RAPD- and actA gene-typing of Listeria monocytogenes isolates of human listeriosis, the intestinal contents of cows and beef

Seventy-five L. monocytogenes isolates of human listeriosis, the intestinal contents of cows and beef were divided into 5 major clusters, 17 sub-clusters and 28 minor clusters by typing using random amplification of polymorphic DNA (RAPD). According to their major RAPD category, L. monocytogenes isolates serotyped as 1/2b and 4b were distinguished from L. monocytogenes isolates of serovars 1/2a and 1/2c. Moreover serovar 4b was distinguished from serovar 1/2b by a difference in the RAPD sub-cluster category. All L. monocytogenes were found to possess either actA gene Type I or II, and only one actA gene type was detected in each RAPD minor cluster. actA gene Type II was observed in 32.0%, 38.5% and 18.9% of isolates from humans, cows and beef, respectively, and was detected more frequently in serovar 4b (46.9%) than in serovars 1/2a (22.2%), 1/2b (7.7%) and 1/2c (0.0%). Twenty (80%) of 25 human isolates fell within three minor RAPD types (II-d (16%), V-p-1 (36%), V-p-2 (28%)). Two isolates from humans and beef were found to have the same RAPD type (Type IV-k-1), actA gene type (Type I) and serovar (1/2b). Our results suggest that only a few genotypes of L. monocytogenes are predominant in human listeriosis in Japan, although the human isolates were collected over a broad span of time and a wide geographical range. Our results also suggest that RAPD-, actA gene- and sero-typing can be useful for epidemiological analysis.     

Characteristics and frequency of detection of fecal Listeria monocytogenes shed by livestock, wildlife, and humans

Listeria monocytogenes is a facultative intracellular pathogen that can be carried asymptomatically in various animals and can be shed in feces. We investigated the prevalence and characteristics of L. monocytogenes isolated from livestock, wildlife, and human potential sources of contamination in 2 areas in Ontario, Canada. From February 2003 to November 2005, a total of 268 fecal samples were collected from different animals. Listeria monocytogenes was isolated using selective enrichment, isolation, and confirmation procedures, and 15 samples (6%) yielded to the isolation of 84 confirmed strains. Listeria monocytogenes was isolated from livestock (beef and dairy), wildlife (deer, moose, otter, and raccoon), and human (biosolids and septic) fecal sources. Thirty-two isolates were from serovar 1/2a, 34 from serovar 1/2b, 1 from serovar 3a, and 17 from serovar 4b. Listeria monocytogenes populations were resolved into 13 EcoRI ribotypes, and 18 ApaI and 18 AscI pulsotypes, with Simpson indexes of discrimination of 0.878 and 0.907, respectively. A majority (59%) of L. monocytogenes isolates exhibited potential virulence linked to the production of a functional internalin A, which was supported by higher entry into Caco-2 cells (9.3%) than isolates producing truncated and secreted internalin A (1.3% of entry). Listeria monocytogenes fecal isolates were on average resistant to 6.4 +/- 2.5 antibiotics out of 17 tested, and potentially virulent isolates exhibited an enhanced resistance to kanamycin, gentamicin, streptomycin, and rifampicin. Livestock, wildlife, and human L. monocytogenes fecal communities exhibited overlapping but distinct populations, and some genotypes and phenotypes were similar to those previously described for surface water isolates in the same area.     

进口水产品中单增李斯特菌的分子流行病学特点

摘要：[目的] 探明进口水产品中单增李斯特菌的污染状况、致病性和分子特征。[方法] 针对2007年7月至2008年11月间从29个国家进口的1275批水产品,进行单增李斯特菌鉴定、谱系与血清型分析、小鼠毒力试验与多位点序列分析。[结果] 检出单增李斯特菌33批次（2.6%）,其中以4b型为主（65.2%）,而1/2a型、1/2b型与1/2c型仅分别占13.0%、17.4%与4.4%。这些分离株对小鼠均具有与强毒参考株相当的毒力。基于actA-hisJ-ribC-sigB的多位点序列分析可将32个菌株分为23个序列型,分辨力达0.97。其中3个序列型包含3个以上分离株,其中序列型9属于流行性克隆I。[结论] 进口水产品中单增李斯特菌污染率与国内水产品相近,但血清型分布以4b型为主,且有流行性克隆I检出,因此要加强对进口水产品中单增李斯特菌的监测。     

Pulsed-field fingerprinting of listeriae: identification of genomic divisions for Listeria monocytogenes and their correlation with serovar.

Clamped homogeneous electric field (CHEF) electrophoresis was optimized for genomic analyses of Listeria monocytogenes. Various human, animal, food, and environmental isolates, as well as strains representing other Listeria species, were separately digested with rarely cutting endonucleases. Of 176 L. monocytogenes strains analyzed, the enzymes AscI and ApaI established 63 and 72 unique restriction endonuclease digestion profiles (REDP), respectively. The 22 non-L. monocytogenes strains exhibited 18 AscI and 19 ApaI unique REDP. Statistical analyses of REDP information using the Dice coincidence index and principal component analysis revealed two distinct genomic divisions of L. monocytogenes that also correlated with the flagellar (H) antigen type: division I contained serovar 1/2a, 1/2c, 3a, and 3c stains and division II contained serovar 1/2b, 3b, 4b, 4d, and 4e strains. Division I isolates digested with ApaI were further grouped into cluster IA (serovar 1/2c and 3c) and cluster IB (serovar 1/2a and 3a) strains. Likewise, division II isolates digested with ApaI were further grouped into cluster IIA (serovar 1/2b and 3b) and cluster IIB (serovar 4b, 4d, and 4e) strains. These data indicate that genotypic data generated by CHEF can be directly related to phenotypic data generated by serotyping for establishing the overall relatedness of isolates. Moreover, these data further substantiate that CHEF analysis is a reproducible and highly discriminating method for characterizing L. monocytogenes strains at the molecular level.     

Prevalence and fingerprinting of Listeria monocytogenes strains isolated from raw whole milk in farm bulk tanks and in dairy plant receiving tanks

The incidence of Listeria species in raw whole milk from farm bulk tanks and from raw milk in storage at a Swedish dairy plant was studied. Listeria monocytogenes was found in 1.0% and Listeria innocua was found in 2.3% of the 294 farm bulk tank (farm tank) milk specimens. One farm tank specimen contained 60 CFU of L. monocytogenes ml(-1). L. monocytogenes was detected in 19.6% and L. innocua was detected in 8.5% of the milk specimens from the silo receiving tanks at the dairy (dairy silos). More dairy silo specimens were positive for both Listeria species during winter than during summer. Restriction enzyme analysis and pulsed-field gel electrophoresis were applied to 65 isolates of L. monocytogenes, resulting in 16 different clonal types. Two clonal types were shared by the farm tank milk and the dairy silo milk. All except one clonal type belonged to serovar 1/2a. In the dairy silo milk five clonal types were found more frequently and for a longer period than the others. No Listeria species were found in any other samples from the plant.     

Genetic characterization of Listeria monocytogenes food isolates and pathogenic potential within serovars 1/2a and 1/2b

A total of 39 Listeria monocytogenes strains isolated from raw milk, smoked meat, chicken carcass and reference strains, belonging to serovars 1/2a, 4a, 1/2b, 3b and 4b, were analysed by RAPD and by polymorphisms of the virulent genes inlAB and iap. Ten isolates, belonging to serovars 1/2a and 1/2b and, collected from raw milk and smoked meat, were further tested for pathogenicity by IP injection into mice. The clustering of the 39 L. monocytogenes strains in 3 groups at 0.45 similarity level, based on molecular typing, was observed. Distribution of serovars in these clusters was in agreement with the proposed three Listeria monocytogenes lineages. Within serovar 1/2b, the 50% lethal dose (LD50) ranged from 8.4 x 10(4) to 1.7 x 10(6) cfu.ml(-1). One of the serovar 1/2b strains, isolated from smoked meat, exhibited the lowest virulence potential evaluated by LD50 and by mean time to death (MTD) and, from this point of view, was completely different from the other strains. Our results suggest the existence of heterogeneity in virulence levels within serovars 1/2a and 1/2b. However, when comparing the isolates based on genotyping, virulence indicators and food origin, no relation could be assessed.     

Heteroduplex mobility assay for the identification of Listeria sp and Listeria monocytogenes strains: application to characterisation of strains from sludge and food samples

One hundred and ten Listeria sp. isolates from sewage sludge were identified according to phenotypic and genotypic methods. The Listeria sp. strains isolated from five types of sludge from three sewage treatment plants in Angers (France) and the surrounding area included L. monocytogenes (55.5%), L. innocua (29.1%), L. seeligeri (13.6%) and L. welshimeri (1.8%). The majority of L. monocytogenes strains belonged to serotypes 4b, 1/2b and 1/2a. Moreover, a heteroduplex mobility assay based on the 16S rRNA sequences was tested for its ability to identify the six species of the genus Listeria. This study, performed on 283 Listeria sp. strains from human, food and sewage sludge samples, showed that all the species were distinguishable from one another. L. innocua and L. seeligeri showed respectively three and two distinct banding patterns. Within L. monocytogenes, four groups (I-IV) were defined. The majority of food and environmental isolates were clustered in group I and it is noteworthy that group IV clustered epidemiologic isolates and strains belonging to serotypes 4b, 1/2a and 1/2b.     

Proteomic expression profiles of virulent and avirulent strains of Listeria monocytogenes isolated from macrophages

Listeria monocytogenes is able to survive and proliferate within macrophages. In the current study, the ability of three L. monocytogenes strains (serovar 1/2a strain EGDe, serovar 4b strain F2365, and serovar 4a strain HCC23) to proliferate in the murine macrophage cell line J774.1 was analyzed. We found that the avirulent strain HCC23 was able to initiate an infection but could not establish prolonged infection within the macrophages. By contrast, strains EGDe and F2365 proliferated within macrophages for at least 7 h. We further analyzed these strains by comparing their protein expression profiles at 0 h, 3 h, and 5 h post-infection using multidimensional protein identification technology coupled with electrospray ionization tandem mass spectrometry. Our results indicated that similar metabolic and cell wall associated proteins were expressed by all three strains at 3 h post-infection. However, increased expression of stress response and DNA repair proteins was associated with the ability to proliferate in macrophages at 5 h post-infection. By comparing the protein expression patterns of these three L. monocytogenes strains during intracellular growth in macrophages, we were able to detect biological differences that may determine the ability of L. monocytogenes to survive in macrophages.     

Amplified fragment length polymorphism (AFLP) analysis of Listeria monocytogenes

An agarose gel based single enzyme AFLP method using EcoR1 digestion of Listeria monocytogenes DNA was developed for epidemiological typing. The method was evaluated with 84 L. monocytogenes cultures, and results were compared with those obtained with serotyping, phage-typing and cadmium and arsenic resistance typing. The AFLP method was reproducible and 14 different banding patterns comprising between five and eight DNA fragments were produced. All except two of the AFLP patterns were serorype specific. Different AFLP patterns were recognised within serovar 4b (four patterns), 1/2a (two patterns), 1/2b (six patterns): single patterns were obtained from cultures of serovars 1/2c, 3a, 3b and 3c. There were associations with AFLP results and those from phage-typing and cadmium and arsenic resistance typing, although each method showed some independence. This preliminary evaluation suggests that this AFLP method will be useful for epidemiological typing of L. monocytogenes.     

Analysis of surface proteins of Listeria in relation to species, serovar and pathogenicity.

SDS extracts of whole bacteria, representing five species and 15 serovars of Listeria, were analysed by SDS-PAGE and by immunoblotting with serum directed against whole formalin-treated L. monocytogenes. Profiles of L. monocytogenes were very different from those of other species of Listeria (i.e. L. innocua,L.welshimeri, L. seeligeri and L. ivanovii). This low degree of similarity between species was found even in the case of common serovars. Within the species L. monocytogenes, protein patterns were characterized, on the one hand, by a high degree of homogeneity between all strains of the same serovar and, on the other hand, by large differences between serovars, especially between sv. 1/2 and 4b. Thus we have identified major, surface-located protein antigens, specific for L. monocytogenes, either common to all serovars (64 and 68 kDa) or characteristic of certain serovars: 98 kDa for sv. 1/2 and 3; 76 and 78 kDa for sv. 4b, 4d and 4e; and 80 and 100 kDa for sv. 4a and 4c. Moreover, some of these bands (68 and 98 kDa) might be related to virulence, since differences were noticed between the profiles of haemolytic L. monocytogenes vs. 1/2a differing only in their virulence for immunocompromised mice. All these results confirmed, for the first time, the classification of Listeria obtained previously by genomic studies. They should help in the identification of new virulence factors and the development of easier and more specific methods of detection and identification.     

DNA fragments from regions involved in surface antigen expression specifically identify Listeria monocytogenes serovar 4 and a subset thereof: cluster IIB (serotypes 4b, 4d, and 4e).

Listeria monocytogenes serotype 4b has frequently been implicated in sporadic as well as epidemic listeriosis. On the basis of pulsed-field fingerprinting, serotype 4b strains, along with strains of serotypes 4d and 4e, constitute one genomic cluster (IIB). We have identified two genomic regions essential for the expression of surface antigens which previously were shown to be specific to cluster IIB strains. A DNA probe of 1.1 kb derived from one of the regions (probe 1) hybridized only with strains of serotypes 4b, 4d, and 4e in Southern blots and dot blots. A different DNA probe of 0.3 kb (probe 2), derived from the other region, hybridized with all serovar 4 strains (serotypes 4b, 4a, 4c, 4d, and 4e). All other L. monocytogenes serotypes were negative with probe 1 or 2. Use of probe 1 in Southern blots of EcoRI-digested genomic DNA revealed a restriction fragment length polymorphism in serotype 4b strains, with the hybridizing EcoRI fragments being 4.5 kb (strains of the epidemic clone) and either 4.5 or 5.0 kb (all other serotype 4b strains). Although the probes hybridized with a special group of Listeria innocua strains which also expressed the surface antigens, the latter could be readily distinguished by the size of the hybridizing EcoRI fragment with probe 1 (ca. 2.2 kb). These data suggest that the combined use of these probes with L. monocytogenes can readily and specifically identify cluster IIB strains as well as the entire serovar 4 complex.     

Screening and characterization of monoclonal antibodies to the surface antigens of Listeria monocytogenes serotype 4b

Aims:  This study aims to develop and characterize monoclonal antibodies (Mabs) with high specificity and affinity for surface antigens of an epidemiologically important serotype 4b of Listeria monocytogenes .
Methods and Results:  Hybridoma clones were derived from B lymphocytes of mice immunized with L. monocytogenes serotype 4b and screened against this strain by an enzyme-linked immunosorbent assay. Twenty-nine clones secreting Mabs reactive with formalin-killed bacteria were obtained; 15, 8, 5 and 1 Mabs were immunoglobulin subclasses IgG2a, IgG2b, IgM and IgG1, respectively. Immuofluoresence or immunogold labelling demonstrated all except five IgM and one IgG2a Mabs bound to the surface of a live L. monocytogenes serotype 4b. The majority of the 23 surface-binding Mabs recognized linear epitopes on a 77-kDa protein. These surface-binding Mabs exhibited little or no cross-reactivity with non-4b serotypes (1/2a, 1/2b, 3a, etc.) of L. monocytogenes , five other Listeria species, Escherichia coli O157:H7 and Salmonella enterica serovar Typhimurium.
Conclusions:  The Mabs recognizing a 77-kDa surface protein are novel antibodies with specificity and affinity for L. monocytogenes serotype 4b.
Significance and Impact of the Study:  These anti-77 kDa surface protein Mabs may be explored as reagents for the development of Mabs-based diagnostic immunoassays for L. monocytogenes serotype 4b strains.     

Invasion assay of Listeria monocytogenes using Vero and Caco-2 cells

The invasion ability of Listeria monocytogenes into cultured cells has been used to evaluate its pathogenicity. In this study, invasive ability was investigated using Vero and Caco-2 cell lines. The form of invasion showed no morphological differences between both cell lines inoculated with L. monocytogenes L89-H2 or L96-23C1 strains when double fluorescence stained with rhodamine and FITC or with Giemsa staining. Recovery count and recovery rate of L. monocytogenes from Vero cells was related to the number of inoculated bacteria (2 x 10(5) to 2 x 10(7)/ml) in a bell-shape pattern, though the relationship was unclear in Caco-2 cells. Recovery rate of L. monocytogenes was higher in Vero cells than Caco-2 cells at a multiplicity of infection (MOI) 10, though the rates in both cells showed different stable stages over a considerably wide range of MOI. The recovery rate of all five L. monocytogenes strains from listeriosis patients was 15% at MOI 10 from infected Vero cells, while meat-derived strains showed variable rates regardless of the serovar. These results suggest that the Vero cell line is suitable for an invasion assay and that a recovery rate of 15% may be the critical limit for the expression of pathogenicity in the host.     

Distribution and characteristics of Listeria monocytogenes isolates from surface waters of the South Nation River watershed, Ontario, Canada

Listeria monocytogenes is a facultative intracellular pathogen thought to be widely distributed in the environment. We investigated the prevalence and characteristics of L. monocytogenes isolates from surface waters derived from catchments within the South Nation River watershed (Ontario, Canada). This watershed is dominated by urban and rural development, livestock and crop production, and wildlife habitats. From June to November 2005, a total of 314 surface water samples were collected biweekly from 22 discrete sampling sites characterized by various upstream land uses. Presumptive Listeria spp. were isolated using a selective enrichment and isolation procedure, and 75 L. monocytogenes isolates were identified based on colony morphology, hemolytic activity, and amplification of three pathogenicity genes: iap, inlA, and hlyA. Thirty-two of 314 (10%) surface water samples were positive for the presence of L. monocytogenes, but detection ranged between 0 and 27% depending on the sampling date. Isolates belonging to serovar group 1/2a, 3a (50%) and group 4b, 4d, 4e (32%) were dominant. L. monocytogenes populations were resolved into 13 EcoRI ribotypes and 21 ApaI and 21 AscI pulsotypes. These had Simpson indexes of discrimination of up to 0.885. Lineage I-related isolates were dominant (61%) during the summer, whereas lineage II isolates were dominant (77%) in the fall. Isolates were, on average, resistant to 6.1 +/- 2.1 antibiotics out of 17 tested. Half of the L. monocytogenes isolates exhibited potential virulence linked to the production of a functional internalin A, and some isolates were found to be moderately to highly virulent by in vitro Caco-2 plaque formation assay (up to 28% of entry). There was a statistically significant link between the occurrence of L. monocytogenes and proximity to an upstream dairy farm and degree of cropped land. Our data indicate that L. monocytogenes is widespread in the studied catchments, where it could represent a public health issue related to agricultural land use.     

Minimum growth temperatures of Listeria monocytogenes and non-haemolytic Listeria

Minimum growth temperatures and those of decreased growth were determined for 100 strains of listerias. The ability of 78 strains of Listeria monocytogenes isolated from animals and 22 non-haemolytic strains to grow at low temperatures was studied, using a flooding technique, in a plate-type continuous temperature gradient incubator at temperatures between -1.6 and 14.5 degrees C. The mean minimum temperature for L. monocytogenes was +1.7 +/- 0.5 degrees C. The growth of non-haemolytic listerias was unobservable at +1.7 +/- 0.5 degrees C. The L. monocytogenes strains grew at about 0.6 degrees C lower than the non-pathogenic strains. No differences in growth temperatures were observed among L. monocytogenes strains isolated from different sources. The serovars with the OI antigen grew at lower temperatures (+1.0 +/- 0.3 degrees C) than the other common serovar 4b (+1.3 +/- 0.4 degrees C). The results indicate that L. monocytogenes grows better than non-haemolytic strains under cold conditions. The possible role of haemolysins as growth factors is also discussed.     

Differentiation of Listeria monocytogenes serovars by using artificial neural network analysis of Fourier-transformed infrared spectra

A classification system based on Fourier transform infrared (FTIR) spectroscopy combined with artificial neural network analysis was designed to differentiate 12 serovars of Listeria monocytogenes using a reference database of 106 well-defined strains. External validation was performed using a test set of another 166 L. monocytogenes strains. The O antigens (serogroup) of 164 strains (98.8%) could be identified correctly, and H antigens were correctly determined in 152 (91.6%) of the test strains. Importantly, 40 out of 41 potentially epidemic serovar 4b strains were unambiguously identified. FTIR analysis is superior to PCR-based systems for serovar differentiation and has potential for the rapid, simultaneous identification of both species and serovar of an unknown Listeria isolate by simply measuring a whole-cell infrared spectrum.     

Host ranges of Listeria-specific bacteriophages from the turkey processing plant environment in the United States

Even though at least 400 Listeria phages have been isolated from various sources, limited information is available on phages from the food processing plant environment. Phages in the processing plant environment may play critical roles in determining the Listeria population that becomes established in the plant. In this study, we pursued the isolation of Listeria-specific phages from environmental samples from four turkey processing plants in the United States. These environmental samples were also utilized to isolate Listeria spp. Twelve phages were isolated and classified into three groups in terms of their host range. Of these, nine (group 1) showed a wide host range, including multiple serotypes of Listeria monocytogenes, as well as other Listeria spp. (L. innocua, L. welshimeri, L. seeligeri, and L. ivanovii). The remaining phages mostly infected L. monocytogenes serotype 4b as well as L. innocua, L. ivanovii, and/or L. welshimeri. All but one of the strains of the serotype 4b complex (4b, 4d, 4e) from the processing plant environment could be readily infected by the wide-host-range phages isolated from the environment of the processing plants. However, many strains of other serotypes (1/2a [or 3a] and 1/2b [or 3b]), which represented the majority of L. monocytogenes strains isolated from the environmental samples, were resistant to infection by these phages. Experiments with two phage-resistant strains showed reduced phage adsorption onto the host cells. These findings suggest that phage resistance may be an important component of the ecology of L. monocytogenes in the turkey processing plants.     

Direct detection of bacterial pathogens in representative dairy products using a combined bacterial concentration-PCR approach

AIMS: To develop a simple, rapid method to concentrate and purify bacteria and their nucleic acids from complex dairy food matrices in preparation for direct pathogen detection using polymerase chain reaction (PCR). METHODS AND RESULTS: Plain non-fat yogurt and cheddar cheese were each seeded with Listeria monocytogenes or Salmonella enterica serovar. Enteritidis in the range of 10(1)-10(6) CFU per 11-g sample. Samples were then processed for bacterial concentration using high-speed centrifugation (9700 g) followed by DNA extraction, PCR amplification, and amplicon confirmation by hybridization. Bacterial recoveries after centrifugation ranged from 53 to >100% and 71 to >100% for serovar. Enteritidis and L. monocytogenes, respectively, in the non-fat yogurt samples; and from 77 to >100% and 69 to >100% for serovar. Enteritidis and L. monocytogenes, respectively, in the cheddar cheese samples. There were no significant differences in recovery efficiency at different inocula levels, and losses to discarded supernatants were always <5%, regardless of dairy product or pathogen. CONCLUSIONS: When followed by pathogen detection using PCR and confirmation by amplicon hybridization, detection limits of 10(3) and 10(1) CFU per 11-g sample were achieved for L. monocytogenes and serovar. Enteritidis, respectively, in both product types and without prior cultural enrichment. SIGNIFICANCE AND IMPACT OF THE STUDY: This study represents progress toward the rapid and efficient direct detection of pathogens from complex food matrices at detection limits approaching those that might be anticipated in naturally contaminated products.     

Estimating size and diversity of nitrifying communities in deodorizing filters using PCR and immunofluorescence

Thirty isolates of Listeria monocytogenes and 18 of L. innocua obtained from different short-ripened cheeses manufactured in Asturias (northern Spain), were compared with each other and with reference strains using serotype, phage type and pulsed-field restriction endonuclease digestion profiles analysis of the total DNA. Restriction enzymes Apa I and Sma I defined five clusters in L. monocytogenes ( m1 to m5 ) and two main clusters in L. innocua ( i1 and i2 ). Cluster i2 was further arranged into three subclusters ( i2a , i2b and i2c ) based on the different Eco 52I ( Xma III) and Crf 42I ( Sac II) patterns of its isolates. Clusters of L. innocua were clearly different whereas those of L. monocytogenes were more closely related to each other. In this latter species, serotype 4b isolates ( m4 and m5 ) constituted a more homogeneous group than serogroup 1 isolates ( m1 , m2 and m3 ). Cluster m3 contained two strains of serotype 1/2a whereas m1 and m2 harboured strains of both serotypes, 1/2a and 1/2b. Therefore, the combined use of restriction patterns and serotype may be useful to differentiate L. monocytogenes strains showing identical restriction profiles but differing in serotype. The cheese source of Listeria strains proved that isolates from cluster m1 were repeatedly detected as a contaminant in the same type of cheese. Comparison of L. monocytogenes Apa I profiles showed a genetic proximity of m4 and m5 to the recognized pathogenic strains ATCC 13932 and NCTC 11994, responsible for meningitis cases in other countries. Finally, bacteriophage typing data indicated that m4 , the sole phage typable group, had a phage type resembling that of strains causing the Auckland (New Zealand) outbreak of listeriosis in 1969. These data suggest a wide distribution of closely related types which might cause, under several circumstances, sporadic cases of listeriosis.     

Ability of the Listeria monocytogenes strain Scott A to cause systemic infection in mice infected by the intragastric route

Listeriosis is an important food-borne disease that causes high rates of morbidity and mortality. For reasons that are not clear, most large outbreaks of human listeriosis involve Listeria monocytogenes serotype 4b. Relatively little is known about the pathogenesis of listeriosis following gastrointestinal exposure to food-borne disease isolates of L. monocytogenes. In the present study, we investigated the pathogenesis of systemic infection by the food-borne isolate Scott A in an intragastric (i.g.) mouse challenge model. We found that the severity of infection with L. monocytogenes Scott A was increased in mice made neutropenic by administration of monoclonal antibody RB6-8C5. This observation was similar to a previous report on a study with the laboratory strain L. monocytogenes EGD. Prior administration of sodium bicarbonate did not enhance the virulence of L. monocytogenes strain Scott A for i.g. inoculated mice. Following i.g. inoculation of mice, two serotype 4b strains of L. monocytogenes (Scott A and 101M) achieved a greater bacterial burden in the spleen and liver and elicited more severe histopathological damage to those organs than did a serotype 1/2a strain (EGD) and a serotype 1/2b stain (CM). Of the four strains tested, only strain CM exhibited poor survival in synthetic gastric fluid in vitro. The other three strains exhibited similar patterns of survival at pHs of greater than 5 and relatively rapid (<30 min) loss of viability at pHs of less than 5.0. Growth of L. monocytogenes Scott A at temperatures of 12.5 to 37 degrees C did not affect its ability to cause systemic infection in i.g. inoculated mice. These observations suggest that the serotype 4b L. monocytogenes strains Scott A and 101M possess one or more virulence determinants that make them better able to cause systemic infection following inoculation via the g.i. tract than do the serotype 1/2 strains EGD and CM.