatures (1 to 45°C), and low-temperature growth contributes to the food safety hazards associated with contamination of ready-to-eat foods with this pathogen. To assess the impact of oxidative stress responses on the ability of L. monocytogenes to grow at low temperatures and to tolerate repeated freeze-thaw stress (cryotolerance), we generated and characterized a catalase-deficient mutant of L. monocytogenes F2365 harboring a mariner-based transposon insertion in the catalase gene (kat). When grown aerobically on blood-free solid medium, the kat mutant exhibited impaired growth, with the extent of impairment increasing with decreasing temperature, and no growth was detected at 4°C. Aerobic growth in liquid was impaired at 4°C, especially under aeration, but not at higher temperatures (10, 25, or 37°C). Genetic complementation of the mutant with the intact kat restored normal growth, confirming that inactivation of this gene was responsible for the growth impairment. In spite of the expected impact of oxidative stress responses on cryotolerance, cryotolerance of the kat mutant was not affected.Listeria monocytogenes is a Gram-positive, facultative intracellular food-borne pathogen that has the ability to cause a severe disease (listeriosis) in humans and animals (13, 28, 30). L. monocytogenes is ubiquitously distributed in the environment and has the ability to grow over a wide range of temperatures (between 1 and 45°C) (13). Growth at low temperature has important implications for environmental persistence of the organism and for contamination of cold-stored, ready-to-eat foods, thus contributing to the food safety hazards associated with L. monocytogenes (19).L. monocytogenes is subjected to oxidative stress during both extracellular and intracellular growth and has evolved several responses to minimize the impact of reactive oxygen species (ROS). Catalase and superoxide dismutase (SOD) work synergistically in detoxification of ROS: superoxide anions are converted to H₂O₂ by SOD, with subsequent conversion of H₂O₂ into water and oxygen by catalase (22). Exposure to ROS may be especially acute during intracellular infection as well as under certain environmental conditions, such as those involved in repeated freezing and thawing (15, 16, 23, 29, 33).Previous studies revealed that the ability of L. monocytogenes to survive repeated freezing and thawing (cryotolerance) was markedly dependent on growth temperature, with bacteria grown at 37°C having significantly higher cryotolerance than those grown at either 4 or 25°C (1). However, mechanisms underlying Listeria''s cryotolerance have not been identified. Since oxidative damage is considered to take place during freezing and thawing, determinants such as catalase may be involved in cryotolerance.The catalase of L. monocytogenes has been investigated primarily in terms of its potential role in pathogenesis, with somewhat conflicting results. The isolation of catalase-negative strains from human listeriosis patients has led to the speculation that catalase is not required for human virulence (4, 8, 12, 31). On the other hand, under certain conditions (e.g., reduced serum levels), catalase-negative strains were impaired in their ability to survive in activated macrophages in comparison to catalase-positive strains (32). Furthermore, the catalase gene kat was among those for which expression was induced in infected cell cultures and in the spleens of mice infected with L. monocytogenes EGD-e, suggesting possible contributions to pathogenesis (5, 9).The potential role of catalase in environmental adaptations of L. monocytogenes such as growth at low temperature and cryotolerance was not addressed in these earlier investigations. In this study, we have characterized an isogenic mutant of L. monocytogenes F2365 to determine the involvement of catalase in growth at different temperatures, survival in selected foods, and cryotolerance of L. monocytogenes.     

Response of Listeria monocytogenes to Disinfection Stress at the Single-Cell and Population Levels as Monitored by Intracellular pH Measurements and Viable-Cell Counts

Listeria monocytogenes has a remarkable ability to survive and persist in food production environments. The purpose of the present study was to determine if cells in a population of L. monocytogenes differ in sensitivity to disinfection agents as this could be a factor explaining persistence of the bacterium. In situ analyses of Listeria monocytogenes single cells were performed during exposure to different concentrations of the disinfectant Incimaxx DES to study a possible population subdivision. Bacterial survival was quantified with plate counting and disinfection stress at the single-cell level by measuring intracellular pH (pH_i) over time by fluorescence ratio imaging microscopy. pH_i values were initially 7 to 7.5 and decreased in both attached and planktonic L. monocytogenes cells during exposure to sublethal and lethal concentrations of Incimaxx DES. The response of the bacterial population was homogenous; hence, subpopulations were not detected. However, pregrowth with NaCl protected the planktonic bacterial cells during disinfection with Incimaxx (0.0015%) since pH_i was higher (6 to 6.5) for the bacterial population pregrown with NaCl than for cells grown without NaCl (pH_i 5 to 5.5) (P < 0.05). The protective effect of NaCl was reflected by viable-cell counts at a higher concentration of Incimaxx (0.0031%), where the salt-grown population survived better than the population grown without NaCl (P < 0.05). NaCl protected attached cells through drying but not during disinfection. This study indicates that a population of L. monocytogenes cells, whether planktonic or attached, is homogenous with respect to sensitivity to an acidic disinfectant studied on the single-cell level. Hence a major subpopulation more tolerant to disinfectants, and hence more persistent, does not appear to be present.Listeria monocytogenes is a food-borne, human pathogen that has a remarkable ability to colonize food-processing environments (5, 16, 20, 21, 26, 29). Some L. monocytogenes strains can persist for years in food-processing plants (11, 14, 20, 27), and specific molecular subtypes can repeatedly be isolated from the processing environment (29) despite being very infrequent in the outdoor environment (9). This ability to persist has, hitherto, not been linked to any specific genetic or phenotypic trait.It has been suggested that persistent L. monocytogenes strains may be more tolerant or resistant to cleaning and especially disinfectants used in the food industry. Aase et al. (1) found increased tolerance to both benzalkonium chloride and ethidium bromide in L. monocytogenes isolates that had persisted for more than 4 years; however, other studies have not been able to link persistence and tolerance to disinfectants (6, 10, 11, 13). We recently compared disinfection sensitivities of persistent and presumed nonpersistent L. monocytogenes strains using viable-cell counts and did not find the latter group more sensitive to the two disinfectants Triquart SUPER and Incimaxx DES than persistent strains (13). However, we found that for all subtypes of L. monocytogenes, growth with NaCl increased the tolerance of planktonic L. monocytogenes cells to Incimaxx DES, whereas spot-inoculated, dried L. monocytogenes cells were not protected by NaCl against disinfection.There is no doubt that L. monocytogenes will be completely inactivated at the disinfectant concentrations recommended for use in the food industry; however, the efficiency of the disinfectant is very much influenced by the presence of organic material being inactivated by the presence of food debris. Hence, it is likely that the bacterial cell in a food production environment may be exposed to concentrations at a sublethal level. It is currently not known if treatment with a sublethal concentration of disinfectant affects the entire bacterial population or only attacks a fraction of the cell population, leaving another fraction of cells unaffected. In case of the latter, some bacterial cells may be able to survive the disinfection treatment. The potential presence of such tolerant subpopulations could, ultimately, ensure that the genome is propagated, leading to persistence.The presence of a more tolerant subpopulation can be determined on the single-cell level. Flow cytometry is a rapid method useable for measurements at the single-cell resolution (22); however, it cannot monitor the same single cells over time. Optical microscopy combined with microfluidic devices that allow measurement of growth of single cells is a useful technique (2), and in situ analyses of the physiological condition of single cells by the fluorescence ratio imaging microscopy (FRIM) technique represents another elegant approach (25). FRIM enables studies of dynamic changes with high sensitivity and on the single-cell level in important physiological parameters: e.g., intracellular pH (pH_i). Listeria maintains its pH_i within a narrow range of 7.6 to 8 at extracellular pH (pH_ex) values of 5.0 to 8.0 (4, 25) and at pH_ex 4.0 with the presence of glucose (23). It is believed that viable cells need to maintain a transmembrane pH gradient with their pH_i above the pH_ex, and failure to maintain pH_i homeostasis indicates that the bacterial cell is severely stressed and ultimately leads to loss of cell viability. FRIM has been used to determine the pH_i of L. monocytogenes after exposure to osmotic and acid stress (7, 23). Also, the dissipation of the pH gradient in L. monocytogenes after exposure to different bacteriocins has been determined with FRIM (4, 12). Hornbæk et al. (12) found that treatment with subinhibitory concentrations of leucocin and nisin gave rise to two subpopulations: one consisting of cells with a dissipated pH gradient (ΔpH) and the other consisting of cells that maintained ΔpH, which could indicate phenotypic heterogeneity.The aim of the present study was to investigate the physiological effects of the disinfectant Incimaxx DES at sublethal and lethal concentrations on single cells and the population level of a persistent L. monocytogenes strain to study a possible subdivision of sensitivity in the population. We also addressed the potential protective effect of NaCl against disinfection and compared sensitivities in a population of planktonic and attached bacteria. We applied the in situ technique FRIM and compared the pH_i measurements with the traditional viable-cell-count method.(Part of the results have been presented at a poster session at the 95th International Association for Food Protection annual meeting in Columbus, OH, 3 to 6 August 2008.)     

Role of Extracellular DNA during Biofilm Formation by Listeria monocytogenes

Listeria monocytogenes is a food-borne pathogen that is capable of living in harsh environments. It is believed to do this by forming biofilms, which are surface-associated multicellular structures encased in a self-produced matrix. In this paper we show that in L. monocytogenes extracellular DNA (eDNA) may be the only central component of the biofilm matrix and that it is necessary for both initial attachment and early biofilm formation for 41 L. monocytogenes strains that were tested. DNase I treatment resulted in dispersal of biofilms, not only in microtiter tray assays but also in flow cell biofilm assays. However, it was also demonstrated that in a culture without eDNA, neither Listeria genomic DNA nor salmon sperm DNA by itself could restore the capacity to adhere. A search for additional necessary components revealed that peptidoglycan (PG), specifically N-acetylglucosamine (NAG), interacted with the DNA in a manner which restored adhesion. If a short DNA fragment (less than approximately 500 bp long) was added to an eDNA-free culture prior to addition of genomic or salmon sperm DNA, adhesion was prevented, indicating that high-molecular-weight DNA is required for adhesion and that the number of attachment sites on the cell surface can be saturated.The food-borne pathogen Listeria monocytogenes is known to persist in food processing plants (28, 48), and it has been reported that some strains of this species are capable of forming biofilms (2, 16). The mechanisms of biofilm formation have not been elucidated, but this process seems to depend on factors such as temperature and inducing compounds (14). One inducing compound is NaCl (22), but ethanol, isopropanol (14), quorum sensing (36), and an increasing temperature (8, 14, 38) also seem to enhance attachment and biofilm formation, whereas an acidic pH reduces adhesion (17, 38, 43). Furthermore, at 30°C flagellum-based motility seems to be a specific determinant for the initial adhesion (23, 42) and biofilm formation (23); however, it has recently been reported that in time nonflagellated mutants can produce hyperbiofilms (42).Since bacteria adhering to surfaces, both in biofilms and as single cells, exhibit increased resistance to sanitizers and antimicrobial agents (10, 41), examining the essential steps in adhesion and biofilm formation is important in order to develop new and improved sanitation processes.Extracellular DNA (eDNA) is a ubiquitous component of the organic matter pool in soil, marine, and freshwater habitats (26), but it is also found in environments as diverse as tissue cultures and the blood of mammals (11, 25). The presence of eDNA in the matrix of multicellular structures has recently been reported to influence the initial attachment and/or biofilm structure of Pseudomonas (1, 47), Streptococcus (29), and Staphylococcus (21, 33, 34) species.The prevalence of eDNA in nature appears to be associated with both lysis of cells and active secretion. The concentrations of eDNA released can be up to 2 μg g⁻¹ soil (30) and up to 0.5 g (m²)⁻¹ in the top few centimeters of deep-sea sediment (where more than 90% of the DNA is extracellular) (5). In the deep sea eDNA plays a key role in the ecosystem, functioning as a nitrogen and phosphorus reservoir (5). At present, there are different theories concerning both the function and the release of eDNA in multicellular structures. The presence of eDNA could be a result of either cell lysis (33, 34) or vesicle release (47), whereas active transport is a more speculative explanation. The role of eDNA in biofilm structure has not been revealed yet, but various functions, including a role as a structural component, an energy and nutrition source, or a gene pool for horizontal gene transfer (HGT) in naturally competent bacteria, can be envisaged.Until now there have been no studies of L. monocytogenes eDNA as a possible matrix component in relation to adhesion and biofilm development. In this study, we determined for the first time the presence of L. monocytogenes eDNA, its origin, and its role as a matrix component for both single-cell adhesion and biofilm formation using static assays, as well as flow cell systems. Furthermore, we showed that an additional component is necessary for eDNA-mediated adhesion.     

Influence of Stress on Single-Cell Lag Time and Growth Probability for Listeria monocytogenes in Half Fraser Broth

The impacts of 12 common food industry stresses on the single-cell growth probability and single-cell lag time distribution of Listeria monocytogenes were determined in half Fraser broth, the primary enrichment broth of the International Organization for Standardization detection method. First, it was determined that the ability of a cell to multiply in half Fraser broth is conditioned by its history (the probability for a cell to multiply can be decreased to 0.05), meaning that, depending on the stress in question, the risk of false-negative samples can be very high. Second, it was established that when cells are injured, the single-cell lag times increase in mean and in variability and that this increase represents a true risk of not reaching the detection threshold of the method in the enrichment broth. No relationship was observed between the impact on single-cell lag times and that on growth probabilities. These results emphasize the importance of taking into account the physiological state of the cells when evaluating the performance of methods to detect pathogens in food.Listeria monocytogenes has been involved in severe food-borne outbreaks with high mortality rates. This pathogen is widespread in many environments (16) and can be isolated from a large variety of foods which are the major routes of infection in humans. Ready-to-eat foods that can support the growth of L. monocytogenes may pose a major risk for public health, and the European Union legislation generally requires absence in 25 g at the production stage as a food safety criterion for this type of food (4).In food, L. monocytogenes is often affected by one or more stresses caused by a variety of processing treatments, including heating, freezing, and exposure to acids and to high osmotic pressures (15, 25, 29, 39). Recovering stressed L. monocytogenes from food is of great importance in food safety since sublethally injured bacteria may repair themselves under suitable conditions and regain or even increase their pathogenicity (19, 30).The injury of microbial cells has two major consequences for pathogen behavior in enrichment broths. First, injured cells become sensitive to selective components present in enrichment broths to which they normally show resistance (9, 10, 11, 42). Therefore, some cells of the stressed bacterial population do not initiate growth in enrichment broth, eventually resulting in an inefficient detection of pathogenic bacteria in food samples (50). This phenomenon can explain results obtained in several studies showing the effect of inoculum size on the growth limits of bacterial populations (26, 27, 37). Second, due to repair time, stressed cells show a longer lag phase than do healthy cells (5, 7, 37). This situation results in a true risk of not reaching the bacterial concentration necessary for the detection of the pathogen (in the range of 10² to 10⁴ CFU ml⁻¹) within the enrichment duration.The recent development of gene-based or immunologically based procedures, such as PCR, gene probes, and enzyme-linked immunosorbent assay, has facilitated the development of more-rapid methods which can identify positive samples in considerably shorter time periods. Nevertheless, these relatively rapid tests also require efficient enrichment steps to increase target organism numbers to detectable levels.At the moment of pathogen detection, low numbers of sublethally injured cells, as often encountered in naturally contaminated foods, show a wide distribution of lag-phase durations (45) and may not be able to multiply in broth containing selective components (11, 42). The challenge of the enrichment stage is to obtain appropriate enrichment conditions (2) which will favor pathogen resuscitation and limit the food microflora growth.In our study, we have focused on the primary enrichment phase of the International Organization for Standardization 11290-1 L. monocytogenes detection method (3), i.e., the half Fraser broth (1/2FB). The objectives were to investigate the impact of 12 different stresses on the single-cell growth probability and single-cell lag time of L. monocytogenes in 1/2FB. The intraspecific variability and the impact of food components and background microflora on single-cell growth probability were also studied.     

Probabilistic Model for Listeria monocytogenes Growth during Distribution,Retail Storage,and Domestic Storage of Pasteurized Milk

A survey on the time-temperature conditions of pasteurized milk in Greece during transportation to retail, retail storage, and domestic storage and handling was performed. The data derived from the survey were described with appropriate probability distributions and introduced into a growth model of Listeria monocytogenes in pasteurized milk which was appropriately modified for taking into account strain variability. Based on the above components, a probabilistic model was applied to evaluate the growth of L. monocytogenes during the chill chain of pasteurized milk using a Monte Carlo simulation. The model predicted that, in 44.8% of the milk cartons released in the market, the pathogen will grow until the time of consumption. For these products the estimated mean total growth of L. monocytogenes during transportation, retail storage, and domestic storage was 0.93 log CFU, with 95th and 99th percentiles of 2.68 and 4.01 log CFU, respectively. Although based on EU regulation 2073/2005 pasteurized milk produced in Greece belongs to the category of products that do not allow the growth of L. monocytogenes due to a shelf life (defined by law) of 5 days, the above results show that this shelf life limit cannot prevent L. monocytogenes from growing under the current chill chain conditions. The predicted percentage of milk cartons—initially contaminated with 1 cell/1-liter carton—in which the pathogen exceeds the safety criterion of 100 cells/ml at the time of consumption was 0.14%. The probabilistic model was used for an importance analysis of the chill chain factors, using rank order correlation, while selected intervention and shelf life increase scenarios were evaluated. The results showed that simple interventions, such as excluding the door shelf from the domestic storage of pasteurized milk, can effectively reduce the growth of the pathogen. The door shelf was found to be the warmest position in domestic refrigerators, and it was most frequently used by the consumers for domestic storage of pasteurized milk. Furthermore, the model predicted that a combination of this intervention with a decrease of the mean temperature of domestic refrigerators by 2°C may allow an extension of pasteurized milk shelf life from 5 to 7 days without affecting the current consumer exposure to L. monocytogenes.L. monocytogenes is an important safety concern for the dairy industry. Several listeriosis outbreaks have been associated with the consumption of dairy products, including pasteurized milk (13, 22). An effective control of L. monocytogenes in pasteurized milk should be based on the selection of raw milk and the controls of the processing, packaging, distribution, and storage conditions. In general, the pathogen is effectively controlled during pasteurization. However, its presence in the finished product is possible as a result of postpasteurization contamination from sources in the plant environment. Considering that the levels of postpasteurization contamination are usually very low, the extent of L. monocytogenes growth during distribution, retail storage, and domestic storage is of major significance for the safety status of pasteurized milk at the time of consumption.The growth of L. monocytogenes during distribution and storage of pasteurized milk can be evaluated using the available predictive models. During the last decade, a large number of mathematical models for L. monocytogenes growth have been published (9, 11, 16, 19, 21, 24, 26, 31, 38), and some of them have been targeted to pasteurized milk (1, 40, 49). However, since the available data show that conditions that prevail during the chill chain vary significantly (8, 17, 23, 27, 28, 34, 44, 45, 48), the value of a deterministic application of these models as a tool in safety management of pasteurized milk would be limited.In recent years the need for taking into account the variabilities of the various factors in predictive microbiology has been increasingly recognized, leading to a more sophisticated modeling approach called probabilistic or stochastic modeling. The main characteristic of probabilistic modeling is the specific quantification of variabilities using probability distributions for the input data rather than point estimates. The importance of characterizing variability was stressed by Nauta (41), who illustrated the differences in decisions that could result if variability is ignored. Probabilistic modeling is being used with increasing frequency in the area of food safety. It has been extensively applied in quantitative microbial risk assessments (12, 18, 20), in quality and safety management systems (25, 30, 32), and recently for more specific topics, such as the evaluation of the effects of food processing (39) and the compliance of food products to safety criteria set by regulations (33).In the present study a probabilistic modeling approach was applied for evaluating the growth of Listeria monocytogenes in pasteurized milk from production to the time of consumption based on a Monte Carlo simulation. The objectives of the study were (i) to estimate the growth of the pathogen at the various stages of the chill chain, including transportation to retail, retail storage, and domestic storage, (ii) to analyze the importance of the chill chain factors, (iii) to evaluate the effects of selected intervention scenarios related to the improvement of the chill chain and handling conditions, and (iv) to evaluate the effect of a potential extension of milk shelf life on the growth of Listeria monocytogenes.