Evaluation of the Organon-Teknika MICRO-ID LISTERIA system.

The MICRO-ID LISTERIA system, designed to identify Listeria isolates to species level within 24 h, was compared with conventional biochemical identification. MICRO-ID LISTERIA used in combination with the CAMP test correctly identified 409 (98.8%) of 414 strains isolated from human, animal, food, and environmental sources belonging to the seven species currently defined within the genus Listeria. The kit was easy to use and simple to interpret. However, 8 of the 15 tests (i.e., phenylalanine deaminase, hydrogen sulfide, indole, ornithine decarboxylase, lysine decarboxylase, malonate, urease, and o-nitrophenyl-beta-D-galactopyranoside) were considered superfluous for the differentiation of Listeria spp. The CAMP test was indispensable when using the MICRO-ID LISTERIA system, in particular to differentiate CAMP test-positive L. monocytogenes from the nonhemolytic, rhamnose-positive L. innocua. The hemolytic L. seeligeri and L. ivanovii strains and the nonhemolytic, non-rhamnose-acidifying L. welshimeri strains could also be differentiated from one another only on the basis of their CAMP test results. The very few strains of L. grayi and L. murrayi were easily differentiated from the other nonhemolytic species. Catalase-negative cocci should not be tested, because 12 out of 19 catalase-negative strains (all enterococci) in our test were misidentified as Listeria spp. The MICRO-ID LISTERIA system identified strains within 18 to 24 h and is thus less time-consuming than conventional tests. The system could, therefore, be used together with correctly done CAMP tests for the rapid identification of Listeria isolates, especially food and environmental isolates, for which rapid species differentiation is important.     

API Listeria, a new and promising one-day system to identify Listeria isolates.

API Listeria is a new 10-test strip for 24-h biochemical identification of Listeria isolates. With this commercial system, 85% of 646 Listeria strains, including atypical isolates selected for this study, were recognized at the species and subspecies level without a complementary test. A new test differentiates Listeria monocytogenes from L. innocua on the basis of the absence of arylamidase from the former. With this system, 97.7% (252 of 258) of the L. monocytogenes strains tested were correctly identified and differentiated from 99.4% (175 of 176) of the L. innocua strains also tested. Gram-positive bacteria other than Listeria spp. gave quite different biochemical patterns. This system considerably reduced the time needed for conventional identification, since results were available within 18 to 24 h.     

API Listeria, a new and promising one-day system to identify Listeria isolates.

API Listeria is a new 10-test strip for 24-h biochemical identification of Listeria isolates. With this commercial system, 85% of 646 Listeria strains, including atypical isolates selected for this study, were recognized at the species and subspecies level without a complementary test. A new test differentiates Listeria monocytogenes from L. innocua on the basis of the absence of arylamidase from the former. With this system, 97.7% (252 of 258) of the L. monocytogenes strains tested were correctly identified and differentiated from 99.4% (175 of 176) of the L. innocua strains also tested. Gram-positive bacteria other than Listeria spp. gave quite different biochemical patterns. This system considerably reduced the time needed for conventional identification, since results were available within 18 to 24 h.     

Direct identification in food samples of Listeria spp. and Listeria monocytogenes by molecular methods

A new molecular approach for the detection and identification of Listeria spp. and Listeria monocytogenes in food is presented here. The method is based on the PCR amplification of a fragment of the iap gene from the five species belonging to the genus and on the analysis of the PCR products obtained by denaturing gradient gel electrophoresis (DGGE). The protocol was first optimized by using strains from international collections. Based on the differences present in the sequences amplified, it was possible to obtain species-specific DGGE migration that allowed fast and easy identification of L. monocytogenes, L. innocua, L. welshimeri, L. seeligeri, and L. ivanovii. Moreover, for L. monocytogenes serotypes, partial differentiation was possible. The optimized protocol was used for identification of Listeria strains traditionally isolated from food and for direct detection and identification of Listeria members in food after an overnight enrichment. Identification of 48 food isolates and direct detection of Listeria spp. in 73 food samples show the potential of the method that can be used as a fast screening test to investigate the presence of Listeria spp. and L. monocytogenes in food.     

Characteristics of maltose transporter activity in an ale and larger strain of the yeast Saccharomyces cerevisiae

R.R. BEUMER, M.C. TE GIFFEL, M.T.C. KOK AND F.M. ROMBOUTS. 1996. All confirmation and identification methods used in this study can be used for the screening of suspected colonies on isolation media for Listeria spp. In traditional enrichment procedures the Microscreen Listeria latex test gives fast results. The DNA probes (Accuprobe and Gene-Trak) are very specific in detecting Listeria monocytogenes . For identification of Listeria spp. both tests (API and Micro-ID) performed equally well. Preference may be given to the API test, since differentiation of L. monocytogenes from L. innocua is based on the absence of arylamidase, through which tests for haemolytic activity and/or CAMP reactions can be omitted. However, the use of Enhanced Haemolysis Agar as isolation medium makes further testing essentially superfluous, since L. monocytogenes strains can be differentiated from L. innocua .     

Reliable and rapid identification of Listeria monocytogenes and Listeria species by artificial neural network-based Fourier transform infrared spectroscopy

Differentiation of the species within the genus Listeria is important for the food industry but only a few reliable methods are available so far. While a number of studies have used Fourier transform infrared (FTIR) spectroscopy to identify bacteria, the extraction of complex pattern information from the infrared spectra remains difficult. Here, we apply artificial neural network technology (ANN), which is an advanced multivariate data-processing method of pattern analysis, to identify Listeria infrared spectra at the species level. A hierarchical classification system based on ANN analysis for Listeria FTIR spectra was created, based on a comprehensive reference spectral database including 243 well-defined reference strains of Listeria monocytogenes, L. innocua, L. ivanovii, L. seeligeri, and L. welshimeri. In parallel, a univariate FTIR identification model was developed. To evaluate the potentials of these models, a set of 277 isolates of diverse geographical origins, but not included in the reference database, were assembled and used as an independent external validation for species discrimination. Univariate FTIR analysis allowed the correct identification of 85.2% of all strains and of 93% of the L. monocytogenes strains. ANN-based analysis enhanced differentiation success to 96% for all Listeria species, including a success rate of 99.2% for correct L. monocytogenes identification. The identity of the 277-strain test set was also determined with the standard phenotypical API Listeria system. This kit was able to identify 88% of the test isolates and 93% of L. monocytogenes strains. These results demonstrate the high reliability and strong potential of ANN-based FTIR spectrum analysis for identification of the five Listeria species under investigation. Starting from a pure culture, this technique allows the cost-efficient and rapid identification of Listeria species within 25 h and is suitable for use in a routine food microbiological laboratory.     

Discovery of natural atypical nonhemolytic Listeria seeligeri isolates

We found seven Listeria isolates, initially identified as isolates with the Xyl(+) Rha(-) biotype of Listeria welshimeri by phenotypic tests, which exhibited discrepant genotypic properties in a well-validated Listeria species identification oligonucleotide microarray. The microarray gives results of these seven isolates being atypical hly-negative L. seeligeri isolates, not L. welshimeri isolates. The aberrant L. seeligeri isolates were d-xylose fermentation positive, l-rhamnose fermentation negative (Xyl(+) Rha(-)), and nonhemolytic on blood agar and in the CAMP test with both Staphylococcus aureus (S(-) reaction) and Rhodococcus equi (R(-) reaction). All genes of the prfA cluster of L. seeligeri, located in the prs-ldh region, including the orfA2, orfD, prfA, orfE, plcA, hly, orfK, mpl, actA, dplcB, plcB, orfH, orfX, orfI, orfP, orfB, and orfA genes, were checked by PCR and direct sequencing for evidence of their presence in the atypical isolates. The prs-prfA cluster-ldh region of the L. seeligeri isolates was approximately threefold shorter due to the loss of orfD, prfA, orfE, plcA, hly, orfK, mpl, actA, dplcB, plcB, orfH, orfX, and orfI. The genetic map order of the cluster genes of all the atypical L. seeligeri isolates was prs-orfA2-orfP-orfB-orfA-ldh, which was comparable to the similar region in L. welshimeri, with the exception of the presence of orfA2. DNA sequencing and phylogenetic analysis of 17 housekeeping genes indicated an L. seeligeri genomic background in all seven of the atypical hly-negative L. seeligeri isolates. Thus, the novel biotype of Xyl(+) Rha(-) Hly(-) L. seeligeri strains can only be distinguished from Xyl(+) Rha(-) L. welshimeri strains genotypically, not phenotypically. In contrast, the Rha(+) Xyl(+) biotype of L. welshimeri would not present an identification issue.     

Rapid identification of Listeria species by using restriction fragment length polymorphism of PCR-amplified 23S rRNA gene fragments

A molecular method based on restriction fragment length polymorphism (RFLP) of PCR-amplified fragments of the 23S rRNA gene was designed to rapidly identify Listeria strains to the species level. Two fragments (S1, 460 bp, and S2, 890 bp) were amplified from boiled DNA. S2 was cut with the restriction enzymes XmnI or CfoI and, if needed, S1 was digested by either AluI or ClaI. This method was first optimized with six reference strains and then applied to 182 isolates collected from effluents of treatment plants. All isolates were also identified by the API Listeria kit, hemolysis, and phosphatidylinositol-specific phospholipase C production (PI-PLC) on ALOA medium. The PCR-RFLP method unambiguously identified 160 environmental strains, including 131 in concordance with the API system, and revealed that 22 isolates were mixed cultures of Listeria monocytogenes and Listeria innocua. Discrepant results were resolved by a multiplex PCR on the iap gene, which confirmed the PCR-RFLP data for 49 of the 51 discordances, including the 22 mixed cultures. Sequencing of the 16S rRNA gene for 12 selected strains and reconstruction of a phylogenetic tree validated the molecular methods, except for two unclassifiable strains. The 158 single identifiable isolates were 92 L. monocytogenes (including seven nonhemolytic and PI-PLC-negative strains), 61 L. innocua, 4 Listeria seeligeri, and 1 Listeria welshimeri strain. The PCR-RFLP method proposed here provides rapid, easy-to-use, inexpensive, and reliable identification of the six Listeria species. Moreover, it can detect mixtures of Listeria species and thus is particularly adapted to environmental and food microbiology.     

Listeria monocytogenes in spontaneous abortions in humans and its detection by multiplex PCR

AIM: To assess the extent of Listeria monocytogenes in causation of human spontaneous abortions by isolation methods and PCR analysis for the presence of virulence-associated genes. METHODS AND RESULTS: A total of 305 samples comprising blood, urine, placental bits, faecal and vaginal swabs were collected from 61 patients with spontaneous abortions. Listeria spp. were isolated from 10 samples collected from nine (14.8%) patients. Confirmation of these isolates was based on biochemical tests, haemolysis on blood agar, CAMP test, phosphatidylinositol-specific phospholipase C (PI-PLC) assay followed by in vivo pathogenicity tests and multiplex PCR to detect virulence-associated genes (prfA, plcA, hlyA, actA and iap). Three isolates were confirmed as L. monocytogenes. Of these, two isolates turned out to be pathogenic and found to posses all five genes. However, the remaining two haemolytic L. monocytogenes isolates lacking the plcA gene and activity in the PI-PLC assay were found to be nonpathogenic by in vivo tests. CONCLUSIONS: The occurrence of pathogenic L. monocytogenes in cases of spontaneous abortions was 3.3%. It seems that the plcA gene and its expression have an important role as essential virulence determinants in pathogenic Listeria spp. SIGNIFICANCE AND IMPACT OF THE STUDY: The recovery of pathogenic L. monocytogenes isolates from cases of spontaneous abortion indicates the significance of listeric infection in pregnant women.     

Natural atypical Listeria innocua strains with Listeria monocytogenes pathogenicity island 1 genes

Identification of bona fide Listeria isolates into the six species of the genus normally requires only a few tests. Aberrant isolates do occur, but even then only one or two extra confirmatory tests are generally needed for identification to species level. We have discovered a hemolytic-positive, rhamnose and xylose fermentation-negative Listeria strain with surprising recalcitrance to identification to the species level due to contradictory results in standard confirmatory tests. The issue had to be resolved by using total DNA-DNA hybridization testing and then confirmed by further specific PCR-based tests including a Listeria microarray assay. The results show that this isolate is indeed a novel one. Its discovery provides the first fully documented instance of a hemolytic Listeria innocua strain. This species, by definition, is typically nonhemolytic. The L. innocua isolate contains all the members of the PrfA-regulated virulence gene cluster (Listeria pathogenicity island 1) of L. monocytogenes. It is avirulent in the mouse pathogenicity test. Avirulence is likely at least partly due to the absence of the L. monocytogenes-specific allele of iap, as well as the absence of inlA, inlB, inlC, and daaA. At least two of the virulence cluster genes, hly and plcA, which encode the L. monocytogenes hemolysin (listeriolysin O) and inositol-specific phospholipase C, respectively, are phenotypically expressed in this L. innocua strain. The detection by PCR assays of specific L. innocua genes (lin0198, lin0372, lin0419, lin0558, lin1068, lin1073, lin1074, lin2454, and lin2693) and noncoding intergenic regions (lin0454-lin0455 and nadA-lin2134) in the strain is consistent with its L. innocua DNA-DNA hybridization identity. Additional distinctly different hemolytic L. innocua strains were also studied.     

The microbial ecology of high-risk, chilled food factories; evidence for persistent Listeria spp. and Escherichia coli strains

AIMS: The intention of this study was to provide evidence of any Listeria spp. or Escherichia coli strain persistence, and to identify whether strains of these organisms adapt to specific environmental or product niches in food factories. METHODS AND RESULTS: A 3-year assessment of the microbial ecology of four, ready-to-eat food-processing factories was undertaken in which approx. 196 000 and 75 000 product and environmental samples were examined for Escherichia coli and Listeria spp. respectively. A total of 152 E. coli isolates (44 environmental and 108 product in 62 ribogroups) and 260 Listeria spp. isolates (174 environmental and 86 product in 30 ribogroups) were identified and ribotyped. The overall prevalence of E. coli (0.08%), all Listeria spp. (0.35%) and L. monocytogenes (0.23%) was very low. Some 10 E. coli ribogroups and 14 Listeria spp. ribogroups showed evidence for persistence, defined as the isolation of the same strain, from the same site, over a prolonged time period. The majority of E. coli strains were product niche oriented whilst the majority of Listeria spp. strains were environmental niche oriented. CONCLUSION: Current UK high-risk food factory designs, personnel hygiene and cleaning and disinfection regimes are sufficient to control Listeria spp. and E. coli to very low levels. SIGNIFICANCE AND IMPACT OF THE STUDY: Persistent strains of these organisms, however, can remain within factory high-risk production areas over considerable time periods, warranting an examination of the strain persistence mechanisms and alternative hygiene controls.     

Antimicrobial resistance of Listeria spp. recovered from processed bison

AIMS: The current study examined the antimicrobial susceptibility of 86 Listeria spp. isolated from processed bison carcasses. MATERIALS AND METhods: Susceptibility to 25 antimicrobial agents was determined using E-test and National Antimicrobial Resistance Monitoring System (NARMS) panels. Most Listeria isolates (88-98%) exhibited resistance to bacitracin, oxacillin, cefotaxime, and fosfomycin. Resistance to tetracycline (18.6%) was also common. Of the 16 tetracycline-resistant Listeria isolates, 15 carried tetM and 2 contained integrase of Tn1545 transposons. Rifampicin and trimethoprim-sulfamethoxazole were the most active antimicrobial agents against Listeria spp., with a MIC(90) of 0.38 microg ml(-1). Ampicillin, erythromycin, penicillin, gentamicin, and tobramycin also exhibited good activity against Listeria spp., with MIC(90) not exceeding 1 microg ml(-1). Differences in resistance among Listeria spp. was displayed, as Listeria innocua strains were more resistant than other Listeria species. CONCLUSIONS: The study showed that Listeria monocytogenes strains from bison were susceptible to the antibiotics most commonly used to treat human listeriosis. However, the presence of antimicrobial resistance in L. innocua indicates the potential for transfer of resistance and a conjugative transposon to L. monocytogenes. SIGNIFICANCE AND IMPACT OF THE STUDY: The findings of our study will provide useful information for the development of public health policy in the use of antimicrobials in food animal production.     

Haemolytic reaction of Listeria monocytogenes on bilayer Columbia Agar plates with defibrinated guinea-pig blood

Haemolysis on blood agar is an important test for the differentation of pathogenic from non-pathogenic Listeria species. The food-borne pathogen Listeria monocytogenes can only be differentiated from harmless L. innocua by its weak β-haemolysis on blood agar. This study demonstrates that L. monocytogenes , which is weakly haemolytic on sheep or horse blood agar forms clear zones of haemolysis on guinea-pig blood agar, which renders haemolysis enhancing methods, such as the CAMP test unnecessary.     

RECOVERY and SPECIATION of LISTERIA FROM RAW and COOKED MEAT and POULTRY PRODUCTS

Recovery of Listeria from raw and cooked meat products was compared using Fraser broth (FB) enrichment incubated at 30 and 35C for 24 and 48 h. the Micro-ID Listeria test strip for biochemical characterization of Listeria was also compared with conventional tests. Listeria spp. were recovered from 33, 47, and 20 of the raw chicken, raw beef and cooked meat products, respectively. No false-negative reactions were observed and more total Listeria -positive samples were found using FB incubated for 48 h compared with 24 h. Samples incubated at 35C had fewer false negative tubes than those incubated at 30C. More false-positive FB tubes were observed after 48 h than after 24 h incubation. Over half of the cooked samples did not hydrolyze the esculin and turn the tubes black, and therefore did not have to be streaked onto selective plates. However, with raw chicken or beef because of the large number of false-positive FB tubes, almost all tubes had to be streaked onto selective plates and very little advantage was gained from using the FB. the Micro-ID Listeria test kit gave a 100% correlation with conventional biochemical reactions for pure cultures of Listeria isolated from the three categories of meat products in this study. When used in conjunction with hemolysis plates and CAMP reactions, this test identifies species of Listeria isolates within 24 h of visible colony formation.     

Comparison of the efficacy of different techniques, culture media, and sources of blood in determining the hemolytic activity of Listeria spp.

Hemolytic activity is a fundamental criterion for the differentiation of Listeria species; therefore, a simple and inexpensive procedure to clearly distinguish hemolytic strains from each other and from nonhemolytic strains would be of great aid. We compared the efficacy of several techniques, culture media, and types of blood in demonstrating the hemolysis of Listeria spp. The hemolytic activities of Listeria monocytogenes and Listeria seeligeri were more easily detected with a red blood cell top-layer (RBCTL) technique and with a microplate technique than when the strains were streaked on blood agar (BA). Listeria ivanovii produced a marked hemolysis regardless of the technique employed. In general, the hemolytic activity of these three species was stronger on media containing brain heart infusion (BHI) agar and (or) potassium tellurite (PT). However, Listeria innocua produced questionable hemolytic reactions when nonselective culture media with BHI and PT were utilized, limiting the advantages gained by employing the two compounds. The RBCTL and the BA techniques disclosed greater hemolytic activity for L. monocytogenes, L. seeligeri, and L. ivanovii with sheep and guinea pig blood than with horse and human blood. When the microplate technique was used, all four kinds of blood were equally effective.     

Evaluation of the MicroFoss system for the detection of Listeria species in environmental samples

The MicroFoss system was evaluated for its ability to detect Listeria species in environmental samples. The sensitivity and specificity of the MicroFoss were determined in relation to a standard culture method for Listeria detection. The sensitivities of both the MicroFoss and standard culture methods were similar (88.4%-MicroFoss, 90.7%-Culture) based on the total number of positive results obtained by both methods. The MicroFoss system detected Listeria spp. in 12 samples, which were not detected by culture, and the culture method detected Listeria spp. in 15 samples, which were not detected by the MicroFoss method. This was likely due to uneven distribution of low levels of Listeria organisms in the split sponge samples used to assess the performance of these test methods. The specificity value determined for the MicroFoss system was 92.7%. The majority of microbes causing false positive results in the MicroFoss system were Bacillus species, which were readily distinguishable from Listeria species by a simple Gram stain and morphological features. Listeria monocytogenes (89.4%-MicroFoss, 88.0%-Culture) and Listeria innocua (8.8%-MicroFoss, 7.7%-Culture) were the most common isolates of Listeria detected by the two test methods, with L. monocytogenes being the most predominant isolate detected. The highly comparable results and rapid nature of the MicroFoss system demonstrate its effectiveness as a detection system for species of Listeria in environmental samples. The fact that the sensitivity of the MicroFoss system was similar to that of the culture method and the Listeria results were obtained within 48 h of testing, support the use of the MicroFoss as an alternative rapid method for screening large numbers of environmental samples for Listeria spp.     

Hemolytic activity reevaluation of putative nonpathogenic Listeria monocytogenes strains.

Identification of 12 strains originally characterized as nonpathogenic Listeria monocytogenes was reassured following the evaluation of their hemolytic capability with a newly developed horse blood agar plate. Seven of the strains were observed consistently to be hemolytic and confirmed as L. monocytogenes with the use of two commercial systems: the Gene-Trak L. monocytogenes-specific colorimetric DNA hybridization assay and the API Listeria system. Except for one strain that formed typical smooth colonies, these hemolytic strains formed rough colonies on a selective medium, lithium chloride-ceftazidime agar. The rest of the strains were nonhemolytic and did not hybridize with the DNA probe; they were identified as Listeria innocua on the basis of their API Listeria system biochemical profile. All but one of these nonhemolytic strains formed smooth colonies on lithium chloride-ceftazidime agar.     

Diversity of Listeria species in urban and natural environments

A total of 442 Listeria isolates, including 234 Listeria seeligeri, 80 L. monocytogenes, 74 L. welshimeri, 50 L. innocua, and 4 L. marthii isolates, were obtained from 1,805 soil, water, and other environmental samples collected over 2 years from four urban areas and four areas representing natural environments. Listeria spp. showed similar prevalences in samples from natural (23.4%) and urban (22.3%) environments. While L. seeligeri and L. welshimeri were significantly associated with natural environments (P ≤ 0.0001), L. innocua and L. monocytogenes were significantly associated with urban environments (P ≤ 0.0001). Sequencing of sigB for all isolates revealed 67 allelic types with a higher level of allelic diversity among isolates from urban environments. Some Listeria spp. and sigB allelic types showed significant associations with specific urban and natural areas. Nearest-neighbor analyses also showed that certain Listeria spp. and sigB allelic types were spatially clustered within both natural and urban environments, and there was evidence that these species and allelic types persisted over time in specific areas. Our data show that members of the genus Listeria not only are common in urban and natural environments but also show species- and subtype-specific associations with different environments and areas. This indicates that Listeria species and subtypes within these species may show distinct ecological preferences, which suggests (i) that molecular source-tracking approaches can be developed for Listeria and (ii) that detection of some Listeria species may not be a good indicator for L. monocytogenes.     

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.     

Bacteriophage typing of Listeria species.

A bacteriophage typing scheme for differentiating Listeria isolates from dairy products and various other foodstuffs was developed. Sixteen selected phages isolated from both environmental sources and lysogenic strains were used for typing and, according to their lytic spectra, divided into four groups. Thus far, 41 distinct patterns of lysis were seen when this set was used in typing 57 defined reference strains, representing all five confirmed species and 16 serotypes in addition to 454 Listeria isolates of primarily foodborne origin. Overall, typability was 84.5%; i.e., a strain was lysed by at least one phage at 100x routine test dilution. Strains belonging to serovar 3 were mostly resistant to lysis by the phages employed. The results were highly reproducible, as determined in retyping trials several weeks later. Some phages isolated from environmental sources showed a wider lytic spectrum than did those isolated from lysogenic strains. In accordance with this, the phages were found in different clusters within a computer-generated linkage map. Species specificity and serovar specificity of the lytic reaction were not found. None of the phages was able to lyse strains of Listeria grayi, Listeria murrayi or Jonesia denitrificans. This phage typing system may provide important information for a means of recognizing and eliminating sources of contamination by Listeria spp. within dairy plant equipment.