Listeria monocytogenesin Multiple Habitats and Host Populations: Review of Available Data for Mathematical Modeling

A new and efficient method to obtain benzalkonium chloride adapted cells of Listeria monocytogenes

A new method to obtain benzalkonium chloride (BAC) adapted L. monocytogenes cells was developed. A factorial design was used to assess the effects of the inoculum size and BAC concentration on the adaptation (measured in terms of lethal dose 50 -LD50-) of 6 strains of Listeria monocytogenes after only one exposure. The proposed method could be applied successfully in the L. monocytogenes strains with higher adaptive capacity to BAC. In those cases, a significant empirical equation was obtained showing a positive effect of the inoculum size and a positive interaction between the effects of BAC and inoculum size on the level of adaptation achieved. However, a slight negative effect of BAC, due to the biocide, was also significant. The proposed method improves the classical method based on successive stationary phase cultures in sublethal BAC concentrations because it is less time-consuming and more effective. For the laboratory strain L. monocytogenes 5873, by applying the new procedure it was possible to increase BAC-adaptation 3.69-fold in only 33 h, whereas using the classical procedure 2.61-fold of increase was reached after 5 days. Moreover, with the new method, the maximum level of adaptation was determined for all the strains reaching surprisingly almost the same concentration of BAC (mg/l) for 5 out 6 strains. Thus, a good reference for establishing the effective concentrations of biocides to ensure the maximum level of adaptation was also determined.

LISTERIA MONOCYTOGENES: OCORRÊNCIA EM PRODUTOS LÁCTEOS E SUAS IMPLICAÇÕES EM SAÚDE PÚBLICA

RESUMO Listeria monocytogenes é o agente causador da listeriose, uma grave doença de origem alimentar que causa severas infecções em humanos com altas taxas de mortalidade. O leite e seus derivados estão entre os produtos alimentícios mais frequentemente envolvidos na transmissão de L. monocytogenes. A listeriose acomete, sobretudo, indivíduos imunodeprimidos, grávidas, recém-nascidos e idosos, o que ressalta o caráter oportunista deste micro-organismo e sua importância para a saúde pública. No presente trabalho, faz-se uma revisão narrativa crítica sobre o risco à saúde humana decorrente da ingestão de leite e derivados contaminados por L. monocytogenes, bem como se discutem os fatores que determinam a contaminação por L. monocytogenes na cadeia de produção e distribuição de leite e derivados. São apresentados e avaliados os dados de ocorrência de L. monocytogenes em leite cru e em produtos lácteos no Brasil, tendo em vista seu potencial de envolvimento em casos de listeriose humana. Adicionalmente, são indicadas as principais áreas de pesquisa e atuação para prevenir a contaminação de L. monocytogenes em produtos lácteos.

Blue and red light modulates SigB-dependent gene transcription, swimming motility and invasiveness in Listeria monocytogenes

BACKGROUND

In a number of gram-positive bacteria, including Listeria, the general stress response is regulated by the alternative sigma factor B (SigB). Common stressors which lead to the activation of SigB and the SigB-dependent regulon are high osmolarity, acid and several more. Recently is has been shown that also blue and red light activates SigB in Bacillus subtilis.

METHODOLOGY/PRINCIPAL FINDINGS

By qRT-PCR we analyzed the transcriptional response of the pathogen L. monocytogenes to blue and red light in wild type bacteria and in isogenic deletion mutants for the putative blue-light receptor Lmo0799 and the stress sigma factor SigB. It was found that both blue (455 nm) and red (625 nm) light induced the transcription of sigB and SigB-dependent genes, this induction was completely abolished in the SigB mutant. The blue-light effect was largely dependent on Lmo0799, proving that this protein is a genuine blue-light receptor. The deletion of lmo0799 enhanced the red-light effect, the underlying mechanism as well as that of SigB activation by red light remains unknown. Blue light led to an increased transcription of the internalin A/B genes and of bacterial invasiveness for Caco-2 enterocytes. Exposure to blue light also strongly inhibited swimming motility of the bacteria in a Lmo0799- and SigB-dependent manner, red light had no effect there.

CONCLUSIONS/SIGNIFICANCE

Our data established that visible, in particular blue light is an important environmental signal with an impact on gene expression and physiology of the non-phototrophic bacterium L. monocytogenes. In natural environments these effects will result in sometimes random but potentially also cyclic fluctuations of gene activity, depending on the light conditions prevailing in the respective habitat.

How a routine checking of Escherichia coli in retailed food of animal origin can protect consumers against exposition to Campylobacter spp. and Listeria monocytogenes?

BACKGROUND/AIM

According to the literature that has been published over the last two decades Campylobacter spp i Listeria monocitogens can be identified as causes of numerous diseases derived by consuming food of animal origin. The purpose of this paper was to find out how established national microbiological criteria of the Republic of Serbia on food safety in retailed food of animal origin could contribute to consumer's protection against exposition to foodborne pathogens such as Campylobacter spp. and Listeria monocytogenes.

METHODS

During a routine microbiological safety control of randomly selected 60 samples of fresh poultry meat, 30 samples of other fresh meat readymade for grilling, 30 samples of sausage products, 37 samples of heat-treated meat, 39 samples of toppings for fast food of animal origin and 31 samples of dairy products a national food safety criteria (Escherichia coli, aerobic plate count, Salmonella spp., coagulasa positive Staphylococcus, Proteus spp., sulphito-reducting Clostridia) were applied and, as well as, testing to Campylobacter spp. and Listeria monocitogens. In determination of Campylobacter spp. and Listeria monocytogenes, food quality control methods of the Food and Agriculture Organization (FAO) were applied, while in determination of the other above motioned bacteria, national provisions on microbiological methods were applied who are adjusted to the FAO ones.

RESULTS

Related to the national criteria on microbiological food safety, 88 (38.8%) samples, out of the total 227 tested, were rejected. When to these results, the results of laboratory tests on Listeria monocytogens were added, a terminal number of rejected samples were not changed. When to these results, the results of Campylobacter spp. testing were added, 91 (40.1%) out of the 227 samples were unsatisfied. Results of logistic regression model with occurrence of Escherichia coli as dependent variable indicated that Escherichia coli was 4.5 times likely to occur among samples with Campylobacter spp. than among samples without Campylobacter spp. (OR = 4.515, 95% CI: 1.019-20.002). Sensitivity of the fitted model (Hosmer-Lemeshowp = 0.268) was 76.8% and its specificity was 75.0%. At the same time Escherichia coli was confound in all (100%) food samples that were contaminated by Listeria monocytogenes.

CONCLUSION

Statistical analysis indicated that Escherichia coli was completely sensitive to identify all samples contaminated with Listeria monocytogenas and highly sensitive to identify samples contaminated with Campylobacter spp. Nevertheless, 3 (1.3%) of the tested samples were not covered with Escherichia coli.

The relationship of silage temperature with the microbiological status of the face of corn silage bunkers

A method is proposed to quantify aerobic deterioration of corn silage forage quality as related to the temperature of silage mass in the bunker. Aerobic deterioration, apart from causing nutritional value losses, affects the hygienic quality of silages through the accumulation of pathogenic organisms and their toxins. A survey was carried out in northern Italy that involved a detailed examination of silage bunker from each of 54 dairy farms. Samples from the core, the peripheral areas within 1m from the silo walls, and the molded spots, when present, were collected. The sample and silage temperatures across the working face were measured at depths of 200 mm at 11 locations and at 7 elevations. The temperature of the central zone of the silo was defined as the reference temperature (T(ref40)). The difference between the temperature of the silage sample and the T(ref40) was used as a heating index associated with aerobic deterioration (dT(ref40)). The working face area with visible molds was measured. The samples were analyzed for DM content; pH; water activity; nitrates; lactic, acetic, and butyric acids; and microbiological count of yeasts, molds, and clostridia spores. The core samples always showed a pH below 4.0 and a dT(ref40) below 2 degrees C, whereas the silages from the peripheral areas were split into 2 groups, one that had a pH lower than 4 and a dT(ref40) lower than 3.5 degrees C (53%) and one that had a pH higher than 4 and a dT(ref40) higher than 5 degrees C (47%). Most of the silages from the peripheral areas (94%) and all the silages from the moldy spots that have a dT(ref40) above 5 degrees C had a pH higher than 4.5. Furthermore, a positive dT(ref40) higher than 5 degrees C corresponds to a higher yeast count than 5 log cfu/g in most of the silages from the peripheral areas (93% of samples) and in almost all the silages from the molded spots. The evaluation of the extension of the visible molded areas combined with temperature measurement at 200 mm behind the feed-out face could offer a good indication of the health status of silage during consumption. Furthermore, this method could be useful to detect early stages of the aerobic deterioration process and to improve silage management.

Maltose and maltodextrin utilization by Listeria monocytogenes depend on an inducible ABC transporter which is repressed by glucose

Background

In the environment as well as in the vertebrate intestine, Listeriae have access to complex carbohydrates like maltodextrins. Bacterial exploitation of such compounds requires specific uptake and utilization systems.

Methodology/Principal Findings

We could show that Listeria monocytogenes and other Listeria species contain genes/gene products with high homology to the maltodextrin ABC transporter and utilization system of B. subtilis. Mutant construction and growth tests revealed that the L. monocytogenes gene cluster was required for the efficient utilization of maltodextrins as well as maltose. The gene for the ATP binding protein of the transporter was located distant from the cluster. Transcription analyses demonstrated that the system was induced by maltose/maltodextrins and repressed by glucose. Its induction was dependent on a LacI type transcriptional regulator. Repression by glucose was independent of the catabolite control protein CcpA, but was relieved in a mutant defective for Hpr kinase/phosphorylase.

Conclusions/Significance

The data obtained show that in L. monocytogenes the uptake of maltodextrin and, in contrast to B. subtilis, also maltose is exclusively mediated by an ABC transporter. Furthermore, the results suggest that glucose repression of the uptake system possibly is by inducer exclusion, a mechanism not described so far in this organism.

High-throughput genome sequencing of two Listeria monocytogenes clinical isolates during a large foodborne outbreak

BACKGROUND

A large, multi-province outbreak of listeriosis associated with ready-to-eat meat products contaminated with Listeria monocytogenes serotype 1/2a occurred in Canada in 2008. Subtyping of outbreak-associated isolates using pulsed-field gel electrophoresis (PFGE) revealed two similar but distinct AscI PFGE patterns. High-throughput pyrosequencing of two L. monocytogenes isolates was used to rapidly provide the genome sequence of the primary outbreak strain and to investigate the extent of genetic diversity associated with a change of a single restriction enzyme fragment during PFGE.

RESULTS

The chromosomes were collinear, but differences included 28 single nucleotide polymorphisms (SNPs) and three indels, including a 33 kbp prophage that accounted for the observed difference in AscI PFGE patterns. The distribution of these traits was assessed within further clinical, environmental and food isolates associated with the outbreak, and this comparison indicated that three distinct, but highly related strains may have been involved in this nationwide outbreak. Notably, these two isolates were found to harbor a 50 kbp putative mobile genomic island encoding translocation and efflux functions that has not been observed in other Listeria genomes.

CONCLUSIONS

High-throughput genome sequencing provided a more detailed real-time assessment of genetic traits characteristic of the outbreak strains than could be achieved with routine subtyping methods. This study confirms that the latest generation of DNA sequencing technologies can be applied during high priority public health events, and laboratories need to prepare for this inevitability and assess how to properly analyze and interpret whole genome sequences in the context of molecular epidemiology.

Genome organization and characterization of the virulent lactococcal phage 1358 and its similarities to Listeria phages

Virulent phage 1358 is the reference member of a rare group of phages infecting Lactococcus lactis. Electron microscopy revealed a typical icosahedral capsid connected to one of the smallest noncontractile tails found in a lactococcal phage of the Siphoviridae family. Microbiological characterization identified a burst size of 72 virions released per infected host cell and a latent period of 90 min. The host range of phage 1358 was limited to 3 out of the 60 lactococcal strains tested. Moreover, this phage was insensitive to four Abi systems (AbiK, AbiQ, AbiT, and AbiV). The genome of phage 1358 consisted of a linear, double-stranded, 36,892-bp DNA molecule containing 43 open reading frames (ORFs). At least 14 ORFs coded for structural proteins, as identified by SDS-PAGE coupled to liquid chromatography-tandem mass spectrometry (LC-MS/MS) analyses. The genomic organization was similar to those of other siphophages. All genes were on the same coding strand and in the same orientation. This lactococcal phage was unique, however, in its 51.4% GC content, much higher than those of other phages infecting this low-GC Gram-positive host. A bias for GC-rich codons was also observed. Comparative analyses showed that several phage 1358 structural proteins shared similarity with two Listeria monocytogenes phages, P35 and P40. The possible origin and evolution of lactococcal phage 1358 is discussed.

Effects of mussel processing soils on the adherence of Listeria monocytogenes to polypropylene and stainless steel

A comparative study of adhesion kinetics of L. monocytogenes (strains CECT 5873, CECT 936, CECT 911, and CECT 4032, representing serotypes 1/2a, 1/2b, 1/2c, and 4b, respectively) to polypropylene (PP) and stainless steel (SS) under two surface contamination conditions in plants processing cooked mussel was carried out. The conditions were either (i) contamination of clean surfaces with mussel cooking juice carrying L. monocytogenes or (ii) contamination with L. monocytogenes after soiling with mussel cooking juice, i.e., conditioning film (CF). The kinetics of adhesion were successfully described by a modified logistic model. Adhesion to PP was higher than to SS in all strains, except CECT 5873. Adhesion was initially higher in the presence of CF, but numbers of adherent cells decreased sharply in the late phase of study in three of eight cases as a result of cell detachment. Combinations of strain, surface material, and surface conditioning where adhesion was most enhanced were defined as the worst-case scenarios (CECT 911-PP, 4032-PP-CF, 5873-SS, and 4032-SS-CF). Subsequently, adhesion in worst-case scenarios was compared with a similar contamination event taking place in plants processing live mussels, using intervalvar water of mussel as a food residue matrix. Adhesion levels were higher in intervalvar water than in cooking juice, especially in both cases with no CF; this was attributed to more space available for adhesion or to physicochemical conditions enhancing cells to adhere.

Modeling of spatially referenced environmental and meteorological factors influencing the probability of Listeria species isolation from natural environments

Many pathogens have the ability to survive and multiply in abiotic environments, representing a possible reservoir and source of human and animal exposure. Our objective was to develop a methodological framework to study spatially explicit environmental and meteorological factors affecting the probability of pathogen isolation from a location. Isolation of Listeria spp. from the natural environment was used as a model system. Logistic regression and classification tree methods were applied, and their predictive performances were compared. Analyses revealed that precipitation and occurrence of alternating freezing and thawing temperatures prior to sample collection, loam soil, water storage to a soil depth of 50 cm, slope gradient, and cardinal direction to the north are key predictors for isolation of Listeria spp. from a spatial location. Different combinations of factors affected the probability of isolation of Listeria spp. from the soil, vegetation, and water layers of a location, indicating that the three layers represent different ecological niches for Listeria spp. The predictive power of classification trees was comparable to that of logistic regression. However, the former were easier to interpret, making them more appealing for field applications. Our study demonstrates how the analysis of a pathogen's spatial distribution improves understanding of the predictors of the pathogen's presence in a particular location and could be used to propose novel control strategies to reduce human and animal environmental exposure.

Listeria monocytogenes in the Irish dairy farm environment

Listeria monocytogenes is a potentially lethal foodborne pathogen commonly found in the environment. European Union hygiene legislation places responsibility for safety on primary production facilities, including farms, as part of a policy to introduce traceability throughout the food chain. This study aimed to determine the occurrence of L. monocytogenes in the Irish dairy farm environment and in particular the milking facility. Two hundred ninety-eight environmental samples were collected from 16 farms in the southern region of Ireland. A number of farms within the group supply raw milk to the unpasteurized milk cheese industry. The samples taken included cow feces, milk, silage, soil, water, etc. Samples were enriched in Listeria enrichment broth and incubated for 48 h, followed by plating on chromogenic agar Listeria Ottavani & Agosti and further incubation of the plates for 24 to 48 h. Presumptive L. monocytogenes isolates were purified and confirmed by PCR targeting the hly gene. Overall, 19% of the samples (57 of 298) were positive for L. monocytogenes. These were serotyped using conventional and PCR methods; serotypes 1/2a, 1/2b, and 4b made up 78% of the typeable isolates. A correlation was found between the level of hygiene standards on the farm and the occurrence of L. monocytogenes. There was little difference in the occurrence of L. monocytogenes between farms supplying milk to the unpasteurized milk cheese industry and those supplying milk for processing. This study demonstrates the prevalence of L. monocytogenes in the dairy farm environment and the need for good hygiene practices to prevent its entry into the food chain.

A New Oxygen Barrier Film Reduces Aerobic Deterioration in Farm-Scale Corn Silage

Recently, many studies have focused on the aerobic deterioration of corn silage at the farm level, because a large part of the product stored in horizontal silos is exposed to air and is more prone to spoilage. The most important factor influencing the preservation of forage ensiling is the degree of anaerobiosis that is usually achieved with sheets of polyethylene. A new black-on-white (125-microm) coextruded oxygen barrier (OB) film has been developed for silage sealing and was tested in the present experiment to assess the effects on fermentation quality, dry matter losses, and yeast and mold counts at opening of whole-crop corn bunker silos compared with conventional polyethylene (ST) film. Two trials were carried out on 2 commercial farms. The bunkers were divided into 2 parts along the length so that half of the feedout face would be covered with ST film and the other half with OB film. Eight plastic net bags with well-mixed fresh material were weighed and buried in the upper layer of the bunker, and 4 bags were buried in the central part. The silos were opened for summer consumption and were fed out at different rates (19 vs. 33 cm/d). The bags were unloaded, weighed, and subsampled to analyze the DM content, pH, lactic and monocarboxylic acids, ammonia, yeast and mold counts, and aerobic stability. The pH of the peripheral silage was different under the 2 films, with a lower value in the OB treatment. The OB film on farm 1 affected the silage dry matter losses, which were reduced 3.7 times in comparison with the ST film sealing. On farm 2, although the dry matter losses were numerically higher in the silage sealed with the ST film compared with OB film (9.0 vs. 5.9%, respectively), the difference was not statistically significant. However, the corn silage sealed with the ST film was less stable than the silage sealed with the OB film. The results indicate that the new OB film is a promising tool to constrain spoilage and dry matter losses under critical farm conditions, when inadequate amounts of silage are removed daily. The OB film further improved the stability of the corn silage in the peripheral areas of the silos even when a proper harvest-to-feedout management was implemented.

Markov chain approach to analyze the dynamics of pathogen fecal shedding--example of Listeria monocytogenes shedding in a herd of dairy cattle

Fecal shedding is an important mechanism of spreading of a number of human and animal pathogens. Understanding of the dynamics of pathogen fecal shedding is critical to be able to control or prevent the spread of diseases caused by these pathogens. The objective of this study was to develop a model for analysis of the dynamics of pathogen fecal shedding. Fecal shedding of Listeria monocytogenes in dairy cattle was used as a model system. A Markov chain model (MCM) with two states, shedding and non-shedding, has been developed for overall L. monocytogenes fecal shedding (all L. monocytogenes subtypes) and fecal shedding of three L. monocytogenes subtypes (ribotypes 1058A, 1039E and 1042B) using data from one study farm. The matrices of conditional probabilities of transition between shedding and non-shedding states for different sets of covariates have been estimated by application of logistic regression. The covariate-specific matrices of conditional probabilities, describing the presence of different risk factors, were used to estimate (i) the stationary prevalence of dairy cows that shed any L. monocytogenes subtype or ribotypes 1058A, 1039E, and 1042B, (ii) the duration of overall and subtype specific fecal shedding, and (iii) the duration of periods without shedding. A non-homogeneous MCM was constructed to study how the prevalence of fecal shedders changes over time. The model was validated with data from the study farm and published literature. The results of our modeling work indicated that (i) the prevalence of L. monocytogenes fecal shedders varies over time and can be higher than 90%, (ii) L. monocytogenes subtypes exhibit different dynamics of fecal shedding, (iii) the dynamics of L. monocytogenes fecal shedding are highly associated with contamination of silage (fermented feed) and cows' exposure to stress, and (iv) the developed approach can be readily used to study the dynamics of fecal shedding in other pathogen-host-environment systems.