Persistence Of Listeria Monocytogenes Research Articles

Predominance and Distribution of a Persistent Listeria monocytogenes Clone in a Commercial Fresh Mushroom Processing Environment

A longitudinal study was conducted to determine the prevalence of Listeria spp. in a commercial fresh mushroom slicing and packaging environment. Samples were collected at three different sampling periods within a 13-month time interval. Of the 255 environmental samples collected, 18.8% tested positive for L. monocytogenes, 4.3% for L. innocua, and 2.0% for L. grayi. L. monocytogenes was most often found on wet floors within the washing and slicing and packaging areas. Each of the 171 L. monocytogenes isolates found in the environment could be placed into one of three different serotypes; 1/2c was predominant (93.6%), followed by 1/2b (3.5%) and 1/2a (2.9%). Of 58 isolates subtyped using multi-virulence-locus sequence typing, all 1/2c isolates were identified as virulence type (VT) 11 (VT11), all 1/2b isolates were VT105, and 1/2a isolates were either VT107 or VT56. VT11 was designated as the predominant and persistent clone in the environment because it was isolated repeatedly at numerous locations throughout the study. The overall predominance and persistence of VT11 indicates that it likely colonized the mushroom processing environment. Areas adjacent to the trench drain in the washing and slicing area and a floor crack in the packaging area may represent primary harborage sites (reservoirs) for VT11. Improvements made to sanitation procedures by company management after period 2 coincided with a significant (P ≤ 0.001) reduction in the prevalence of L. monocytogenes from 17.8% in period 1 and 30.7% in period 2 to 8.5% in period 3. This suggests that targeted cleaning and sanitizing procedures can be effective in minimizing the occurrence of L. monocytogenes contamination in processing facilities. Additional research is needed to understand why VT11 was predominant and persistent in the mushroom processing environment.

Whole-Genome Sequencing Allows for Improved Identification of Persistent Listeria monocytogenes in Food-Associated Environments.

While the food-borne pathogen Listeria monocytogenes can persist in food associated environments, there are no whole-genome sequence (WGS) based methods to differentiate persistent from sporadic strains. Whole-genome sequencing of 188 isolates from a longitudinal study of L. monocytogenes in retail delis was used to (i) apply single-nucleotide polymorphism (SNP)-based phylogenetics for subtyping of L. monocytogenes, (ii) use SNP counts to differentiate persistent from repeatedly reintroduced strains, and (iii) identify genetic determinants of L. monocytogenes persistence. WGS analysis revealed three prophage regions that explained differences between three pairs of phylogenetically similar populations with pulsed-field gel electrophoresis types that differed by ≤3 bands. WGS-SNP-based phylogenetics found that putatively persistent L. monocytogenes represent SNP patterns (i) unique to a single retail deli, supporting persistence within the deli (11 clades), (ii) unique to a single state, supporting clonal spread within a state (7 clades), or (iii) spanning multiple states (5 clades). Isolates that formed one of 11 deli-specific clades differed by a median of 10 SNPs or fewer. Isolates from 12 putative persistence events had significantly fewer SNPs (median, 2 to 22 SNPs) than between isolates of the same subtype from other delis (median up to 77 SNPs), supporting persistence of the strain. In 13 events, nearly indistinguishable isolates (0 to 1 SNP) were found across multiple delis. No individual genes were enriched among persistent isolates compared to sporadic isolates. Our data show that WGS analysis improves food-borne pathogen subtyping and identification of persistent bacterial pathogens in food associated environments.

Health Risks Associated with the Occurrence and Persistence of Listeria monocytogenes in Dairy Plants

Listeria monocytogenes is a rod-shaped Gram-positive, non-spore pathogen that can be widely distributed in the environment, including soil and surface water used for agricultural purposes, wastewater, surfaces of materials used in food industries, and food products [1]. The distribution of L. monocytogenes in several habitats is associated to its great survival capacity in wide ranges of pH (4.0-9.5), temperature (1-45°C), and water activity as low as 0.92 [2]. The disease caused by L. monocytogenes, or Listeriosis, primarily affects pregnant women, newborns and immunocompromised adults may cause sepsis, abortion and infection of the central nervous system and resulting in high morbidity and mortality [3]. Although having a low incidence compared to other foodborne diseases, such as salmonellosis or campylobacteriosis, Listerioris is considered as a major public health issue due to its high hospitalization rate (94%), and high case-fatality proportion (12.8 to 17% of cases) [4].

Monitoring occurrence and persistence of Listeria monocytogenes in foods and food processing environments in the Republic of Ireland.

Although rates of listeriosis are low in comparison to other foodborne pathogenic illness, listeriosis poses a significant risk to human health as the invasive form can have a mortality rate as high as 30%. Food processors, especially those who produce ready-to-eat (RTE) products, need to be vigilant against Listeria monocytogenes, the causative pathogen of listeriosis, and as such, the occurrence of L. monocytogenes in food and in the food processing environment needs to be carefully monitored. To examine the prevalence and patterns of contamination in food processing facilities in Ireland, 48 food processors submitted 8 samples every 2 months from March 2013 to March 2014 to be analyzed for L. monocytogenes. No positive samples were detected at 38% of the processing facilities tested. Isolates found at the remaining 62% of facilities were characterized by serotyping and Pulsed Field Gel Electrophoresis (PFGE). A general L. monocytogenes prevalence of 4.6% was seen in all samples analyzed with similar rates seen in food and environmental samples. Differences in prevalence were seen across different food processors, food sectors, sampling months etc. and PFGE analysis allowed for the examination of contamination patterns and for the identification of several persistent strains. Seven of the food processing facilities tested showed contamination with persistent strains and evidence of bacterial transfer from the processing environment to food (the same pulsotype found in both) was seen in four of the food processing facilities tested.

Classification of Listeria monocytogenes persistence in retail delicatessen environments using expert elicitation and machine learning.

Increasing evidence suggests that persistence of Listeria monocytogenes in food processing plants has been the underlying cause of a number of human listeriosis outbreaks. This study extracts criteria used by food safety experts in determining bacterial persistence in the environment, using retail delicatessen operations as a model. Using the Delphi method, we conducted an expert elicitation with 10 food safety experts from academia, industry, and government to classify L. monocytogenes persistence based on environmental sampling results collected over six months for 30 retail delicatessen stores. The results were modeled using variations of random forest, support vector machine, logistic regression, and linear regression; variable importance values of random forest and support vector machine models were consolidated to rank important variables in the experts' classifications. The duration of subtype isolation ranked most important across all expert categories. Sampling site category also ranked high in importance and validation errors doubled when this covariate was removed. Support vector machine and random forest models successfully classified the data with average validation errors of 3.1% and 2.2% (n = 144), respectively. Our findings indicate that (i) the frequency of isolations over time and sampling site information are critical factors for experts determining subtype persistence, (ii) food safety experts from different sectors may not use the same criteria in determining persistence, and (iii) machine learning models have potential for future use in environmental surveillance and risk management programs. Future work is necessary to validate the accuracy of expert and machine classification against biological measurement of L. monocytogenes persistence.

Listeria Monocytogenes Persistence in Ready-to-Eat Sausages and in Processing Plants.

Listeria monocytogenes is of major concern in the fermented meat products and is able to persist in their processing environments. The aim of the present work was to evaluate the virulence profile and the persistence capacity of L. monocytogenes strains isolated in Sardinian fermented sausages processing plants. Food (ground meat, sausages at the end of acidification and ripening stage) and environmental samples (a total of n. 385), collected from 4 meat processing plants located in Sardinia (Italy), were examined to detect L. monocytogenes presence. All the L. monocytogenes isolates were identified by polymerase chain reaction (PCR) method. A subset of strains was also characterised by multiplex PCR-based serogrouping, using the lmo0737, lmo1118, ORF2819 and ORF2110 genes. Three different multiplex PCRs were used to obtain the virulence profiles by the rrn, hlyA, actA, prfA, inlA, inlB, iap, plcA, plcB and mpl marker genes. Furthermore, in vitro biofilm forming ability and resistance to disinfectants were carried out on microtiter plate. The overall prevalence was 31.5% in food, and 68.5% in environmental samples. The prevalent serotype resulted 1/2c (43%), followed by 1/2a (40%), 4b (8.6%), and 1/2b (8.6%). The amplification products of the virulence genes were found in all the isolates with the following prevalence: 77.1% hlyA; 100% rrn; 100% prfA; 97.1% iap; 65.7% inlB; 88.6% inlA; 100% plcA; 100% plcB and 74.3% mpl. As for biofilm forming ability, 37.1% of the strains were positive and resulted weak producer, but all the isolates were sensible to disinfectants showing a reduction of L. monocytogenes growth after each incubation time. More appropriate technologies and application of measures of hygienic control should be implemented to prevent the L. monocytogenes growth and cross-contamination in salsiccia sarda processing plants.

Listeria monocytogenes Persistence in Food-Associated Environments: Epidemiology, Strain Characteristics, and Implications for Public Health

Over the last 10 to 15 years, increasing evidence suggests that persistence of Listeria monocytogenes in food processing plants for years or even decades is an important factor in the transmission of this foodborne pathogen and the root cause of a number of human listeriosis outbreaks. L. monocytogenes persistence in other food-associated environments (e.g., farms and retail establishments) may also contribute to food contamination and transmission of the pathogen to humans. Although L. monocytogenes persistence is typically identified through isolation of a specific molecular subtype from samples collected in a given environment over time, formal (statistical) criteria for identification of persistence are undefined. Environmental factors (e.g., facilities and equipment that are difficult to clean) have been identified as key contributors to persistence; however, the mechanisms are less well understood. Although some researchers have reported that persistent strains possess specific characteristics that may facilitate persistence (e.g., biofilm formation and better adaptation to stress conditions), other researchers have not found significant differences between persistent and nonpersistent strains in the phenotypic characteristics that might facilitate persistence. This review includes a discussion of our current knowledge concerning some key issues associated with the persistence of L. monocytogenes, with special focus on (i) persistence in food processing plants and other food-associated environments, (ii) persistence in the general environment, (iii) phenotypic and genetic characteristics of persistent strains, (iv) niches, and (v) public health and economic implications of persistence. Although the available data clearly indicate that L. monocytogenes persistence at various stages of the food chain contributes to contamination of finished products, continued efforts to quantitatively integrate data on L. monocytogenes persistence (e.g., meta-analysis or quantitative microbial risk assessment) will be needed to advance our understanding of persistence of this pathogen and its economic and public health impacts.

Biofilm Formation under Different Temperature Conditions by a Single Genotype of Persistent Listeria monocytogenes Strains

Some Listeria monocytogenes strains, termed persistent strains, originate from the same processing plant and have the ability to survive and grow over extended periods of time at contamination sources. In order to evaluate biofilm formation by such persistent strains, we isolated the pathogen from chicken samples collected from the same retail shop in repeated visits over 6 months. Strains that were of serotype 1/2b and were assigned to the same genotype by multi-virulence-locus sequence typing analysis were isolated on repeated occasions from December 1997 to June 1998 and thus were defined as persistent strains. In the present study, biofilm formation by the persistent strains was evaluated using microplates at 30 and 37°C. The biofilm-forming capability was measured after cells attaching to the microplate well were stained with crystal violet. Comparison of biofilm formation at 30°C among the persistent strains showed that a significantly higher amount of the stain was obtained from the persistent strains isolated from December to March than from those isolated from April to June. However, no significant difference in biofilm formation at 30°C was observed between persistent and nonpersistent groups of L. monocytogenes strains. In contrast, biofilm formation at 37°C was consistent among the persistent strains, and they produced significantly more biofilm at 37°C than did the nonpersistent strains. The persistent strains were also found to change their biofilm-forming ability in a temperature-dependent manner, which may suggest that the persistent strains alter their biofilm formation in response to changing environmental factors.

Implementation of Statistical Tools To Support Identification and Management of Persistent Listeria monocytogenes Contamination in Smoked Fish Processing Plants

Listeria monocytogenes persistence in food processing plants is a key source of postprocessing contamination of ready-to-eat foods. Thus, identification and elimination of sites where L. monocytogenes persists (niches) is critical. Two smoked fish processing plants were used as models to develop and implement environmental sampling plans (i) to identify persistent L. monocytogenes subtypes (EcoRI ribotypes) using two statistical approaches and (ii) to identify and eliminate likely L. monocytogenes niches. The first statistic, a binomial test based on ribotype frequencies, was used to evaluate L. monocytogenes ribotype recurrences relative to reference distributions extracted from a public database; the second statistic, a binomial test based on previous positives, was used to measure ribotype occurrences as a risk factor for subsequent isolation of the same ribotype. Both statistics revealed persistent ribotypes in both plants based on data from the initial 4 months of sampling. The statistic based on ribotype frequencies revealed persistence of particular ribotypes at specific sampling sites. Two adaptive sampling strategies guided plant interventions during the study: sampling multiple times before and during processing and vector swabbing (i.e., sampling of additional sites in different directions [vectors] relative to a given site). Among sites sampled for 12 months, a Poisson model regression revealed borderline significant monthly decreases in L. monocytogenes isolates at both plants (P = 0.026 and 0.076). Our data indicate elimination of an L. monocytogenes niche on a food contact surface; niches on nonfood contact surfaces were not eliminated. Although our data illustrate the challenge of identifying and eliminating L. monocytogenes niches, particularly at nonfood contact sites in small and medium plants, the methods for identification of persistence we describe here should broadly facilitate science-based identification of microbial persistence.

Persistent Listeria monocytogenes subtypes isolated from a smoked fish processing facility included both phage susceptible and resistant isolates

Contamination of Ready-To-Eat foods with Listeria monocytogenes can typically be traced back to post-processing contamination from environmental sources; contamination is often linked to subtypes that persist in food associated environments. Although phage-based biocontrol strategies have been proposed for controlling this pathogen, information on the efficacy of phage treatment against diverse L. monocytogenes subtypes from food associated environments is still limited. We identified subtypes that were repeatedly found (“persistent”) in a smoked fish processing facility by using EcoRI ribotyping data for isolates obtained in 1998–2009. PFGE analysis of 141 isolates (9 ribotypes) supported persistence for up to 11 years. Characterization of selected isolates, representing persistent subtypes, against a panel of 28 listeriaphages showed a wide range of likelihood of phage susceptibility, ranging from 4.6% (for 7 ribotype DUP-1043A isolates) to 95.4% (for 7 ribotype DUP-1044A isolates). In challenge studies with 105 and 106 CFU/ml L. monocytogenes, using phage cocktails and a commercial phage product at different phage-host ratios, one isolate (ribotype DUP-1043A) was not affected by any treatment. A reduction in L. monocytogenes counts of up to 4 log units was observed, after 8 h of treatment, in isolates of two ribotypes, but subsequent re-growth occurred. Survivor isolates obtained after 24 h of treatment showed decreased susceptibility to individual phages included in the phage cocktail, suggesting rapid emergence of resistant subtypes.

Genome Sequencing Identifies Two Nearly Unchanged Strains of Persistent Listeria monocytogenes Isolated at Two Different Fish Processing Plants Sampled 6 Years Apart

Listeria monocytogenes is a food-borne human-pathogenic bacterium that can cause infections with a high mortality rate. It has a remarkable ability to persist in food processing facilities. Here we report the genome sequences for two L. monocytogenes strains (N53-1 and La111) that were isolated 6 years apart from two different Danish fish processers. Both strains are of serotype 1/2a and belong to a highly persistent DNA subtype (random amplified polymorphic DNA [RAPD] type 9). We demonstrate using in silico analyses that both strains belong to the multilocus sequence typing (MLST) type ST121 that has been isolated as a persistent subtype in several European countries. The purpose of this study was to use genome analyses to identify genes or proteins that could contribute to persistence. In a genome comparison, the two persistent strains were extremely similar and collectively differed from the reference lineage II strain, EGD-e. Also, they differed markedly from a lineage I strain (F2365). On the proteome level, the two strains were almost identical, with a predicted protein homology of 99.94%, differing at only 2 proteins. No single-nucleotide polymorphism (SNP) differences were seen between the two strains; in contrast, N53-1 and La111 differed from the EGD-e reference strain by 3,942 and 3,471 SNPs, respectively. We included a persistent L. monocytogenes strain from the United States (F6854) in our comparisons. Compared to nonpersistent strains, all three persistent strains were distinguished by two genome deletions: one, of 2,472 bp, typically contains the gene for inlF, and the other, of 3,017 bp, includes three genes potentially related to bacteriocin production and transport (lmo2774, lmo2775, and the 3'-terminal part of lmo2776). Further studies of highly persistent strains are required to determine if the absence of these genes promotes persistence. While the genome comparison did not point to a clear physiological explanation of the persistent phenotype, the remarkable similarity between the two strains indicates that subtypes with specific traits are selected for in the food processing environment and that particular genetic and physiological factors are responsible for the persistent phenotype.

Biofilm recovery from chitosan action: A possible clue to understand Listeria monocytogenes persistence in food plants

Listeria monocytogenes is able to form biofilms on surfaces of food industry premises. Once established at certain plant sites, this pathogen can persist there for months or years, risking the transfer to raw materials or processed food by direct contact or other mechanisms. Though persistence of certain strains of L. monocytogenes isolated from food industry surfaces has been reported in several cases, experimental evidence has so far been inconclusive about the association of persistence to features such as better biofilm forming ability or higher resistance to disinfectants. This work has aimed at checking another possible cause of persistence, the ability to recover after exposure to antimicrobial compounds. Damaged cells in the biofilms formed by twelve L. monocytogenes strains previously isolated from an Iberian pig slaughterhouse and processing plant, six persistent and six non persistent, were compared in their recovery of viable cell counts during the 24 h following exposure to chitosan or peracetic acid. Persistent strains showed a better resuscitation rate after chitosan damage.

Effectiveness of two sanitation procedures for decreasing the microbial contamination levels (including Listeria monocytogenes) on food contact and non-food contact surfaces in a dessert-processing factory

The aim of this study was to determine the presence and persistence of Listeria monocytogenes (LM) after using two sanitization protocols in different parts of a dessert-processing factory. The sampling for LM detection was performed: a) on various food contact and non-food surfaces; b) after and before treatment using two different sanitization protocols. LM was not detected on food contact surfaces while its identification was restricted to non-food contact surfaces (mostly the floor). Both sanitizing protocols managed to reduce the LM load (as well for the rest of the microbial parameter studied) but not to eradicate the microorganism completely.

Increased in vitro adherence and on-farm persistence of predominant and persistent Listeria monocytogenes strains in the milking system.

Dairy farms are a reservoir for Listeria monocytogenes, and the reduction of this pathogen at the farm level is important for reducing human exposure. The objectives of this research were to study the diversity of L. monocytogenes strains on a single dairy farm, assess strain dynamics within the farm, identify potential sources of L. monocytogenes in bulk tank milk and milk filters, and assess the adherence abilities of representative strains. A total of 248 L. monocytogenes isolates were analyzed by pulsed-field gel electrophoresis (PFGE). Combined AscI and ApaI restriction analysis yielded 40 PFGE types (strains). The most predominant strains were T (28.6%), D (22.6%), and F (14.9%). A high level of heterogeneity of strains among isolates from fecal (Simpson's index of diversity [SID] = 0.96) and environmental (SID = 0.96) samples was observed. A higher homogeneity of strains was observed among isolates from milk filters (SID = 0.71) and bulk tank milk (SID = 0.65). Six of 17 L. monocytogenes isolates (35.3%) were classified in an in vitro assay as having a "low adherence ability," 9 (52.9%) were classified as having a "medium adherence ability," and 2 (11.8%) were classified as having a "high adherence ability." The L. monocytogenes strains that were predominant and persistent showed significantly better adherence than did strains that were only sporadic, predominant, or persistent (P = 0.0006). Our results suggest that the milking system was exposed to several L. monocytogenes strains from different sources. Only 3 strains, however, were successful in persisting within the milking system, suggesting that some strains are more suitable to that particular ecological environment than others.

Diverse Geno- and Phenotypes of Persistent Listeria monocytogenes Isolates from Fermented Meat Sausage Production Facilities in Portugal

The persistence of Listeria monocytogenes in food-associated environments represents a key factor in transmission of this pathogen. To identify persistent and transient strains associated with production of fermented meat sausages in northern Portugal, 1,723 L. monocytogenes isolates from raw material and finished products from 11 processors were initially characterized by random amplification of polymorphic DNA (RAPD), PCR-based molecular serotyping, and epidemic clone characterization, as well as cadmium, arsenic, and tetracycline resistance typing. Pulsed-field gel electrophoresis (PFGE) typing of 240 representative isolates provided evidence for persistence of L. monocytogenes for periods of time ranging from 10 to 32 months for all seven processors for which isolates from different production dates were available. Among 50 L. monocytogenes isolates that included one representative for each PFGE pattern obtained from a given sample, 12 isolates showed reduced invasion efficiency in Caco-2 cells, including 8 isolates with premature stop codons in inlA. Among 41 isolates representing sporadic and persistent PFGE types, 22 isolates represented lysogens. Neither strains with reduced invasion nor lysogens were overrepresented among persistent isolates. While the susceptibility of isolates to lysogenic phages also did not correlate with persistence, it appeared to be associated with molecular serotype. Our data show the following. (i) RAPD may not be suitable for analysis of large sets of L. monocytogenes isolates. (ii) While a large diversity of L. monocytogenes subtypes is found in Portuguese fermented meat sausages, persistence of L. monocytogenes in this food chain is common. (iii) Persistent L. monocytogenes strains are diverse and do not appear to be characterized by unique genetic or phenotypic characteristics.

Environmental prevalence and persistence of Listeria monocytogenes in cold-smoked trout processing plants

The presence of Listeria monocytogenes on the surfaces of equipment and worker’s hands during different production stages, as well as on fish skin and meat during processing and storage of cold-smoked trout, was investigated. Listeria monocytogenes was recovered from 10 (6.06%) of a total 165 cotton-swabbed samples collected from the surfaces of equipment and worker’s hands at two separate processing facilities. Of 105 samples collected from fish skin and meat during various production steps in both processing plants, 14 (13.33%) were confirmed culture-positive for L. monocytogenes, with recovery being most frequently in samples collected in the area before vacuum packaging. Recovery rates at two different Serbian processing plants did not differ (p<0.05), but suggested that different L. monocytogenes serotypes appeared to be resident within each processing plant and may have contributed to the final product contamination. From all smoked trout samples collected during 7, 14, 21 and 28 days of storage at 4ºC only two were culture-positive for L. monocytogenes serotype 1/2a and both of these were collected on the 7th day of storage. Conversely, 4, 3 and 1 sample(s) were contaminated with L. monocytogenes serotypes 1/2a and 1/2b after 7, 14 and 21 days storage at 10ºC. Listeria monocytogenes was not recovered from smoked trout stored 28 days at 10ºC. Results emphasize the importance of adhering to strict hygienic and quality control standards throughout the processing environment.

Comparison of invasiveness among surface-adherent variants of Listeria monocytogenes in Caco-2 cell culture assays

The persistence of Listeria monocytogenes in meat processing plants is believed to be partly linked with their adherence properties to abiotic surfaces that may allow contamination of ready-to-eat (RTE) foods. Isolates of raw meats, RTE meats and environmental samples from RTE meat processing facilities that demonstrated differences in adherence to abiotic surfaces (glass, plastic, stainless steel, and rubber) were tested for cellular adherence and invasion in the Caco-2 human cell line. Strains of L. monocytogenes were classified into strong (CW50, 99-38, CW77, SM5 and CW62) and weakly adherent (CW34, CW35, CW72, SM3, J7 and J126) based on a microplate adherence assay using the fluorescing substrate, 5,6-CFDA. These strains were tested for adherence and invasion in cell culture assays with Caco-2 cells. At long incubation time (2 h) and high multiplicity of infection (MOI, 100:1) we observed equivalent cellular adherence and invasiveness. After optimizing conditions for time of infection (15-, 30-, 60-, 90-, and 120-min) and MOI (100:1, 10:1, 1:1, and 0.1:1), we found that under the conditions of 15-min infection time at an MOI of 10:1 ( Listeria:Caco-2 cells), we again observed equivalent cellular adherence by the two distinct surface-adherent groups, but we only recovered invading Listeria from the group that was strongly adherent to environmental surfaces. The data indicate that a surface factor that provides for strong environmental surface adherence may also be involved with internal cellular virulence and survival and implicates strongly adherent strains as possibly being of greater invasiveness than weakly adherent strains.

An 8-Year Surveillance of the Diversity and Persistence of Listeria monocytogenes in a Chilled Food Processing Plant Analyzed by Amplified Fragment Length Polymorphism

Contamination routes of Listeria monocytogenes were examined in a chilled food processing plant that produced ready-to-eat and ready-to-reheat meals during an 8-year period by amplified fragment length polymorphism (AFLP) analysis. A total of 319 L. monocytogenes isolates were recovered from raw materials (n = 18), the environment (n = 77), equipment (n = 193), and products (n = 31), and 18 different AFLP types were identified, five of which were repeatedly found to be persistent types. The three compartments (I to III) of the plant showed markedly different contamination statuses. Compartment I, which produced cooked meals, was heavily contaminated with three persistent AFLP types. AFLP type A1 dominated, and it comprised 93% of the isolates of the compartment. Compartment II, which produced uncooked chilled food, was contaminated with four persistent and five nonpersistent AFLP types. The equipment of compartment III, which produced cooked ready-to-reheat meals, was free of contamination. In compartments that produced cooked meals, the cleaning routines, product types, and lack of compartmentalization seemed to predispose production lines to persistent contamination. The most contaminated lines harbored L. monocytogenes in coolers, conveyors, and packing machines. Good compartmentalization limited the flow of L. monocytogenes into the postheat-treatment area and prevented the undesired movement of equipment and personnel, thus protecting the production lines from contamination. In compartment II, grated cheese was shown to cause product contamination. Therefore, special attention should be paid to continuous quality control of raw ingredients when uncooked ready-to-eat foods are produced. In compartment II, reconstruction of the production line resulted in reduced prevalence rates of L. monocytogenes and elimination of two persistent AFLP types.

Ultrasonic Cleaning of Conveyor Belt Materials Using Listeria monocytogenes as a Model Organism

Persistent Listeria monocytogenes contamination of food industry equipment is a difficult problem to solve. Ultrasonic cleaning offers new possibilities for cleaning conveyors and other equipment that are not easy to clean. Ultrasonic cleaning was tested on three conveyor belt materials: polypropylene, acetal, and stainless steel (cold-rolled, AISI 304). Cleaning efficiency was tested at two temperatures (30 and 45°C) and two cleaning times (30 and 60 s) with two cleaning detergents (KOH, and NaOH combined with KOH). Conveyor belt materials were soiled with milk-based soil and L. monocytogenes strains V1, V3, and B9, and then incubated for 72 h to attach bacteria to surfaces. Ultrasonic cleaning treatments reduced L. monocytogenes counts on stainless steel 4.61 to 5.90 log units; on acetal, 3.37 to 5.55 log units; and on polypropylene, 2.31 to 4.40 log units. The logarithmic reduction differences were statistically analyzed by analysis of variance using Statistical Package for the Social Sciences software. The logarithmic reduction was significantly greater in stainless steel than in plastic materials (P < 0.001 for polypropylene, P = 0.023 for acetal). Higher temperatures enhanced the cleaning efficiency in tested materials. No significant difference occurred between cleaning times. The logarithmic reduction was significantly higher (P = 0.013) in cleaning treatments with potassium hydroxide detergent. In this study, ultrasonic cleaning was efficient for cleaning conveyor belt materials.

Persistence of Listeria and Salmonella During Swine Manure Treatment

Swine manures are known to be a source of a variety of human and animal pathogens. Of particular concern are bacteria causing human food-borne illnesses such as Salmonella and Listeria species. The effects of commonly used manure treatments on the persistence of these pathogens have rarely been compared. The objective of this study was to compare the persistence of Listeria monocytogenes and Salmonella typhimurium during the treatment of swine manure by the most commonly used manure management methods: liquid storage, aerated liquid storage, thermophilic composting (55°C) and manure packing (low temperature composting). Swine manure obtained from Ohio State University's Agricultural Technical Institute swine farm was inoculated with L. monocytogenes and S. typhimurium to obtain a final concentration of 106 CFU/g of each organism. To simulate liquid storage, manure was diluted with water and stored aerated and unaerated at 20-25°C. To simulate pack storage and thermophilic composting, the same manure was amended with sawdust to provide 60% moisture and incubated in triplicate aerated 4-liter compost reactors at 25 and 55°C, respectively. The treatments were sampled on days 0, 3, 7, 14, 28 and 56 and analyzed for the most probable numbers (MPN) of Listera and Salmonella using standard methods. L. monocytogenes was culturable in all the treatments for 28 days and in compost mixes through day 56 at both temperatures. The MPN/g of L. monocytogenes in the 55°C compost and the aerated liquid declined by 3 to 4 logs after 3 days, but not in the compost stored at 25°C or the unaerated liquid. Likewise after 3 days, Salmonella MPNs dropped to 44 MPN g−1 in the 55°C compost and 1.2 × 104 MPN g−1 in the aerated liquid but were unchanged in the sawdust manure mix at 25°C and the unaerated liquid. Salmonella persisted for up to 28 days in compost at 55°C, 42 days in aerated liquid and up to 56 days in both compost at 25°C and the unaerated liquid. Taken together, the results indicate that both pathogens were destroyed most rapidly under thermophilic composting conditions and that they persisted the longest in pack storage or low temperature composting conditions. However, although L. monocytogenes declined by 4-5 logs after 14 days in all treatments, it persisted at low levels (<1000 MPN g−1) longer in composts than in liquid manure treatments.