Variability in biofilm production by Listeria monocytogenes correlated to strain origin and growth conditions

Unique response characteristics in persistent strains of Listeria monocytogenes exposed to low pH

Some Listeria monocytogenes strains are persistent in food processing environments, where this pathogen may be subjected to various stresses. This study aimed to elucidate the response of persistent strains of L. monocytogenes to low pH and H₂O₂ exposure. Almost all of the persistent strains examined were highly susceptible to low pH, whereas H₂O₂ susceptibility was comparable to that of control strains. Two persistent strains isolated from the same sample, however, exhibited lower susceptibility to low pH. These findings suggest an acid-susceptible phenotype predominates in the habitat, indicating that environmental conditions contribute to the establishment of persistence. Representative strains exhibiting acid-susceptible and less acid-susceptible phenotypes were further investigated regarding acid response characteristics. Less acid-susceptible strains exhibited increased survival in acidified brain heart infusion (BHI) broth compared with acidified phosphate-buffered saline (PBS). These strains also exhibited increased survival in acidified PBS containing glucose and glutamate, which are involved in acid response mechanisms, compared with acidified PBS alone. However, neither acidified BHI broth nor exogenous glucose and glutamate increased survival of acid-susceptible strains. An adaptive acid tolerance response of the acid-susceptible phenotype was observed, but this was limited compared with that of the less acid-susceptible phenotype.

Disinfectant Susceptibility of Biofilm Formed by Listeria monocytogenes under Selected Environmental Conditions

Listeria monocytogenes is a one of the most important food-borne pathogens. Its ability to form biofilm contributes to increased resistance to disinfectants and inefficient disinfection, posing a serious threat for the food industry, and in the end the consumer. The aim of this study was the comparison of the biofilm formation ability of L. monocytogenes strains on stainless steel, under different environmental conditions (temperature, pH, NaCl concentration, nutrients availability), and the assessment of biofilm susceptibility to disinfectants. The bactericidal activity of four disinfectants in two concentrations (100% and 50% of working solution) against biofilm was conducted on four clinical strains, four strains isolated from food and one reference strain ATCC 19111. It was found that biofilm susceptibility to disinfectants was influenced by environmental conditions. Biofilm susceptibility correlated with the decrease of temperature, pH, nutrients availability and salinity of the environment. The least sensitive to disinfectants was biofilm produced at pH = 4 (the bacterial number ranged from 0.25 log CFU × cm-2 to 1.72 log CFU × cm-2) whereas the most sensitive was biofilm produced at pH = 9 (5.16 log CFU × cm-2 to 7.84 log CFU × cm-2). Quatosept was the most effective disinfectant, regardless of the conditions. In conclusion, biofilm susceptibility to disinfectants is strain-dependent and is affected by environmental conditions.

Expression levels of the agr locus and prfA gene during biofilm formation by Listeria monocytogenes on stainless steel and polystyrene during 8 to 48 h of incubation 10 to 37 °C

The objective of this study was to compare the gene expression levels of the agr locus and prfA gene during adhesion and biofilm formation by four L. monocytogenes isolates (2 biofilm-forming and 2 non-forming) on stainless steel and polystyrene surfaces at different temperatures (10 °C, 20 °C and 37 °C), and times (8 h, 12 h, 24 h and 48 h). The agrA and prfA genes were expressed at higher levels than the agrBCD genes. The levels of agr locus expression were higher in the biofilm-forming strains, and the greatest difference between biofilm-forming and non-forming isolates was observed for the agrB, agrC and agrD genes. However, no difference in the expression of the prfA gene was seen among the isolates, independent of the biofilm-forming ability. Maximum expression of the agr locus and prfA gene was observed at 37 °C, whereas expression was lowest at 10 °C. The agr locus, and particularly the agrB, agrC and agrD genes, is important in the initial adhesion phase of biofilm production by L. monocytogenes, with this expression independent of prfA. In addition, the agr locus and prfA gene expression levels were strongly influenced by time and temperature.

Genomic characterization of Listeria monocytogenes isolates reveals that their persistence in a pig slaughterhouse is linked to the presence of benzalkonium chloride resistance genes

BACKGROUND

The aim of this study was to characterize the genomes of 30 Listeria monocytogenes isolates collected at a pig slaughterhouse to determine the molecular basis for their persistence.

RESULTS

Comparison of the 30 L. monocytogenes genomes showed that successive isolates (i.e., persistent types) recovered from thew sampling site could be linked on the basis of single nucleotide variants confined to prophage regions. In addition, our study revealed the presence among these strains of the bcrABC cassette which is known to produce efflux pump-mediated benzalkonium chloride resistance, and which may account for the persistence of these isolates in the slaughterhouse environment. The presence of the bcrABC cassette was confirmed by WGS and PCR and the resistance phenotype was determined by measuring minimum inhibitory concentrations. Furthermore, the BC-resistant strains were found to produce lower amounts of biofilm in the presence of sublethal concentrations of BC.

CONCLUSIONS

High resolution SNP-based typing and determination of the bcrABC cassette may provide a means of distinguishing between resident and sporadic L. monocytogenes isolates, and this in turn will support better management of this pathogen in the food industry.

Behavior of Foodborne Pathogens Listeria monocytogenes and Staphylococcus aureus in Mixed-Species Biofilms Exposed to Biocides

In nature and man-made environments, microorganisms reside in mixed-species biofilms, in which the growth and metabolism of an organism are different from these behaviors in single-species biofilms. Pathogenic microorganisms may be protected against adverse treatments in mixed-species biofilms, leading to health risk for humans. Here, we developed two mixed five-species biofilms that included one or the other of the foodborne pathogens Listeria monocytogenes and Staphylococcus aureus The five species, including the pathogen, were isolated from a single food-processing environmental sample, thus mimicking the environmental community. In mature mixed five-species biofilms on stainless steel, the two pathogens remained at a constant level of ∼105 CFU/cm2 The mixed five-species biofilms as well as the pathogens in monospecies biofilms were exposed to biocides to determine any pathogen-protective effect of the mixed biofilm. Both pathogens and their associate microbial communities were reduced by peracetic acid treatments. S. aureus decreased by 4.6 log cycles in monospecies biofilms, but the pathogen was protected in the five-species biofilm and decreased by only 1.1 log cycles. Sessile cells of L. monocytogenes were affected to the same extent when in a monobiofilm or as a member of the mixed-species biofilm, decreasing by 3 log cycles when exposed to 0.0375% peracetic acid. When the pathogen was exchanged in each associated microbial community, S. aureus was eradicated, while there was no significant effect of the biocide on L. monocytogenes or the mixed community. This indicates that particular members or associations in the community offered the protective effect. Further studies are needed to clarify the mechanisms of biocide protection and to identify the species playing the protective role in microbial communities of biofilms.IMPORTANCE This study demonstrates that foodborne pathogens can be established in mixed-species biofilms and that this can protect them from biocide action. The protection is not due to specific characteristics of the pathogen, here S. aureus and L. monocytogenes, but likely caused by specific members or associations in the mixed-species biofilm. Biocide treatment and resistance are a challenge for many industries, and biocide efficacy should be tested on microorganisms growing in biofilms, preferably mixed systems, mimicking the application environment.

Biofilm formation and potential virulence factors of Salmonella strains isolated from ready-to-eat shrimps

Salmonella species is an important foodborne pathogen with the non-typhoidal serovars such as Enteritidis and Typhimurium as the most predominant strains. This study examines the biofilm formation, phenotypic virulence factors and cell surface characteristics of Salmonella strains from ready-to-eat shrimps. The ready-to-eat shrimps were obtained from open markets between November 2016 and October 2017 in Edo and Delta States, Nigeria. The occurrence of Salmonella strains in this study was 210/1440 (14.58%) of the ready-to-eat shrimp’s samples. The identified strains comprise of Salmonella Enteritidis 11, Salmonella Typhimurium 14 and other Salmonella spp. 20. The 45 identified Salmonella strains revealed the following virulence properties: swimming and swarming motility 45(100%); S-layer 39(86.67%); haemolytic activity 40(88.89%); lipase activity 43(95.56%); protease activity 43(95.56%); gelatinase production 43(95.56%); and DNA degrading activity 41(91.11%). The variation in the formation of biofilm-based on the diversity of Salmonella species was observed with higher percentage of Salmonella Typhimurium strains as strong biofilms producers under different environmental conditions. For surface hydrophobicity using bacterial adherence to hydrocarbons, 25(55.56%) were hydrophilic while 20(44.44%) were moderately hydrophobic from the 45 Salmonella isolates. Using salting aggregation test for surface hydrophobicity, all selected isolates 45(100%) was hydrophilic. Autoaggregation index for the 12 selected Salmonella isolates ranged from 15.2–47.2%, while the autoaggragation index for the 12 selected test bacteria ranged from 26.2–71.3%. Coaggragation between the 12 selected test bacteria and 12 Salmonella isolates ranged from 12.5–81.0%. The occurrence of pathogenic species of Salmonella from ready-to-eat shrimps could be detrimental to the consumers. Findings on the physiological conditions of biofilms formed by the foodborne pathogenic Salmonella and the cell surface characteristics therein are crucial for the advancement of methods for controlling Salmonella from ready-to-eat foods.

A Comparative Study of the Susceptibility of Listeria Species to Sanitizer Treatments When Grown under Planktonic and Biofilm Conditions

Listeria species are ubiquitous in nature and can adapt to survive in a variety of niches, including food processing environments. Listeria species that colonize these environments may also have the potential to persist. Food safety strategies designed to manage these niches include regular cleaning and disinfection with proven sanitizers containing biocide-active compounds. Typically, these sanitizers are effective against bacteria growing under planktonic conditions, but their efficacy may be compromised when bacteria are contained in biofilms. The susceptibility of persistent Listeria isolates, i.e., those capable of forming biofilms, to a selection of sanitizers was investigated. A quaternary ammonium compound-based sanitizer was the biocide most effective against planktonic bacteria, with a MIC of 0.0015 to 0.006%. In contrast, ethanol-based sanitizers were the least effective. Although, no triclosan tolerance was observed for planktonic Listeria isolates, triclosan was the only biocide that resulted in a significant biomass reduction. Differences between Listeria species were observed; L. monocytogenes and L. welshimeri biofilms were more tolerant to quaternary ammonium compound-based sanitizers than were L. innocua biofilms. These findings extend our understanding of the application of commonly used sanitizers in the food industry and the efficacy of these sanitizers against Listeria species and their associated biofilms.

Influence of incubation conditions on the formation of model biofilms by Listeria monocytogenes and Salmonella Typhimurium on abiotic surfaces

AIMS

This research aims to develop strongly adherent and mature model biofilms (on a 20 cm² polystyrene surface) for two pathogenic species, i.e. Listeria monocytogenes and Salmonella Typhimurium. These model biofilms can be used as standards to study biofilms or to study/compare the influence of different inactivation technologies.

METHODS AND RESULTS

Three influencing factors on the formation of biofilms are investigated, i.e. growth medium, incubation temperature and incubation time, which are three easily controllable environmental factors. Optical density measurement and plate counts were used to evaluate the adherence and the maturity of the biofilms, respectively. Confocal laser scanning microscopy was used to verify most important findings obtained with previously mentioned assays. Results indicated that mature and strongly adherent L. monocytogenes biofilms are obtained following 13 h of incubation at 30°C with BHI as growth medium. For S. Typhimurium, an incubation period of 19 h at 25°C was required with 20-fold diluted TSB as growth medium.

CONCLUSIONS

Based on previously mentioned assays, a protocol for the formation of reproducible model biofilms was obtained.

SIGNIFICANCE AND IMPACT OF THE STUDY

The developed model biofilms can be applied as a standard to study biofilms (in different research fields) and their subsequent inactivation by different methods. In addition, the results of this study could be used to control biofilm formation (e.g. by setting a maximum allowed surface temperature).

Application of calcium oxide (CaO, heated scallop-shell powder) for the reduction of Listeria monocytogenes biofilms on eggshell surfaces

This study investigated bactericidal activity of 0.05 to 0.50% calcium oxide (CaO) against planktonic cells in tryptic soy broth (TSB) and biofilms of Listeria monocytogenes on eggshell surfaces. The bactericidal activity of CaO against planktonic cells and biofilms of L. monocytogens significantly (P < 0.05) increased log reductions with increasing concentrations of CaO. Exposure to 0.05 to 0.50% CaO for one min reduced planktonic cells in TSB cell suspensions by 0.47 to 3.86 log10CFU/mL and biofilm cells on the shell surfaces by 0.14 to 2.32 log10CFU/cm2. The Hunter colors of eggshells ("L" for lightness, "a" for redness, and "b" for yellowness), shell thickness (puncture force), and sensory quality (egg taste and yolk color) were not changed by 0.05 to 0.50% CaO treatment. The nonlinear Weibull model was used to calculate CR = 3 values as the CaO concentration of 3 log (99.9%) reduction for planktonic cells (R2 = 0.96, RMSE = 0.26) and biofilms (R2 = 0.95, RMSE = 0.18) of L. monocytogens. The CR = 3 value, 0.31% CaO for planktonic cells, was significantly (P < 0.05) lower than 0.57% CaO for biofilms. CaO could be an alternative disinfectant to reduce planktonic cells and biofilms L. monocytogenes on eggshell surface in egg processing plants.

Ability of Shiga toxigenic Escherichia coli to survive within dry-surface biofilms and transfer to fresh lettuce

Biofilms are known to play important roles in bacterial survival and persistence in food-processing environments. This study aimed to determine the ability of the top 7 STEC serotypes to form biofilms on polystyrene (POL) and stainless steel (SS) plates and to quantify their survival and transfer from dry-surface biofilms to lettuce pieces. The ability of 14 STEC strains to form biofilms on these two materials at different exposure times and temperatures was assessed using crystal violet, Congo red and SEM. At 10 °C all serotypes were weak biofilm producers on both surfaces. In contrast, serotypes O45-040, O45-445, O103-102, O103-670 and O157-R508 were strong biofilm producers at 25 °C. Strains O103-102, O103-670, O111-CFS, O111-053 and O157:H7-R508 were expressers of curli. Under scanning electron microscopy, strains O103-670, O111-CFS, O157-R508, and O121-083 formed more discernible multilayer, mature biofilms on SS coupons. Regardless of the surface (POL/SS), all STEC strains were able to transfer viable cells onto fresh lettuce within a short contact time (2 min) to varying degrees (up to 6.35 log cfu/g). On POL, viable cell of almost all serotypes exhibited decreased detachment (p = 0.001) over 6 days; while after 30 days on SS, serotypes O45-040, O103-102, O103-670, O111-053, O111-CFS, O121-083, O145-231 O157:H7-R508 and O157:H7-122 were transferred to lettuce. After enrichment, all 14 STEC strains were recovered from dry-surface biofilms on POL and SS plates after 30 days. Results demonstrated that the top 7 STEC remained viable within dry-surface biofilms for at least 30 days, transferring to lettuce within 2 min of exposure and acting as a source of adulteration.

Growth and Biofilm Formation by Listeria monocytogenes in Catfish Mucus Extract on Four Food Contact Surfaces at 22 and 10°C and Their Reduction by Commercial Disinfectants

The objective of this study was to determine the effect of strain and temperature on growth and biofilm formation by Listeria monocytogenes in high and low concentrations of catfish mucus extract on various food contact surfaces at 10 and 22°C. The second objective of this study was to evaluate the efficacy of disinfectants at recommended concentrations and contact times for removing L. monocytogenes biofilm cells from a stainless steel surface covered with catfish mucus extract. Growth and biofilm formation of all L. monocytogenes strains increased with higher concentrations of catfish mucus extract at both 10 and 22°C. When 15 μg/mL catfish mucus extract was added to 3 log CFU/mL L. monocytogenes, the biofilm levels of L. monocytogenes on stainless steel reached 4 to 5 log CFU per coupon at 10°C and 5 to 6 log CFU per coupon at 22°C in 7 days. With 375 μg/mL catfish mucus extract, the biofilm levels of L. monocytogenes on stainless steel reached 5 to 6 log CFU per coupon at 10°C and 6 to 7.5 log CFU per coupon at 22°C in 7 days. No differences ( P > 0.05) were observed between L. monocytogenes strains tested for biofilm formation in catfish mucus extract on the stainless steel surface. The biofilm formation by L. monocytogenes catfish isolate HCC23 was lower on Buna-N rubber than on stainless steel, polyethylene, and polyurethane surfaces in the presence of catfish mucus extract ( P < 0.05). Contact angle analysis and atomic force microscopy confirmed that Buna-N rubber was highly hydrophobic, with lower surface energy and less roughness than the other three surfaces. The complete reduction of L. monocytogenes biofilm cells was achieved on the stainless steel coupons with a mixture of disinfectants, such as quaternary ammonium compounds with hydrogen peroxide or peracetic acid with hydrogen peroxide and octanoic acid at 25 or 50% of the recommended concentration, in 1 or 3 min compared with use of the quaternary ammonium compounds, chlorine, or acid disinfectants alone, which were ineffective for removing all the L. monocytogenes biofilm cells.

Dynamics of biofilm formation by Listeria monocytogenes on stainless steel under mono-species and mixed-culture simulated fish processing conditions and chemical disinfection challenges

The progressive ability of a six-strains L. monocytogenes cocktail to form biofilm on stainless steel (SS), under fish-processing simulated conditions, was investigated, together with the biocide tolerance of the developed sessile communities. To do this, the pathogenic bacteria were left to form biofilms on SS coupons incubated at 15°C, for up to 240h, in periodically renewable model fish juice substrate, prepared by aquatic extraction of sea bream flesh, under both mono-species and mixed-culture conditions. In the latter case, L. monocytogenes cells were left to produce biofilms together with either a five-strains cocktail of four Pseudomonas species (fragi, savastanoi, putida and fluorescens), or whole fish indigenous microflora. The biofilm populations of L. monocytogenes, Pseudomonas spp., Enterobacteriaceae, H₂S producing and aerobic plate count (APC) bacteria, both before and after disinfection, were enumerated by selective agar plating, following their removal from surfaces through bead vortexing. Scanning electron microscopy was also applied to monitor biofilm formation dynamics and anti-biofilm biocidal actions. Results revealed the clear dominance of Pseudomonas spp. bacteria in all the mixed-culture sessile communities throughout the whole incubation period, with the in parallel sole presence of L. monocytogenes cells to further increase (ca. 10-fold) their sessile growth. With respect to L. monocytogenes and under mono-species conditions, its maximum biofilm population (ca. 6logCFU/cm²) was reached at 192h of incubation, whereas when solely Pseudomonas spp. cells were also present, its biofilm formation was either slightly hindered or favored, depending on the incubation day. However, when all the fish indigenous microflora was present, biofilm formation by the pathogen was greatly hampered and never exceeded 3logCFU/cm², while under the same conditions, APC biofilm counts had already surpassed 7logCFU/cm² by the end of the first 96h of incubation. All here tested disinfection treatments, composed of two common food industry biocides gradually applied for 15 to 30min, were insufficient against L. monocytogenes mono-species biofilm communities, with the resistance of the latter to significantly increase from the 3rd to 7th day of incubation. However, all these treatments resulted in no detectable L. monocytogenes cells upon their application against the mixed-culture sessile communities also containing the fish indigenous microflora, something probably associated with the low attached population level of these pathogenic cells before disinfection (<10²CFU/cm²) under such mixed-culture conditions. Taken together, all these results expand our knowledge on both the population dynamics and resistance of L. monocytogenes biofilm cells under conditions resembling those encountered within the seafood industry and should be considered upon designing and applying effective anti-biofilm strategies.

Increased Adhesion of Listeria monocytogenes Strains to Abiotic Surfaces under Cold Stress

Food contamination by Listeria monocytogenes remains a major concern for some food processing chains, particularly for ready-to-eat foods, including processed foods. Bacterial adhesion on both biotic and abiotic surfaces is a source of contamination by pathogens that have become more tolerant or even persistent in food processing environments, including in the presence of adverse conditions such as cold and dehydration. The most distinct challenge that bacteria confront upon entry into food processing environments is the sudden downshift in temperature, and the resulting phenotypic effects are of interest. Crystal violet staining and the BioFilm Ring Test® were applied to assess the adhesion and biofilm formation of 22 listerial strains from different serogroups and origins under cold-stressed and cold-adapted conditions. The physicochemical properties of the bacterial surface were studied using the microbial adhesion to solvent technique. Scanning electron microscopy was performed to visualize cell morphology and biofilm structure. The results showed that adhesion to stainless-steel and polystyrene was increased by cold stress, whereas cold-adapted cells remained primarily in planktonic form. Bacterial cell surfaces exhibited electron-donating properties regardless of incubation temperature and became more hydrophilic as temperature decreased from 37 to 4°C. Moreover, the adhesion of cells grown at 4°C correlated with affinity for ethyl acetate, indicating the role of cell surface properties in adhesion.

Listeria monocytogenes in Food-Processing Facilities, Food Contamination, and Human Listeriosis: The Brazilian Scenario

Listeria monocytogenes is a foodborne pathogen that contaminates food-processing environments and persists within biofilms on equipment, utensils, floors, and drains, ultimately reaching final products by cross-contamination. This pathogen grows even under high salt conditions or refrigeration temperatures, remaining viable in various food products until the end of their shelf life. While the estimated incidence of listeriosis is lower than other enteric illnesses, infections caused by L. monocytogenes are more likely to lead to hospitalizations and fatalities. Despite the description of L. monocytogenes occurrence in Brazilian food-processing facilities and foods, there is a lack of consistent data regarding listeriosis cases and outbreaks directly associated with food consumption. Listeriosis requires rapid treatment with antibiotics and most drugs suitable for Gram-positive bacteria are effective against L. monocytogenes. Only a minority of clinical antibiotic-resistant L. monocytogenes strains have been described so far; whereas many strains recovered from food-processing facilities and foods exhibited resistance to antimicrobials not suitable against listeriosis. L. monocytogenes control in food industries is a challenge, demanding proper cleaning and application of sanitization procedures to eliminate this foodborne pathogen from the food-processing environment and ensure food safety. This review focuses on presenting the L. monocytogenes distribution in food-processing environment, food contamination, and control in the food industry, as well as the consequences of listeriosis to human health, providing a comparison of the current Brazilian situation with the international scenario.

Variations in biofilm formation, desiccation resistance and Benzalkonium chloride susceptibility among Listeria monocytogenes strains isolated in Canada

Listeria monocytogenes is a pathogenic foodborne microorganism noted for its ability to survive in the environment and food processing facilities. Survival may be related to the phenotype of individual strains including the ability to form biofilms and resist desiccation and/or sanitizer exposure. The objectives of this research were to compare 14 L. monocytogenes strains isolated from blood (3), food (6) and water (5) with respect to their benzalkonium chloride (BAC) sensitivity, desiccation resistance, and ability to form biofilm. Correlations were tested between those responses, and the presence of the SSI-1 (Stress Survival Islet) and LGI1/CC8 (Listeria Genomic Island 1 in a clonal complex 8 background) genetic markers. Genetic sequences from four strains representing different phenotypes were also probed for predicted amino acid differences in biofilm, desiccation, and membrane related genes. The water isolates were among the most desiccation susceptible strains, while strains exhibiting desiccation resistance harboured SSI-1 or both the SSI-1 and LGI1/CC8 markers. BAC resistance was greatest in planktonic LGI1/CC8 cells (relative to non-LGI1/CC8 cells), and higher BAC concentrations were also needed to inhibit the formation of biofilm by LGI1/CC8 strains during incubation for 48h and 6days compared to other strains. Formation of biofilm on stainless steel was not significantly (p>0.05) different among the strains. Analysis of genetic sequence data from desiccation and BAC sensitive (CP4 5-1, CP5 2-3, both from water), intermediate (Lm568, food) and desiccation and BAC resistant (08 5578, blood, human outbreak) strains led to the finding of amino acid differences in predicted functional protein domains in several biofilm, desiccation and peptidoglycan related genes (e.g., lmo0263, lmo0433, lmo0434, lmo0771, lmo0973, lmo1080, lmo1224, lmo1370, lmo1744, and lmo2558). Notably, the LGI1/CC8 strain 08-5578 had a frameshift mutation in lmo1370, a gene previously associated with desiccation resistance. In conclusion, the more desiccation and BAC resistant LGI1/CC8 isolates may pose a challenge for sanitation efforts.

Persistent Listeria monocytogenes strains isolated from mussel production facilities form more biofilm but are not linked to specific genetic markers

Contamination of mussels with the human pathogen Listeria monocytogenes occurs during processing in the factory, possibly from bacteria persisting in the factory's indoor and outdoor areas. In this study, a selection of persistent (n=8) and sporadic (n=8) L. monocytogenes isolates associated with mussel-processing premises in New Zealand were investigated for their phenotypic and genomic characteristics. To identify traits that favour or contribute to bacterial persistence, biofilm formation, heat resistance, motility and recovery from dry surfaces were compared between persistent and sporadic isolates. All isolates exhibited low biofilm formation at 20°C, however, at 30°C persistent isolates showed significantly higher biofilm formation after 48h using cell enumeration and near significant difference using the crystal violet assay. All 16 isolates were motile at 20°C and 30°C and motility was fractionally higher for sporadic isolates, but no significant difference was observed. We found persistent isolates tend to exhibit greater recovery after incubation on dry surfaces compared to sporadic isolates. Two of the three most heat-resistant isolates were persistent, while four of five isolates lacking heat resistance were sporadic isolates. Comparison of genome sequences of persistent and sporadic isolates showed that there was no overall clustering of persistent or sporadic isolates, and that differences in prophages and plasmids were not associated with persistence. Our results suggest a link between persistence and biofilm formation, which is most likely multifactorial, combining subtle phenotypic and genotypic differences between isolates.

Biofilm-producing ability of Listeria monocytogenes isolates from Brazilian cheese processing plants

The persistence of Listeria monocytogenes in food industry environments has been associated to the ability of specific isolates to produce biofilms. This study aimed to evaluate the biofilm production of 85 L. monocytogenes strains previously isolated from samples of cheese, brine and the environment of two cheese processing plants located in São Paulo, Brazil. The L. monocytogenes isolates belonged to serotypes 4b, 1/2b and 1/2c, yielded 30 different pulsotypes by pulsed-field gel electrophoresis (PFGE), and were submitted to biofilm-formation assays on polystyrene microplates and stainless steel coupons incubated statically at 35±0.5°C for 48h. All isolates from different sources showed ability to produce biofilms on polystyrene microplates, from which 21 (24.7%) also produced biofilms on stainless steel. Four isolates (4.7%) belonging to four different pulsotypes were classified as strong biofilms-producers on polystyrene microplates, while isolates belonging to four pulsotypes previously evaluated as persistent had weak or moderate ability to produce biofilms on polystyrene microplates. No relationship between the serotypes or pulsotypes and their biofilm-forming ability was observed. This study highlights the high variability in the biofilm production among L. monocytogenes strains collected from cheese and cheese-production environment, also indicating that strong biofilm-formation ability is not a key factor for persistence of specific isolates in cheese processing plants.

Variability of Listeria monocytogenes strains in biofilm formation on stainless steel and polystyrene materials and resistance to peracetic acid and quaternary ammonium compounds

Listeria monocytogenes is a foodborne pathogen able to tolerate adverse conditions by forming biofilms or by deploying stress resistant mechanisms, and thus manages to survive for long periods in food processing plants. This study sought to investigate the correlation between biofilm forming ability, tolerance to disinfectants and cell surface characteristics of twelve L. monocytogenes strains. The following attributes were evaluated: (i) biofilm formation by crystal violet staining method on polystyrene, and by standard cell enumeration on stainless steel and polystyrene; (ii) hydrophobicity assay using solvents; (iii) minimum inhibitory concentration (MIC) and biofilm eradication concentration (BEC) of peracetic acid (PAA) and quaternary ammonium compounds (QACs), and (iv) resistance to sanitizers (PAA 2000ppm; QACs 500ppm) of biofilms on polystyrene and stainless steel. After 72h of incubation, higher biofilm levels were formed in TSB at 20°C, followed by TSB at 37°C (P=0.087) and diluted TSB 1/10 at both 20 (P=0.005) and 37°C (P=0.004). Cells grown at 30°C to the stationary phase had significant electron donating nature and a low hydrophobicity, while no significant correlation of cell surface properties to biofilm formation was observed. Strains differed in MIC_PAA and BEC_PAA by 24- and 15-fold, respectively, while a positive correlation between MIC_PAA and BEC_PAA was observed (P=0.02). The MIC_QACs was positively correlated with the biofilm-forming ability on stainless steel (P=0.03). Regarding the impact of surface type, higher biofilm populations were enumerated on polystyrene than on stainless steel, which were also more tolerant to disinfectants. Among all strains, the greatest biofilm producer was a persistent strain with significant tolerance to QACs. These results may contribute to better understanding of L. monocytogenes behavior and survival on food processing surfaces.

Biofilm formation and cell surface properties of Staphylococcus aureus isolates from various sources

This study investigated biofilm formation, cell surface hydrophobicity, colony spreading, and slime production for 112 Staphylococcus aureus strains isolated from various sources (leaf vegetables, pea leaf, perilla leaf, Kim-bab, person, and animal). When biofilm formation was classified by origin, S. aureus isolated from animal origin showed a significantly higher level of biofilm formation than others (p≤0.05). When S. aureus groups with different levels of biofilm formation (very strong, strong, moderate, and weak) were evaluated for the correlation with cell surface properties, there was a positive correlation between biofilm formation and hydrophobicity (r=0.926). Biofilm formation and colony spreading on tryptic soy broth (without dextrose) also showed positive correlation (r=0.863). In contrast, biofilm formation and slime production were negatively correlated (r=-0.973). Based on these results, the biofilm forming ability of S. aureus differs depending on their origin and might be affected by cell surface properties such as cell surface hydrophobicity.

Molecular analysis of dominant species in Listeria monocytogenes-positive biofilms in the drains of food processing facilities

Listeria monocytogenes exhibits symbiotic codependence with the dominant commensal bacteria, which may help it avoid being removed or inactivated by disinfectants in local environments. In this study, we investigated L. monocytogenes-positive biofilms at food production facilities, and the dominant bacterial species of the biofilms were identified to determine the properties of the microbiological background. For this purpose, the ISO 11290 method was used for the detection and isolation of L. monocytogenes, and the species were further identified based on 16S rRNA and hly genes. 16S rRNA gene-based cloning, terminal restriction fragment length polymorphism, and denaturing gradient gel electrophoresis were combined to evaluate the dominant bacteria of the drain biofilms. Out of 100 drain samples, 8 were naturally contaminated with L. monocytogenes. Three molecular methods consistently showed that Pseudomonas psychrophila, Pseudomonas sp., and Klebsiella oxytoca were dominant species in 3Q, 5Q, and 6Q samples; Aeromonas hydrophila and Klebsiella sp. were significantly dominant in 1-2, 1-3, and 3-2 samples; A. hydrophila and K. oxytoca were dominant in the 2-3 sample; and A. hydrophila and Pseudomonas sp. were prominent in the 3-3 sample. Different biofilms from the same plant shared common bands, suggesting that similar bacteria can be found and can be dominant in different biofilms. This study provides a better understanding of the dominant compositions in these bacterial communities. Further studies to determine the mechanism of co-culture with L. monocytogenes will be of critical importance in predicting effective disinfection strategies.

Biofilm formation of the L. monocytogenes strain 15G01 is influenced by changes in environmental conditions

Listeria monocytogenes 15G01, a strain belonging to the persistent pulsotype 5132, was isolated from a seafood processing plant in New Zealand. Simple monoculture assays using crystal violet staining showed good biofilm formation for this strain and it was therefore chosen to be further investigated in regard to its biofilm forming ability. To evaluate its behaviour in different conditions commonly encountered in food processing environments, biofilm assays and growth studies were performed using common laboratory media under a range of temperatures (20 °C, 30 °C and 37 °C). Furthermore, the effects of incubation time and different environmental conditions including static, dynamic and anaerobic incubation on biofilm formation were investigated. Changes in the environmental conditions resulted in different biofilm phenotypes of L. monocytogenes 15G01. We demonstrated that increasing temperature and incubation time led to a higher biofilm mass and that dynamic incubation has little effect on biofilm formation at 37 °C but encourages biofilm formation at 30 °C. Biofilm production at 20 °C was minimal regardless of the medium used. We furthermore observed that anaerobic environment led to reduced biofilm mass at 30 °C for all tested media but not at 37 °C. Biofilm formation could not be narrowed down to one factor but was rather dependent on multiple factors with temperature and medium having the biggest effects.

Inactivation of Escherichia coli O157:H7 and Listeria monocytogenes in biofilms by pulsed ultraviolet light

BACKGROUND

The inactivation of biofilms formed by pathogenic bacteria on ready-to-eat and minimally processed fruits and vegetables by nonthermal processing methods is critical to ensure food safety. Pulsed ultraviolet (PUV) light has shown promise in the surface decontamination of liquid, powdered, and solid foods. In this study, the antimicrobial efficacy of PUV light treatment on nascent biofilms formed by Escherichia coli O157:H7 and Listeria monocytogenes on the surfaces of food packaging materials, such as low-density polyethylene (LDPE), and fresh produce, such as lettuce (Lactuca sativa) leaves, was investigated.

RESULTS

The formation of biofilms on Romaine lettuce leaves and LDPE films was confirmed by crystal violet and Alcian blue staining methods. Inactivation of cells in the biofilm was determined by standard plating procedures, and by a luminescence-based bacterial cell viability assay. Upon PUV treatment of 10 s at two different light source to sample distances (4.5 and 8.8 cm), viable cell counts of L. monocytogenes and E. coli O157:H7 in biofilms on the lettuce surface were reduced by 0.6-2.2 log CFU mL(-1) and 1.1-3.8 log CFU mL(-1), respectively. On the LDPE surface, the efficiency of inactivation of biofilm-encased cells was slightly higher. The maximum values for microbial reduction on LDPE were 2.7 log CFU mL(-1) and 3.9 log CFU mL(-1) for L. monocytogenes and E. coli O157:H7, respectively. Increasing the duration of PUV light exposure resulted in a significant (P < 0.05) reduction in biofilm formation by both organisms. The results also revealed that PUV treatment was more effective at reducing E. coli biofilms compared with Listeria biofilms. A moderate increase in temperature (~7-15°C) was observed for both test materials.

CONCLUSIONS

PUV is an effective nonthermal intervention method for surface decontamination of E. coli O157:H7 and L. monocytogenes on fresh produce and packaging materials.

Effectiveness of sanitizing products on controlling selected pathogen surrogates on retail deli slicers

The objectives of this study were (i) to assess the efficacy of quaternary ammonium chloride-based wet foam (WF) and dry foam (DF) sanitizer systems (600 ppm) for reducing Listeria innocua (a nonpathogenic surrogate of Listeria monocytogenes) or a 100.0 μg/ml rifampin-resistant Salmonella Typhimurium LT2 (a nonpathogenic surrogate of Salmonella enterica serovar Typhimurium) on niche and transfer point areas of an unwashed retail deli slicer as compared with traditional chlorine (Cl(-)) treatment (200 ppm) and (ii) to compare sanitizer surface contact times (10 and 15 min) for pathogen surrogate control. Turkey frankfurter slurries inoculated with L. innocua or Salmonella Typhimurium were used to inoculate seven high-risk sites on a commercial slicer. After 30 min of bacterial attachment, slicers were dry wiped to remove excess food matter, followed by a randomly assigned sanitizer treatment. Surviving pathogen surrogate cells were enumerated on modified Oxford's agar not containing antimicrobic supplement (L. innocua) or on tryptic soy agar supplemented with 100 μg/ml rifampin (Salmonella Typhimurium LT2). Replicate-specific L. innocua and Salmonella Typhimurium reductions were calculated as log CFU per square centimeter of control minus log CFU per square centimeter of enumerated survivors for each site. For both organisms, all sanitizer treatments differed from each other, with Cl(-) producing the least reduction and WF the greatest reduction. A significant (P < 0.05) site-by-treatment interaction was observed. The results of the study indicate that quaternary ammonium chloride sanitizers (600 ppm) applied by both WF and DF were more effective at reducing L. innocua and Salmonella Typhimurium than a traditional Cl sanitizer (200 ppm) on unwashed slicer surfaces.

Exploring the diversity of Listeria monocytogenes biofilm architecture by high-throughput confocal laser scanning microscopy and the predominance of the honeycomb-like morphotype

Listeria monocytogenes is involved in food-borne illness with a high mortality rate. The persistence of the pathogen along the food chain can be associated with its ability to form biofilms on inert surfaces. While most of the phenotypes associated with biofilms are related to their spatial organization, most published data comparing biofilm formation by L. monocytogenes isolates are based on the quantitative crystal violet assay, which does not give access to structural information. Using a high-throughput confocal-imaging approach, the aim of this work was to decipher the structural diversity of biofilms formed by 96 L. monocytogenes strains isolated from various environments. Prior to large-scale analysis, an experimental design was created to improve L. monocytogenes biofilm formation in microscopic-grade microplates, with special emphasis on the growth medium composition. Microscopic analysis of biofilms formed under the selected conditions by the 96 isolates revealed only weak correlation between the genetic lineages of the isolates and the structural properties of the biofilms. However, a gradient in their geometric descriptors (biovolume, mean thickness, and roughness), ranging from flat multilayers to complex honeycomb-like structures, was shown. The dominant honeycomb-like morphotype was characterized by hollow voids hosting free-swimming cells and localized pockets containing mixtures of dead cells and extracellular DNA (eDNA).

Attached and planktonic Listeria monocytogenes global proteomic responses and associated influence of strain genetics and temperature

Contamination of industrial and domestic food usage environments by the attachement of bacterial food-borne pathogen Listeria monocytogenes has public health and economic implications. Comprehensive proteomics experiments using label-free liquid chromatography/tandem mass spectrometry were used to compare the proteomes of two different L. monocytogenes strains (Siliken_1/2c and F2365_4b), which show very different capacities to attach to surfaces. Growth temperature and strain type were highly influential on the proteomes in both attached and planktonic cells. On the basis of the proteomic data, it is highly unlikely that specific surface proteins play a direct role in adherence to inanimate surfaces. Instead, strain-dependent responses related to cell envelope polymer biosynthesis and stress response regulation likely contribute to a different ability to attach and also to survive external stressors. Collectively, the divergent proteome-level responses observed define strain- and growth-temperature-dependent differences relevant to attachment efficacy, highlight relevant proteins involved in stress protection in attached cells, and suggest that strain differences and growth conditions are important in relation to environmental persistence.

Listeria monocytogenes isolated from food samples from a Romanian black market show distinct virulence profiles

Listeria monocytogenes is a facultative intracellular foodborne pathogen responsible for listeriosis. In a recent study, in which we investigated neglected exogenous routes of transmission of foodborne pathogens into the European Union, we have isolated 15 L. monocytogenes strains in food products, which were imported from the Republic of Moldavia to Romania and illegally sold at a local market. The aim of this study was to characterize the subtype and virulence potential of these 15 L. monocytogenes strains. Multilocus sequence typing revealed that these L. monocytogenes strains belong to six different sequence types (ST2, ST8, ST9, ST20, ST121 and ST155). In addition, in vitro virulence assays using human intestinal epithelial Caco2 and macrophage-like THP1 cells showed a high strain variability regarding the invasion efficiency in Caco2 cells (0.98-2.78%) and the intracellular growth rate in both cell types. Both ST121 strains and the ST9 isolate were unable to invade Caco2 cells, and all ST155 strains showed no proliferation inside macrophages and revealed low cytotoxicity. Furthermore we performed sequence analysis of three main virulence factors: PrfA, internalin A (InlA) and listeriolysin O (LLO). The Romanian food isolates showed a high diversity in the InlA and LLO amino acid sequences, whereas the amino acid sequence of PrfA of all strains was identical. Overall, the amino acid sequences of PrfA, InlA and LLO were identical for strains belonging to the same ST. We detected in total 30 different amino acid substitutions, resulting in seven different InlA variants, two of which have not yet been described. The three strains, which were unable to invade Caco2 cells, harboured a premature stop codon resulting in truncated InlA. Furthermore, we detected four different amino acid substitutions in the LLO sequence, which are present in four variants. The number of LLO mutations correlates negatively with intracellular growth in Caco2 and THP1 cells and subsequently with cytotoxicity. In conclusion, we show that L. monocytogenes isolated from food samples from a Romanian black market show distinct virulence profiles, due to a high diversity in the amino acid sequence of main virulence factors.

The influence of subminimal inhibitory concentrations of benzalkonium chloride on biofilm formation by Listeria monocytogenes

Disinfectants, such as benzalkonium chloride (BAC), are commonly used to control Listeria monocytogenes and other pathogens in food processing plants. Prior studies have demonstrated that the resistance to BAC of L. monocytogenes was associated with the prolonged survival of three strains of molecular serotype 1/2a in an Iberian pork processing plant. Because survival in such environments is related to biofilm formation, we hypothesised that the influence of BAC on the biofilm formation potential of L. monocytogenes might differ between BAC-resistant strains (BAC-R, MIC≥10mg/L) and BAC-sensitive strains (BAC-S, MIC≤2.5mg/L). To evaluate this possibility, three BAC-R strains and eight BAC-S strains, which represented all of the molecular serotype 1/2a strains detected in the sampled plant, were compared. Biofilm production was measured using the crystal violet staining method in 96-well microtitre plates. The BAC-R strains produced significantly (p<0.05) less biofilm than the BAC-S in the absence of BAC, independent of the rate of planktonic growth. In contrast, when the biofilm values were measured in the presence of BAC, one BAC-R strain (S10-1) was able to form biofilm at 5mg/L of BAC, which prevented biofilm formation among the rest of the strains. A genetic determinant of BAC resistance recently described in L. monocytogenes (Tn6188) was detected in S10-1. When a BAC-S strain and its spontaneous mutant BAC-R derivative were compared, resistance to BAC led to biofilm formation at 5mg/L of BAC and to a significant (p<0.05) stimulation of biofilm formation at 1.25mg/L of BAC, which significantly (p<0.05) reduced the biofilm level in the parent BAC-S strain. Our results suggest that the effect of subminimal inhibitory concentrations of BAC on biofilm production by L. monocytogenes might differ between strains with different MICs and even between resistant strains with similar MICs but different genetic determinants of BAC resistance. For BAC-R strains similar to S10-1, subminimal inhibitory BAC may represent an advantage, compensating for the weak biofilm formation level that might be associated with resistance. Biofilm formation in the presence of increased subminimal inhibitory concentrations of the disinfectant may represent an important attribute among certain resistant and persistent strains of L. monocytogenes.

L. monocytogenes in a cheese processing facility: Learning from contamination scenarios over three years of sampling

The aim of this study was to analyze the changing patterns of Listeria monocytogenes contamination in a cheese processing facility manufacturing a wide range of ready-to-eat products. Characterization of L. monocytogenes isolates included genotyping by pulsed-field gel electrophoresis (PFGE) and multi-locus sequence typing (MLST). Disinfectant-susceptibility tests and the assessment of L. monocytogenes survival in fresh cheese were also conducted. During the sampling period between 2010 and 2013, a total of 1284 environmental samples were investigated. Overall occurrence rates of Listeria spp. and L. monocytogenes were 21.9% and 19.5%, respectively. Identical L. monocytogenes genotypes were found in the food processing environment (FPE), raw materials and in products. Interventions after the sampling events changed contamination scenarios substantially. The high diversity of globally, widely distributed L. monocytogenes genotypes was reduced by identifying the major sources of contamination. Although susceptible to a broad range of disinfectants and cleaners, one dominant L. monocytogenes sequence type (ST) 5 could not be eradicated from drains and floors. Significantly, intense humidity and steam could be observed in all rooms and water residues were visible on floors due to increased cleaning strategies. This could explain the high L. monocytogenes contamination of the FPE (drains, shoes and floors) throughout the study (15.8%). The outcome of a challenge experiment in fresh cheese showed that L. monocytogenes could survive after 14days of storage at insufficient cooling temperatures (8 and 16°C). All efforts to reduce L. monocytogenes environmental contamination eventually led to a transition from dynamic to stable contamination scenarios. Consequently, implementation of systematic environmental monitoring via in-house systems should either aim for total avoidance of FPE colonization, or emphasize a first reduction of L. monocytogenes to sites where contamination of the processed product is unlikely. Drying of surfaces after cleaning is highly recommended to facilitate the L. monocytogenes eradication.

Unraveling microbial biofilms of importance for food microbiology

The presence of biofilms is a relevant risk factors in the food industry due to the potential contamination of food products with pathogenic and spoilage microorganisms. The majority of bacteria are able to adhere and to form biofilms, where they can persist and survive for days to weeks or even longer, depending on the microorganism and the environmental conditions. The biological cycle of biofilms includes several developmental phases such as: initial attachment, maturation, maintenance, and dispersal. Bacteria in biofilms are generally well protected against environmental stress, consequently, extremely difficult to eradicate and detect in food industry. In the present manuscript, some techniques and compounds used to control and to prevent the biofilm formation are presented and discussed. Moreover, a number of novel techniques have been recently employed to detect and evaluate bacteria attached to surfaces, including real-time polymerase chain reaction (PCR), DNA microarray and confocal laser scanning microscopy. Better knowledge on the architecture, physiology and molecular signaling in biofilms can contribute for preventing and controlling food-related spoilage and pathogenic bacteria. The present study highlights basic and applied concepts important for understanding the role of biofilms in bacterial survival, persistence and dissemination in food processing environments.

Biofilm formation and persistence on abiotic surfaces in the context of food and medical environments

The biofilm formation on abiotic surfaces in food and medical sectors constitutes a great public health concerns. In fact, biofilms present a persistent source for pathogens, such as Pseudomonas aeruginosa and Staphylococcus aureus, which lead to severe infections such as foodborne and nosocomial infections. Such biofilms are also a source of material deterioration and failure. The environmental conditions, commonly met in food and medical area, seem also to enhance the biofilm formation and their resistance to disinfectant agents. In this regard, this review highlights the effect of environmental conditions on bacterial adhesion and biofilm formation on abiotic surfaces in the context of food and medical environment. It also describes the current and emergent strategies used to study the biofilm formation and its eradication. The mechanisms of biofilm resistance to commercialized disinfectants are also discussed, since this phenomenon remains unclear to date.

Success and failure of colloidal approaches in adhesion of microorganisms to surfaces

Biofilms are communities of cells attached to surfaces, their contributions to biological process may be either a benefit or a threat depending on the microorganism involved and on the type of substrate and environment. Biofilm formation is a complex series of steps; due to the size of microorganisms, the initial phase of biofilm formation, the bacterial adhesion to the surface, has been studied and modeled using theories developed in colloidal science. In this review the application of approaches such as Derjaguin, Landau, Verwey, Overbeek (DLVO) theory and its extended version (xDLVO), to bacterial adhesion is described along with the suitability and applicability of such approaches to the investigation of the interface phenomena regulating cells adhesion. A further refinement of the xDLVO theory encompassing the brush model is also discussed. Finally, the evidences of phenomena neglected in colloidal approaches, such as surface heterogeneity and fluid flow, likely to be the source of failure are defined.