Use of the CAMP test for identification of Listeria monocytogenes

Listeria monocytogenes in foods-From culture identification to whole-genome characteristics

Listeria monocytogenes is an important foodborne pathogen, which is able to persist in the food production environments. The presence of these bacteria in different niches makes them a potential threat for public health. In the present review, the current information on the classical and alternative methods used for isolation and identification of L. monocytogenes in food have been described. Although these techniques are usually simple, standardized, inexpensive, and are routinely used in many food testing laboratories, several alternative molecular-based approaches for the bacteria detection in food and food production environments have been developed. They are characterized by the high sample throughput, a short time of analysis, and cost-effectiveness. However, these methods are important for the routine testing toward the presence and number of L. monocytogenes, but are not suitable for characteristics and typing of the bacterial isolates, which are crucial in the study of listeriosis infections. For these purposes, novel approaches, with a high discriminatory power to genetically distinguish the strains during epidemiological studies, have been developed, e.g., whole-genome sequence-based techniques such as NGS which provide an opportunity to perform comparison between strains of the same species. In the present review, we have shown a short description of the principles of microbiological, alternative, and modern methods of detection of L. monocytogenes in foods and characterization of the isolates for epidemiological purposes. According to our knowledge, similar comprehensive papers on such subject have not been recently published, and we hope that the current review may be interesting for research communities.

Listeria monocytogenes in Export-approved Beef from Mato Grosso, Brazil: Prevalence, Molecular Characterization and Resistance to Antibiotics and Disinfectants

The Brazilian state of Mato Grosso is the largest producer and exporter of beef in the country, but few studies of relevance have been conducted to evaluate the microbiological safety of its products. This study aimed to estimate the prevalence of Listeria monocytogenes (LM) in export-approved beef from Mato Grosso and to characterize the isolates in terms of molecular properties and antimicrobial resistance. From a total of 50 samples analyzed, Listeria sp. was isolated in 18 (36% prevalence). Listeria monocytogenes was confirmed in 6 (12% prevalence). Among the serotype groups assessed by multiplex PCR, serotype 4 (4b, 4d or 4e) was the most prevalent. Although antibiotic resistance was not an issue, two strains isolated from different plants showed high resistance to sodium hypochlorite. Overall, this scenario causes concern because it puts at risk not only the Brazilian customer, but also the population of countries that import beef from Mato Grosso.

Structure determination of CAMP factor of Mobiluncus curtisii and insights into structural dynamics

Bacterial vaginosis (BV) is a common type of vaginal inflammation caused by a proliferation of pathogenic bacteria, among which Mobiluncus curtisii. In our previous studies on M. curtisii genome, we identified the presence of a genomic fragment encoding a 25 kDa pore-forming toxin, the CAMP factor, which is known to be involved in the synergistic lysis of erythrocytes namely CAMP reaction. However, whether this hypothetical gene product has hemolytic activity is unknown. Moreover, its relative structure and function are not yet solved. Here we found that the M. curtisii CAMP factor is a monomer at pH 4.4 and oligomer at pH > 4.6. Hemolysis assays showed that M. curtisii CAMP factor could lyse sheep red blood cells efficiently in pH 5.4-7.4. Negative staining electron microscope analysis of the CAMP factor revealed ring-like structures at pH above 4.6. Additionally, the crystal structure of M. curtisii CAMP factor, determineded at 1.85 Å resolution, reveals a 5 + 3 helix motif. Further functional analysis suggested that the structural rearrangement of the N-terminal domain might be required for protein function. In conclusion, this structure-function relationship study of CAMP factor provides a new perspective of the M. curtisii role in BV development.

Identifying Suitable Listeria innocua Strains as Surrogates for Listeria monocytogenes for Horticultural Products

A laboratory-based study testing 9 Listeria innocua strains independently and a cocktail of 11 Listeria monocytogenes strains was carried out. The aim was to identify suitable L. innocua strain(s) to model L. monocytogenes in inactivation experiments. Three separate inactivation procedures and a hurdle combination of the three were employed: thermal inactivation (55°C), UV-C irradiation (245 nm), and chemical sanitizer (TsunamiTM 100, a mixture of acetic acid, peroxyacetic acid, and hydrogen peroxide). The responses were strain dependent in the case of L. innocua with different strains responding differently to different regimes and L. innocua isolates generally responded differently to the L. monocytogenes cocktail. In the thermal inactivation treatment, inactivation of all strains including the L. monocytogenes cocktail plateaued after 120 min. In the case of chemical sanitizer, inactivation could be achieved at concentrations of 10 and 20 ppm with inactivation increasing with contact time up to 8 min, beyond which there was no significant benefit. All L. innocua strains except PFR16D08 were more sensitive than the L. monocytogenes cocktail to the hurdle treatment. PFR16D08 almost matched the resistance of the L. monocytogenes cocktail but was much more resistant to the individual treatments. A cocktail of two L. innocua strains (PFR 05A07 and PFR 05A10) had the closest responses to the hurdle treatment to those of the L. monocytogenes cocktail and is therefore recommended for hurdle experiments.

A hybrid sub-lineage of Listeria monocytogenes comprising hypervirulent isolates

The foodborne pathogen Listeria monocytogenes (Lm) is a highly heterogeneous species and currently comprises of 4 evolutionarily distinct lineages. Here, we characterize isolates from severe ovine listeriosis outbreaks that represent a hybrid sub-lineage of the major lineage II (HSL-II) and serotype 4h. HSL-II isolates are highly virulent and exhibit higher organ colonization capacities than well-characterized hypervirulent strains of Lm in an orogastric mouse infection model. The isolates harbour both the Lm Pathogenicity Island (LIPI)-1 and a truncated LIPI-2 locus, encoding sphingomyelinase (SmcL), a virulence factor required for invasion and bacterial translocation from the gut, and other non-contiguous chromosomal segments from another pathogenic species, L. ivanovii. HSL-II isolates exhibit a unique wall teichoic acid (WTA) structure essential for resistance to antimicrobial peptides, bacterial invasion and virulence. The discovery of isolates harbouring pan-species virulence genes of the genus Listeria warrants global efforts to identify further hypervirulent lineages of Lm.

Structure determination of the CAMP factor of Streptococcus agalactiae with the aid of an MBP tag and insights into membrane-surface attachment

CAMP factor is a unique α-helical bacterial toxin that is known for its co-hemolytic activity in combination with staphylococcal sphingomyelinase. It was first discovered in the human pathogen Streptococcus agalactiae (also known as group B streptococcus), but homologous genes have been found in many other Gram-positive pathogens. In this study, the efforts that led to the determination of the first structure of a CAMP-family toxin are reported. Initially, it was possible to produce crystals of the native protein which diffracted to near 2.45 Å resolution. However, a series of technical obstacles were encountered on the way to structure determination. Over a period of more than five years, many methods, including selenomethionine labeling, mutations, crystallization chaperones and heavy-atom soaking, were attempted, but these attempts resulted in limited progress. The structure was finally solved using a combination of iodine soaking and molecular replacement using the crystallization chaperone maltose-binding protein (MBP) as a search model. Analysis of native and MBP-tagged CAMP-factor structures identified a conserved interaction interface in the C-terminal domain (CTD). The positively charged surface may be critical for binding to acidic ligands. Furthermore, mutations on the interaction interface at the CTD completely abolished its co-hemolytic activities. This study provides novel insights into the mechanism of the membrane-permeabilizing activity of CAMP factor.

Outbreak of encephalitic listeriosis in red-legged partridges (Alectoris rufa)

An outbreak of neurological disease was investigated in red-legged partridges between 8 and 28 days of age. Clinical signs included torticollis, head tilt and incoordination and over an initial eight day period approximately 30-40 fatalities occurred per day. No significant gross post mortem findings were detected. Histopathological examination of the brain and bacterial cultures followed by partial sequencing confirmed a diagnosis of encephalitis due to Listeria monocytogenes. Further isolates were obtained from follow-up carcasses, environmental samples and pooled tissue samples of newly imported day-old chicks prior to placement on farm. These isolates had the same antibiotic resistance pattern as the isolate of the initial post mortem submission and belonged to the same fluorescent amplified fragment length polymorphism (fAFLP) subtype. This suggested that the isolates were very closely related or identical and that the pathogen had entered the farm with the imported day-old chicks, resulting in disease manifestation in partridges between 8 and 28 days of age. Reports of outbreaks of encephalitic listeriosis in avian species are rare and this is to the best of our knowledge the first reported outbreak in red-legged partridges.

Matrix-assisted Laser Desorption Ionization-Time of Flight Mass Spectrometry (MALDI-TOF MS) Can Precisely Discriminate the Lineages of Listeria monocytogenes and Species of Listeria

The genetic lineages of Listeria monocytogenes and other species of the genus Listeria are correlated with pathogenesis in humans. Although matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF MS) has become a prevailing tool for rapid and reliable microbial identification, the precise discrimination of Listeria species and lineages remains a crucial issue in clinical settings and for food safety. In this study, we constructed an accurate and reliable MS database to discriminate the lineages of L. monocytogenes and the species of Listeria (L. monocytogenes, L. innocua, L. welshimeri, L. seeligeri, L. ivanovii, L. grayi, and L. rocourtiae) based on the S10-spc-alpha operon gene encoded ribosomal protein mass spectrum (S10-GERMS) proteotyping method, which relies on both genetic information (genomics) and observed MS peaks in MALDI-TOF MS (proteomics). The specific set of eight biomarkers (ribosomal proteins L24, L6, L18, L15, S11, S9, L31 type B, and S16) yielded characteristic MS patterns for the lineages of L. monocytogenes and the different species of Listeria, and led to the construction of a MS database that was successful in discriminating between these organisms in MALDI-TOF MS fingerprinting analysis followed by advanced proteotyping software Strain Solution analysis. We also confirmed the constructed database on the proteotyping software Strain Solution by using 23 Listeria strains collected from natural sources.

Cheese-related listeriosis outbreak, Portugal, March 2009 to February 2012

In Portugal, listeriosis has been notifiable since April 2014, but there is no active surveillance programme for the disease. A retrospective study involving 25 national hospitals led to the detection of an outbreak that occurred between March 2009 and February 2012. The amount of time between the start of the outbreak and its detection was 16 months. Of the 30 cases of listeriosis reported, 27 were in the Lisbon and Vale do Tejo region. Two cases were maternal/neonatal infections and one resulted in fetal loss. The mean age of the non-maternal/neonatal cases was 59 years (standard deviation: 17); 13 cases were more than 65 years old. The case fatality rate was 36.7%. All cases were caused by molecular serogroup IVb isolates indistinguishable by pulsed-field gel electrophoresis and ribotype profiles. Collaborative investigations with the national health and food safety authorities identified cheese as the probable source of infection, traced to a processing plant. The magnitude of this outbreak, the first reported food-borne listeriosis outbreak in Portugal, highlights the importance of having an effective listeriosis surveillance system in place for early detection and resolution of outbreaks, as well as the need for a process for the prompt submission of Listeria monocytogenes isolates for routine laboratory typing.

Production and applications of an N-terminally-truncated recombinant beta-haemolysin from Staphylococcus aureus

The beta-haemolysin of Staphylococcus aureus (SA-hlb) is a secreted neutral sphingomyelinase (nSMase) implicated in the pathogenesis of infection and responsible for the characteristic in vitro 'hot-cold' haemolytic ability of the bacterium. Here, we describe the production of a biologically active N-terminally-truncated recombinant SA-hlb protein for use in in vitro assays and as a research tool. Using local isolates of S. aureus, we PCR-amplified an SA-hlb DNA sequence of 891 nucleotides, 99 nucleotides shorter than the full-length molecule, before cloning and sequencing (GenBank accession no. JN580071). The pQE.TriSystem vector (Qiagen, Germany) was used to express recombinant SA-hlb (r-SA-hlb) with a C-terminal 8xHis tag in Escherichia coli JM107 cells. Both JM107 lysate and the purified r-SA-hlb possessed hot-cold lytic activity against sheep and buffalo erythrocytes, which was abolished by incubation at ≥90 °C for 30 min or exposure to dithiothreitol, and could be neutralized by bovine immune sera. Purified r-SA-hlb was also cytotoxic to buffalo mononuclear cells and was effective as a coating antigen for indirect ELISA to screen for reactive sera. Importantly, the r-SA-hlb was suitable for use as a β-toxin in the modified CAMP test. We conclude that the r-SA-hlb protein produced was functionally active and has numerous potential applications.

Cardiolipin synthetase is involved in antagonistic interaction (reverse CAMP phenomenon) of Mycoplasma species with Staphylococcus aureus beta-hemolysis

Mycoplasma hyorhinis has been implicated in a variety of swine diseases. However, little is known about the hemolytic capabilities of Mycoplasma species in general or M. hyorhinis in particular. In this study, we show that M. hyorhinis possesses beta-hemolytic activity which may be involved in the invasion process. M. hyorhinis also possesses antagonistic cooperativity (reverse CAMP phenomenon) with Staphylococcus aureus beta-hemolysis, resulting in the protection of erythrocytes from the beta-hemolytic activity of S. aureus (reverse CAMP). The reversed CAMP phenomenon has been attributed to phospholipase D (PLD) activity. In silico analysis of the M. hyorhinis genome revealed the absence of the pld gene but the presence of the cls gene encoding cardiolipin synthetase, which contains two PLD active domains. The transformation of Mycoplasma gallisepticum that has neither the cls gene nor the reverse CAMP phenomenon with the cls gene from M. hyorhinis resulted in the reverse CAMP phenomenon, suggesting for the first time that reverse CAMP can be induced by cardiolipin synthetase.

Prevalence and distribution of Listeria monocytogenes serotypes and pulsotypes in sows and fattening pigs in farrow-to-finish farms (France, 2008)

This study was undertaken to acquire new data on the prevalence of Listeria monocytogenes in sows and fattening pigs in farrow-to-finish pig farms, and to analyze distribution of serotypes and genotypes of the bacterium within farms. Detection of L. monocytogenes was carried out on 730 pooled feces samples from sows in 73 pig farms and on 172 pooled feces samples from fattening pigs in 43 of these farms. Isolates were serotyped and typed by pulsed-field gel electrophoresis. For sows, 46% of the farms and 11% of the samples were positive for L. monocytogenes. A total of 124 isolates were collected and distributed in four serotypes: 1/2a (41%), 1/2b (36%), 4b (21%), and 1/2c (2%). Positive farms harbored one to three serotypes. The genetic diversity was high; 51 genetic profiles were obtained with 25, 16, 9, and 1 for the serotypes 1/2a, 1/2b, 4b, and 1/2c, respectively. Positive farms harbored 1 to 6 genetic profiles. Isolates showing similar genotypes occurred in several farms. For fattening pigs, 25% of the farms and 14.5% of the samples were positive for L. monocytogenes. The 34 isolates belonged to four serotypes: 1/2a (32%), 1/2b (41%), 4b (24%), and 1/2c (3%). They were distributed in 20 genotypes: 6 for 1/2a; 8 for 1/2b, 5 for 4b, and 1 for 1/2c. Similar serotypes and pulsotypes were recovered in sows and fattening pigs from the same farms, suggesting common sources of contamination.

Semi-automated repetitive-sequence-based polymerase chain reaction compared to pulsed-field gel electrophoresis for Listeria monocytogenes subtyping

Listeriosis is a severe infection that mainly affects pregnant women, neonates, and immuno-compromised adults. The commercially available semi-automated repetitive-sequence-based polymerase chain reaction assay system, DiversiLab, has been successfully used for subtyping several species of bacteria. In this article we compare the DiversiLab System with macrorestriction analysis by pulsed-field gel electrophoresis (PFGE), which is currently the gold standard for molecular subtyping of Listeria monocytogenes. We used a panel of 116 human and food L. monocytogenes isolates for the comparative evaluation. Among these isolates, there were 4 pairs of duplicates, 13 strains were epidemiologically related, and the remaining food isolates were epidemiologically unrelated. The isolates of different serotypes represented distinct DiversiLab types (DTs) and ApaI/AscI-PFGE types except for one DT-containing isolates of two serotypes, 4b and 1/2b. The four duplicates displayed the same DT and ApaI/AscI PFGE type demonstrating the good reproducibility of the two methods. The epidemiologically related strains were clustered in the same DT and PFGE type. The Simpson's index of diversity was 0.954; 0.988; 0.994; and 0.998 for DiversiLab, AscI-PFGE, ApaI-PFGE, and AscI/ApaI-PFGE, respectively. Thus, PFGE was more discriminating than DiversiLab. However, for 1/2a serotype strains, six AscI-PFGE, three ApaI-PFGE, and one ApaI/AscI PFGE type were divided into different DTs. DiversiLab enabled a good discrimination between serotype 1/2a strains. DiversiLab is less labor intensive than PFGE and provides results in <24 hours compared with 30 hours to 3 days for PFGE from the time a pure culture of the bacteria has been obtained. On the basis of these results, DiversiLab may be useful for tracking the source of contamination in food-processing facilities and their environments. Also, DiversiLab may be more appropriate for long-term epidemiological studies where less discrimination is needed.

Evaluation of a multiplex PCR assay as an alternative method for Listeria monocytogenes serotyping

Listeria monocytogenes serotyping is commonly used as the first level of characterisation in the epidemiological surveillance of food and clinical isolates and is therefore widely accepted. The aim of this study was to define a scheme for multiplex molecular serotyping of L. monocytogenes based on a previously described PCR assay and then to evaluate and compare this new procedure with conventional serotyping by agglutination. The study included 1204 Listeria strains collected from food products in France, from March 2005 to October 2006. Two multiplex PCR assays were designed to cluster L. monocytogenes strains into five molecular serogroups: IIa, IIb, IIc, IVa, IVb in agreement with the most commonly encountered serotypes. Amplification of the prfA gene was added to the multiplex PCR to check for L. monocytogenes species; forty-eight (4%) of the isolates tested belonged to the genus Listeria but were not L. monocytogenes. Using this first multiplex PCR, the concordance between conventional and molecular methods was 90.6%, 97.8%, 100% and 100%, for 1/2a, 1/2c, 1/2b and 4b serotypes respectively. False results were observed for some atypical 1/2a, 3a and 1/2c strains. Therefore, this lack of specificity was resolved by using an additional PCR assay based on amplification of the flaA gene, a specific target of 1/2a and 3a strains. When applying the second PCR assay to IIa and IIc molecular serogroup strains, total agreement was obtained between molecular and conventional serotyping methods with a lower level of discrimination for the molecular one. This study proposes to define a strategy for molecular serotyping using both PCR assays: a multiplex and the flaA PCR in order to assign the atypical 1/2a, 3a and 1/2c strains. Moreover, prs gene detection was added for Listeria genus recognition as a positive control in association with flaA detection. Indeed, this molecular serotyping scheme could be considered as a useful and rapid method for first-level characterisation of the most frequently encountered L. monocytogenes serotypes.

Identification, cloning, and expression of the CAMP-like factor autotransporter gene (cfa) of Bartonella henselae

The CAMP reaction was first described by Christie et al. (R. Christie, N. E. Atkins, and E. Munch-Petersen, Aust. J. Exp. Biol. 22:197-200, 1944) as the synergistic lysis of sheep red blood cells by Staphylococcus aureus sphingomyelinase and CAMP factor (cohemolysin), a secreted protein from group B streptococci. We observed a CAMP-like reaction when Bartonella henselae was grown in close proximity to S. aureus on 5% sheep blood agar. This study describes the cloning, sequencing, and characterization of a CAMP-like factor autotransporter gene (cfa) from B. henselae. A cosmid library of B. henselae ATCC 49793 was constructed using SuperCos1 in Escherichia coli XL1-Blue MR. Cosmids were screened for the CAMP reaction, and a quantitative cohemolysis microtiter assay was developed using purified sphingomyelinase. Cosmid clones with the strongest cohemolytic reaction had similar restriction enzyme patterns. A DNA fragment that expressed the cohemolysin determinant was subcloned in a 7,200-bp StuI-BamHI fragment which contained a 6,024-bp open reading frame. The deduced amino acid sequence showed homology to the family of autotransporters. The autotransporters are a group of proteins that mediate their own export through the outer membrane. They contain an N-terminal passenger region, the alpha-domain, and a C-terminal transporter region, the beta-domain. The alpha-domain contained four, nearly identical 42-amino-acid repeats and showed homology to the family of RTX (repeat in toxin) hemolysins. The concentrated supernatant of the recombinant strain expressed a protein with a molecular mass of 180 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis consistent with the calculated molecular weight of the secreted alpha-domain. In conclusion, we have characterized a novel secreted cohemolysin autotransporter protein of B. henselae.

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.

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

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.Key words: Listeria spp., hemolysis, test comparison.

Isolation of catalase-negative Listeria monocytogenes strains from listeriosis patients and their rapid identification by anti-p60 antibodies and/or PCR

Two catalase-negative Listeria monocytogenes serovar 1/2b strains were isolated from listeriosis patients in 1995 in Germany. The infections appeared in individuals from different cities at different seasons and were caused by L. monocytogenes strains of different clonal types. In particular, the catalase reaction of one strain isolated from blood was consistently negative, whereas this reaction was only reversibly blocked when the strain was freshly isolated from ascitic fluid. After subculturing, the catalase-positive reaction was restored. Initially, identification of these isolates was difficult to achieve not only because of the lack of a catalase reaction, which generally distinguishes L. monocytogenes from other morphologically similar pathogenic gram-positive bacteria, but also because other routinely used biochemical tests such as CAMP and the commercial API test gave unclear results. However, rapid and unequivocal identification of these strains was possible by analyzing secretions of the p60 protein in culture supernatants by enzyme-linked immunosorbent assay or Western blot (immunoblot) analysis with our recently developed Listeria- and L. monocytogenes-specific anti-p60 antibodies. Additionally, the identifications were confirmed by Listeria- and L. monocytogenes-specific PCR analyses with primers derived from the iap, hly, and prfA genes. Immunoanalyses also allowed for the differentiation of these two strains, whereas no differentiation was possible by PCR when the internal, variable repetitive iap gene portion was analyzed. However, size variations of the PCR products comprising these gene portions which were obtained from a number of L. monocytogenes strains belonging to the same serotypes indicated that this type of PCR is not only useful for specific identifications but may be used in parallel as an additional marker for epidemiological studies. In conclusion, the data suggest that catalase production should not be taken as a strict criterion for the identification of listeriae. Furthermore, at least the infection caused by the stably catalase-negative strain supports the notion that catalase does not seem to be necessary for the intracellular growth of L. monocytogenes.

Incidence of Listeria spp. in tropical fish

The incidence of Listeria spp. in tropical fish and shellfish was studied. The isolation protocol included a pre-enrichment, followed by two selective enrichment steps and plating on three selective agars. Listeria monocytogenes could be detected in 17.2% of finfish and 12.1% of shellfish. L. innocua was the most common species encountered. In 6.9% finfish and 5.6% shellfish, both L. monocytogenes and L. innocua were detected. Polymerase chain reaction (PCR)-based amplification of internal fragments of the iap gene was found to be useful in differentiation of L. monocytogenes from L. innocua.

Recovery of different Listeria ribotypes from naturally contaminated, raw refrigerated meat and poultry products with two primary enrichment media

Isolation rates for Listeria monocytogenes and the other Listeria spp. typically improve when samples are enriched in more than one primary enrichment medium. This study evaluated the abilities of two primary enrichment media, University of Vermont-modified Listeria enrichment broth (UVM) and Listeria repair broth (LRB), to recover different ribotypes of Listeria spp. from raw meat and poultry samples. Forty-five paired 25-g retail samples of ground beef, pork sausage, ground turkey, and chicken (160 samples) underwent primary enrichment in UVM and LRB (30 degrees C for 24 h) followed by secondary enrichment in Fraser broth (35 degrees C for 24 and 40 h) and plating on modified Oxford agar. After 24 h of incubation of 35 degrees C, 608 Listeria colonies from selected positive samples were biochemically confirmed as L. monocytogenes (245 isolates), L innocua (276 isolates), and L. welshimeri (89 isolates) and then ribotyped with the automated Riboprinter microbial characterization system (E. I. du Pont de Nemours & Co., Inc.). Thirty-six different Listeria strains comprising 16 L. monocytogenes (including four known clinical ribotypes), 12 L. innocua, and 8 L. welshimeri ribotypes were identified from selected positive samples (15 samples of each product type; two UVM and two LRB isolates per sample). Twenty-six of 36(13 L. monocytogenes) ribotypes were detected with both UVM and LRB, whereas 3 of 36 (1 L. monocytogenes) and 7 of 36 (3 L. monocytogenes) Listeria ribotypes were observed with only UVM or LRB, respectively. Ground beef, pork sausage, ground turkey, and chicken yielded 22 (8 L. monocytogenes), 21 (12 L. monocytogenes), 20 (9 L. monocytogenes), and 19 (11 L. monocytogenes) different Listeria ribotypes, respectively, with some Listeria ribotypes confined to a particular product. More importantly, major differences in both the number and distribution of Listeria ribotypes, including previously recognized clinical and nonclinical ribotypes of L. monocytogenes, were observed when 10 UVM and 10 LRB isolates from five samples of each product were ribotyped. When a third set of six samples per product type was examined from which two Listeria isolates were obtained by using only one of the two primary enrichment media, UVM and LRB failed to detect L. monocytogenes (both clinical and nonclinical ribotypes) in two and four samples, respectively. These findings stress the importance of using more than one primary enrichment medium and picking a sufficient number of colonies per sample when attempting to isolate specific L. monocytogenes strains during investigations of food-borne listeriosis.

Influence of environmental parameters on phosphatidylcholine phospholipase C production in Listeria monocytogenes: a convenient method to differentiate L. monocytogenes from other Listeria species

The ability to produce phosphatidylcholine phospholipase C (lecithinase) is associated with virulence in pathogenic species of Listeria. Levels of production vary greatly among members of the genus, and this virulence factor is not readily detectable in many members of the pathogenic species on conventional agar media containing egg yolk, a common substrate for the enzyme. In this study, the influence of a variety of environmental parameters, including temperature, pH, and salt concentration, on the production of lecithinase by a number of strains was evaluated. Lecithinase production by Listeria monocytogenes LO28 in brain heart infusion medium was optimal at 1.75 to 2.0% NaCl; pH 7.0 to 7.3, and 37 to 40 degrees C, and the presence of oxygen had no effect. In a chemically defined medium, the optimal NaCl concentration and temperature were lower at 0.75 to 1.0% NaCl and 33.5 degrees C. As detection of virulence factors is useful to assist in the identification and differentiation of Listeria species, this report shows that lecithinase activity can conveniently be detected within 36 h on a relatively inexpensive medium. Under the conditions described, L. monocytogenes could be distinguished from other members of the genus as a result of distinct lecithin degradation which was not evident in L. innocua, L. seeligeri, L. ivanovii, L. welshimeri, or L. murrayi/grayi.

Cholesterol oxidase from Rhodococcus equi is likely the major factor involved in the cooperative lytic process (CAMP reaction) with Listeria monocytogenes

The CAMP reaction between Listeria monocytogenes and Rhodococcus equi was studied by a diffusion assay. Listeria monocytogenes displayed identical cooperative haemolytic effect with supernatant cultures of R. equi or with commercial cholesterol oxidase (COX). This result, even with enzymes of different sources (commercial COX is obtained from Pseudomonas spp.) suggests that this enzyme secreted by R. equi has a crucial role in the synergistic haemolytic (CAMP) reaction with L. monocytogenes. The mechanism of the cooperative lytic process between L. monocytogenes and R. equi may represent a different and novel mechanism reaction, in which the COX may not act as a conventional second-step factor, and a reaction different to the direct interaction with the cholesterol of the erythrocyte membrane may be involved.