Characterization of Listeria strains isolated from soft cheese

Εvaluation of oxygen availability on growth and inter-strain interactions of L. monocytogenes in/on liquid, semi-solid and solid laboratory media

The coexistence and interactions among Listeria monocytogenes strains in combination with the structural characteristics of foods, may influence their growth capacity and thus, the final levels at the time of consumption. In the present study, we aimed to evaluate the effect of oxygen availability in combination with substrate micro-structure on growth and inter-strain interactions of L. monocytogenes. L. monocytogenes strains, selected for resistance to different antibiotics (to enable distinct enumeration), belonging to serotypes 4b (C5, ScottA), 1/2a (6179) and 1/2b (PL25) and were inoculated in liquid (Tryptic Soy Broth supplemented with Yeast Extract - TSB-YE) and solid (TSB-YE supplemented with 0.6% and 1.2% agar) media (2-3 log CFU/mL, g or cm²), single or as two-strain cultures (1:1 strain-ratio). Aerobic conditions (A) were achieved with constant shaking or surface inoculation for liquid and solid media respectively, while static incubation or pour plated media corresponded to hypoxic environment (H). Anoxic conditions (An) were attained by adding 0.1% w/v sodium thioglycolate and paraffin overlay (for solid media). Growth was assessed during storage at 7 °C (n = 3 × 2). Inter-strain interactions were manifested by the difference in the final population between singly and co-cultured strains. Τhe extent of suppression increased with reduction in agar concentration, while the impact of oxygen availability was dependent on strain combination. During co-culture, in liquid and solid media, 6179 was suppressed by C5 by 4.0 (in TSB-YE under H) to 1.8 log units (in solid medium under An), compared to the single culture, which attained population of ca. 9.4 log CFU/mL or g. The growth of 6179 was also inhibited by ScottA by 2.7 and 1.9 log units, in liquid culture under H and An, respectively. Interestingly, in liquid medium under A, H and An, ScottA was suppressed by C5, by 3.3, 2.4 and 2.3 log units, respectively, while in solid media, growth inhibition was less pronounced. Investigating growth interactions in different environments could assist in explaining the dominance of L. monocytogenes certain serotypes.

Fate of Listeria monocytogenes in the Presence of Resident Cheese Microbiota on Common Packaging Materials

Literature data regarding the survival of microorganisms on materials used for food package purposes are scarce. The aim of the current study is to assess the survival of Listeria monocytogenes on different packaging materials for dairy products during extended storage at different temperatures. Three packaging materials (5 × 5 cm) were contaminated with a cocktail of five strains of Listeria monocytogenes suspended in a cheese homogenate, including the cheese's native microbial population. Contaminated samples were incubated at 37°, 12°, and 4°C and periodically analyzed up to 56 days. The evolution of the total viable count and pathogen population was evaluated. At 37°C, the results showed that Listeria monocytogenes was no longer detected on polyethylene-coated nylon (B) by day 4 and on polyethylene-coated parchment (A) and greaseproof paper (C) by day 7. Interestingly, the initial cell population (ranging between 2.5 and 2.7 log CFU/cm2) of Listeria monocytogenes increased to 3 log CFU/cm2 within 4 days of storage at 12°C on A and C. During storage, the number remained fairly constant at 12°C and 4°C on two materials (A-C) and decreased slowly on the third one (B). This study shows that survival of Listeria monocytogenes on packaging materials for dairy products will be higher when stored at 4 or 12°C compared to 37°C. The survival of Listeria monocytogenes on the packaging materials raises concerns of cross-contamination during food handling and preparation at catering and retail premises and within the home, highlighting the importance of treating the packaging materials as a potential source of cross-contamination. These initial findings may aid in quantifying risks associated with contamination of food packaging materials.

Growth, detection and virulence of Listeria monocytogenes in the presence of other microorganisms: microbial interactions from species to strain level

Like with all food microorganisms, many basic aspects of L. monocytogenes life are likely to be influenced by its interactions with bacteria living in close proximity. This pathogenic bacterium is a major concern both for the food industry and health organizations since it is ubiquitous and able to withstand harsh environmental conditions. Due to the ubiquity of Listeria monocytogenes, various strains may contaminate foods at different stages of the supply chain. Consequently, simultaneous exposure of consumers to multiple strains is also possible. In this context even strain-to-strain interactions of L. monocytogenes play a significant role in fundamental processes for the life of the pathogen, such as growth or virulence, and subsequently compromise food safety, affect the evolution of a potential infection, or even introduce bias in the detection by classical enrichment techniques. This article summarizes the impact of microbial interactions on the growth and detection of L. monocytogenes primarily in foods and food-associated environments. Furthermore it provides an overview of L. monocytogenes virulence in the presence of other microorganisms.

Listeria monocytogenes Strains Underrepresented during Selective Enrichment with an ISO Method Might Dominate during Passage through Simulated Gastric Fluid and In Vitro Infection of Caco-2 Cells

Various Listeria monocytogenes strains may contaminate a single food product, potentially resulting in simultaneous exposure of consumers to multiple strains. However, due to bias in strain recovery, L. monocytogenes strains isolated from foods by selective enrichment (SE) might not always represent those that can better survive the immune system of a patient. We investigated the effect of cocultivation in tryptic soy broth with 0.6% yeast extract (TSB-Y) at 10°C for 8 days on (i) the detection of L. monocytogenes strains during SE with the ISO 11290-1:1996/Amd 1:2004 protocol and (ii) the in vitro virulence of strains toward the Caco-2 human colon epithelial cancer cell line following exposure to simulated gastric fluid (SGF; pH 2.0)-HCl (37°C). We determined whether the strains which were favored by SE would be effective competitors under the conditions of challenges related to gastrointestinal passage of the pathogen. Interstrain competition of L. monocytogenes in TSB-Y determined the relative population of each strain at the beginning of SE. This in turn impacted the outcome of SE (i.e., favoring survival of competitors with better fitness) and the levels exposed subsequently to SGF. However, strong growth competitors could be outcompeted after SGF exposure and infection of Caco-2 cells by strains outgrown in TSB-Y and underdetected (or even missed) during enrichment. Our data demonstrate a preferential selection of certain L. monocytogenes strains during enrichments, often not reflecting a selective advantage of strains during infection. These findings highlight a noteworthy scenario associated with the difficulty of matching the source of infection (food) with the L. monocytogenes isolate appearing to be the causative agent during listeriosis outbreak investigations.IMPORTANCE This report is relevant to understanding the processes involved in selection and prevalence of certain L. monocytogenes strains in different environments (i.e., foods or sites of humans exposed to the pathogen). It highlights the occurrence of multiple strains in the same food as an important aspect contributing to mismatches between clinical isolates and infection sources during listeriosis outbreak investigations.

Characterisation of Listeria monocytogenes Isolates from Poultry by Serotyping and Phage Typing

With the aim of improving the knowledge of Listeria monocytogenestypes distributed in poultry, isolates collected from a hundred eviscerated and refrigerated chicken carcasses from four different slaughterhouse plants in northwest Spain were characterised by serotyping and phage typing. Forty-two phage patterns were observed. Eight phage patterns (43, 43/387, 43/387/4477/1652 and 43/19/387/575 for strains of serogroup 1/2 and 107, 108/107/47, 2389 and 2389/3274 for strains of serogroup 4) were the most common. Thus, thirty strains (56.60% of phage typeable strains) showed the above-mentioned or indistinguishable (1 major phage reaction differences for serogroup 1/2 strains or 2 major differences for serogroup 4) patterns. A wide distribution of some L. monocytogenesphage types was observed because different chicken carcasses (including carcasses from different slaughterhouses) were contaminated with the same or indistinguishable L. monocytogenesphage types. Besides, more than one type (serogroup or phage type) of L. monocytogeneswas detected in 43.75% of positive samples. This finding emphasised the usefulness of subtyping several Listeria monocytogenesisolates from the same food sample in epidemiological studies.

Evaluation of the international phage typing set and some experimental phages for typing of Listeria monocytogenes from poultry in Spain

AIMS

The validity of the international phage set and 13 experimental phages for subtyping Listeria monocytogenes strains isolated from poultry in Spain was investigated.

METHODS AND RESULTS

Ninety-six L. monocytogenes strains (52 from serogroup 1/2 and 44 from serogroup 4) were phage-typed using the international phage set, 10 experimental phages for typing serogroup 1/2 strains (seven isolated in France: 1313, 9425, 1807, 351, 881, 717 and 586-, and three from Denmark: 5775, 12682 and 6223-) and three experimental phages isolated in France for typing serogroup 4 strains (2425 A, 4286 and 197). Percentages of serogroup 1/2, serogroup 4 and total phage-typeable strains were 57.7%, 52.3% and 55.2%, respectively. Important differences in the behaviour of the phages tested were found. The typeability rate, the specificity index and the percentage of strong reactions were greater in the phages of international set than in the experimental phages. The number of phage typeable strains and the number of phage types (42) were not modified by the use of experimental phages.

CONCLUSIONS

The phage set used was not effective for typing L. monocytogenes strains from poultry in Spain, because a low typeability rate was found.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our results suggest the importance of the availability of new phages specific to a geographical area in order to improve the typeability of the system.

Molecular epidemiology of Listeria monocytogenes isolates collected from the environment, raw meat and raw products in two poultry- and pork-processing plants

AIMS

In order to study the transmission of Listeria monocytogenes in a poultry and a pork meat plant, we analysed the contamination by this pathogen over several months.

METHODS AND RESULTS

Five hundred and two isolates of L. monocytogenes were collected and characterized by genotyping and serotyping. Thirty-seven genotypes were obtained by ApaI-restriction analysis-pulsed field gel electrophoresis (REA-PFGE) and 35 by SmaI-REA-PFGE and resulted in 50 combined genotypes. The tracing of the contamination in both plants showed that some clones were able to survive for several months. However, some other clones were found only during processing operations, were not detectable after cleaning and seemed to enter continuously into the plant.

CONCLUSIONS

Some L. monocytogenes strains may persist for a long period in the plant environment. Different genotypes can be associated with poultry as well as pork meat.

SIGNIFICANCE AND IMPACT OF THE STUDY

Listeria monocytogenes contamination can be due to contaminated raw materials, bacterial spread and also ineffective cleaning procedures.

Characterization of Listeria monocytogenes isolated from production lines of fresh and cold-smoked fish

AIMS

The aims of this study were to characterize strains of Listeria monocytogenes isolated from cold-smoking fish plants to establish possible routes of contamination through the processing chain.

METHODS AND RESULTS

Listeria monocytogenes from fresh fish suppliers, raw materials, factory sites and finished products isolated in Portugal (162 isolates) and England (28 isolates) were characterized by serotyping, phage typing, tetracycline, cadmium and arsenic resistance, and plasmid profiling. On the basis of serotyping and phage typing, the isolates were categorized into eight groups. Although cultures within some of the groups could be further differentiated on the basis of plasmid profiling and cadmium and arsenite typing, consideration of all typing data predominantly clustered together isolates from a single location. L. monocytogenes strains: from fresh salmon suppliers were not found in the processing lines; from fresh salmon from different locations differed; and from the water where salmon trout were farmed differed from those isolated from the fish samples.

SIGNIFICANCE AND IMPACT OF THE STUDY

No clear source or route of contamination in the cold-smoked processing chain could be established; however, these results highlight the complexity in tracking this bacterium through food chains.

Listeria monocytogenes in faeces from clinically healthy dairy cows in Sweden

Faecal samples from 102 clinically healthy dairy cows, representing 34 farms in the Swedish province of Uppsala, were analysed for the presence of Listeria spp. using an enrichment procedure. Listeria monocytogenes was isolated from six (6%) and L. innocua from 2 (2%) cows. From each of the 6 samples positive for L. monocytogenes, 5 isolates were further characterised by restriction enzyme analysis using the 3 enzymes Apa I, Sma I, and Asc I, followed by pulsed-field gel electrophoresis. Three of the L. monocytogenes positive cows lived at the same farm, and they all harboured the same clonal type. One of these 3 cows also harboured a further clonal type of L. monocytogenes. The fact that one of the cows harboured 2 different clonal types of L. monocytogenes is important from an epidemiological point of view when routes of infection are to be investigated.

Sources of Listeria monocytogenes contamination in a cold-smoked rainbow trout processing plant detected by pulsed-field gel electrophoresis typing

Sites of Listeria monocytogenes contamination in a cold-smoked rainbow trout (Oncorhynchus mykiss) processing plant were detected by sampling the production line, environment, and fish at different production stages. Two lots were monitored. The frequency of raw fish samples containing L. monocytogenes was low. During processing, the frequency of fish contaminated with L. monocytogenes clearly rose after brining, and the most contaminated sites of the processing plant were the brining and postbrining areas. A total of 303 isolates from the raw fish, product, and the environment were characterized by pulsed-field gel electrophoresis (PFGE). PFGE yielded nine pulsotypes, which formed four clusters. The predominating L. monocytogenes pulsotypes of the final product were associated with brining and slicing, whereas contaminants of raw fish were not detected in the final product. Air-mediated contamination in the plant could not be proved. In accordance with these results, an L. monocytogenes eradication program was planned. The use of hot steam, hot air, and hot water seemed to be useful in eliminating L. monocytogenes. None of the control samples taken in the 5 months after the eradication program was implemented contained L. monocytogenes.

Comparison of five typing methods for the epidemiological study of Listeria monocytogenes

Five typing methods were compared in a study designed to adapt a strategy for epidemiologically typing large numbers of Listeria monocytogenes strains. The methods studied were serotyping, electrophoretic typing of esterases (zymotyping), restriction fragment length polymorphism of ribosomal DNA (ribotyping), random amplified polymorphic DNA (RAPD) and pulsed-field gel electrophoresis (PFGE). Data were analysed by computer-assisted statistical analysis. Included in the analysis were 35 strains of L. monocytogenes, including 14 epidemic strains isolated during outbreaks in France in 1992 and 1993, and 21 strains isolated from food and the environment. Five serotypes, eight zymotypes, ten ribotypes, 13 RAPD patterns and 12 PFGE patterns were identified among the 35 strains. The most discriminating combination of typing methods was ribotyping and PFGE typing [27 types, discriminatory index (D.I.) = 0.978]. A factorial analysis of correspondence for each method differentiated the epidemic strains from the environmental strains. This study shows that computer-assisted statistical treatment of the data, combined with the use of discriminating typing methods, is a powerful tool for the epidemiological analysis of Listeria monocytogenes.

An outbreak of listeriosis suspected to have been caused by rainbow trout

An outbreak of listeriosis in Sweden, consisting of nine cases, was investigated by means of molecular typing of strains from patients and strains isolated from suspected foodstuffs, together with interviews of the patients. Listeria monocytogenes was isolated from six of the patients, and all isolates were of the same clonal type. This clonal type was also isolated from a "gravad" rainbow trout, made by producer Y, found in the refrigerator of one of the patients. Unopened packages obtained from producer Y were also found to contain the same clonal type of L. monocytogenes. Based on the interview results and the bacteriological typing, we suspect that at least six of the nine cases were caused by gravad or cold-smoked rainbow trout made by producer Y. To our knowledge, this is the first rainbow trout-borne outbreak of listeriosis ever reported.

Effect of medium-chain fatty acids in mould ripened cheese on the growth of Listeria monocytogenes

Listeria monocytogenes and List. innocua were isolated from commercial soft ripened and blue-veined cheeses manufactured in France, mainly from Brie cheese made from unpasteurized milk. Five isolates were List. monocytogenes serotype 1/2 and two were List. innocua. Examination of Bleu d'Auvergne cheese with the cryoscanning electron microscope showed that many conidia spores were present in the blue veins in close contact with the cheese surface. There were few conidia spores in the Brie, mostly on the outside of the cheese but not in contact with the surface. High concentrations of free dodecanoic (lauric) acid (1.77-2.50 g/kg cheese) and tetradecanoic (myristic) acid (2.54-6.38 g/kg cheese) were found in the veins of the blue cheese, but concentrations in the white regions were much lower. Free lauric and myristic acids were not detected in the Brie cheeses. There was no difference in the overall fatty acid composition of the fat in the surface ripened and blue-veined cheeses, although higher concentrations of free medium-chain fatty acids were found in a blue cheese compared with a surface ripened cheese. The pH and fat content were higher in regions with obvious fungal growth, the blue veins of Fourme d'Ambert and the rind of Brie. Free lauric acid dissolved in butteroil inhibited multiplication in broth at pH 7.0 of a test strain of List. monocytogenes isolated from Bleu d'Auvergne. Some inhibition was seen with hexanoic, octanoic, decanoic and tetradecanoic acids. We suggest that the presence of localized concentrations of free medium-chain fatty acids (dissolved in the fat) in the blue veins of blue mould ripened cheese could act as natural preservatives and inhibit the growth of listerias in conditions where (if present), one would otherwise expect them to grow.

The clones of Listeria monocytogenes detected in food depend on the method used

Restriction enzyme analysis (REA) with pulsed-field gel electrophoresis (PFGE) has been used to characterize and compare Listeria monocytogenes strains isolated from foods by two methods, an enrichment procedure and a direct plating procedure. In total 151 isolates from nine foods were investigated. In six of the foods (101 strains investigated) only one clone of L. monocytogenes was found irrespective of the method used. In three foods (50 strains investigated) the direct plating procedure yielded more clones than the enrichment procedure. At the most, five clones were detected in the same food. The results presented here indicate that direct plating from the food reveals more L. monocytogenes clones than revealed by an enrichment procedure.

Use of pulsed-field gel electrophoresis to link sporadic cases of invasive listeriosis with recalled chocolate milk

Pulsed-field gel electrophoresis established the linkage between recalled chocolate milk and a multistate invasive listeriosis outbreak during a four-product recall period. Listeria monocytogenes isolates from four hospitalized patients and an environmental dairy sample displayed AscI restriction endonuclease digestion profiles identical to that of the chocolate milk isolate.

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.