Competition of Listeria monocytogenes serotype 1/2a and 4b strains in mixed-culture biofilms

Ultra violet-C pretreatment enhances the antimicrobial efficacy of unpeeled carrots against subsequent contamination with Listeria monocytogenes

To our knowledge, this study is the first to elucidate the bactericidal efficacy of unpeeled carrots (hereafter referred to as carrots) pretreated with Ultra Violet-C (UV-C) against subsequent contamination with Listeria monocytogenes. Carrots pretreated with UV-C (240 mJ/cm2) exhibited a significant antilisterial effect within 2 h. In fact, the population of UV-C-pretreated carrots decreased from 7.94 log CFU/cm2 to levels below the limit of detection (LOD; <1.65 log CFU/cm2) within 24 h. For carrots that were not pretreated with UV-C, 3-4 log reductions were found after 24 h. Carrots pretreated with UV-C exhibited antimicrobial activity against another gram-positive pathogen, Staphylococcus aureus, but not against the gram-negative pathogens, E. coli O157:H7 and Salmonella enterica. Pretreatment with UV-C created a lasting antimicrobial effect as introducing L. monocytogenes on carrots, 72 h post-UV-C treatment, still maintained the antilisterial effect. Notably, all UV-C doses in the range of 48-240 mJ/cm2 induced a lasting antilisterial effect. The bactericidal effects against L. monocytogenes were confirmed in three varieties of washed and unwashed carrots (Danvers, Nantes, and Chantenay). Fluorescence microscopy confirmed the bactericidal effect of UV-C-pretreated carrots on the survival of L. monocytogenes. Conclusively, pretreating carrots with UV-C can reduce the population of L. monocytogenes to levels below the LOD and may further prevent pathogen growth during cold storage. Additional studies are necessary to discern the mechanism underlying the bactericidal efficacy of UV-C-pretreated carrots.

Surveillance and genetic characterization of Listeria monocytogenes in the food chain in Montenegro during the period 2014-2022

Introduction

Listeria monocytogenes is an ubiquitous foodborne pathogen that represents a serious threat to public health and the food industry.

Methods

In this study Whole Genome Sequencing (WGS) was used to characterize 160 L. monocytogenes isolates obtained from 22,593 different food sources in Montenegro during the years 2014-2022.

Results

Isolates belonged to 21 different clonal complexes (CCs), 22 sequence types (STs) and 73 core genome multilocus sequence types (cgMLST) revealing a high diversity. The most prevalent STs were ST8 (n = 29), ST9 (n = 31), ST121 (n = 19) and ST155 (n = 20). All isolates carried virulence genes (VGs), 111 isolates carried mobile genetic elements (MGEs) (ranging from 1 to 7 MGEs) and 101 isolates carried plasmids (ranging from 1 to 3 plasmids). All isolates carried the intrinsic resistance genes fosX and lin. None of the isolates carried acquired antimicrobial resistance genes (ARGs).

Discussion/conclusion

Continuous monitoring and surveillance of L. monocytogenes is needed for improving and ameliorating the public health.

Identification of Protein Biomarkers for Differentiating Listeria monocytogenes Genetic Lineage III

Listeria monocytogenes is the causative agent of listeriosis, a severe foodborne illness characterized by septicemia, meningitis, encephalitis, abortions, and occasional death in infants and immunocompromised individuals. L. monocytogenes is composed of four genetic lineages (I, II, III, and IV) and fourteen serotypes. The aim of the current study was to identify proteins that can serve as biomarkers for detection of genetic lineage III strains based on simple antibody-based methods. Liquid chromatography (LC) with electrospray ionization tandem mass spectrometry (ESI MS/MS) followed by bioinformatics and computational analysis were performed on three L. monocytogenes strains (NRRL B-33007, NRRL B-33014, and NRRL B-33077), which were used as reference strains for lineages I, II, and III, respectively. Results from ESI MS/MS revealed 42 unique proteins present in NRRL B-33077 and absent in NRRL B-33007 and NRRL B-33014 strains. BLAST analysis of the 42 proteins against a broader panel of >80 sequenced strains from lineages I and II revealed four proteins [TM2 domain-containing protein (NRRL B-33077_2770), DUF3916 domain-containing protein (NRRL B-33077_1897), DNA adenine methylase (NRRL B-33077_1926), and protein RhsA (NRRL B-33077_1129)] that have no homology with any sequenced strains in lineages I and II. The four genes that encode these proteins were expressed in Escherichia coli strain DE3 and purified. Polyclonal antibodies were prepared against purified recombinant proteins. ELISA using the polyclonal antibodies against 12 L. monocytogenes lineage I, II, and III isolates indicated that TM2 protein and DNA adenine methylase (Dam) detected all lineage III strains with no reaction to lineage I and II strains. In conclusion, two proteins including TM2 protein and Dam are potentially useful biomarkers for detection and differentiation of L. monocytogenes lineage III strains in clinical, environmental, and food processing facilities. Furthermore, these results validate the approach of using a combination of proteomics and bioinformatics to identify useful protein biomarkers.

Antibacterial and anti-biofilm activities of probiotic Lactobacillus plantarum against Listeria monocytogenes isolated from milk, chicken and pregnant women

Listeria monocytogenes (L. monocytogenes) is a foodborne pathogen that poses significant risks to public health and food safety. The present study aimed to identify the presence of Listeria spp. in various samples, including pasteurized milk, chicken fillets, and stool samples from pregnant women in Sharkia Governorate, Egypt. Additionally, the study identified the serotypes, virulence-associated genes, antimicrobial resistance patterns, and biofilm formation in L. monocytogenes isolates. Moreover, the antibacterial and anti-biofilm activity of Lactobacillus plantarum ATCC 14917 (L. plantarum) against L. monocytogenes isolates was investigated. A cross-sectional study was conducted from August 2021 to January 2022 to collect 300 samples of pasteurized milk, chicken fillets, and stool from pregnant women admitted to outpatient clinics of hospitals. The results showed that 32.7% of the samples were positive for Listeria spp., including L. innocua (48.9%), L. monocytogenes (26.5%), L. ivanovii (14.3%), L. grayi (5.1%), and L. welshimeri (5.1%). Among all L. monocytogenes isolates, hlyA, actA, inlC, and inlJ virulence-associated genes were detected. However, the virulence genes plcB, iap, and inlA were found in 10 (38.5%), 8 (30.8%), and 25 (96.2%) isolates, respectively. The L. monocytogenes isolates classified into four serotypes (1/2a, 1/2b, 1/2c, and 4b), with 1/2a and 4b each identified in 30.8% of the isolates, while 1/2b and 1/2c were identified in 19.2% of the isolates. All L. monocytogenes isolates showed 100% resistance to streptomycin, kanamycin, and nalidix acid, and 92.3% of isolates showed gentamicin resistance. However, all isolates were susceptible to ampicillin and ampicillin/sulbactam. Multidrug resistance (MDR) was observed in 20 (76.9%) L. monocytogenes isolates. The biofilm formation ability of 26 L. monocytogenes isolates was evaluated at different incubation temperatures. At 4°C, 25°C, and 37°C, 53.8, 69.2, and 80.8% of the isolates, respectively, were biofilm producers. Furthermore, 23.1% were strong biofilm producers at both 4°C and 25°C, while 34.6% were strong biofilm formers at 37°C. Treating L. monocytogenes isolates with L. plantarum cell-free supernatant (CFS) reduced the number of biofilm-producing isolates to 15.4, 42.3, and 53.8% at 4°C, 25°C, and 37°C, respectively. L. plantarum's CFS antibacterial activity was tested against six virulent, MDR, and biofilm-forming L. monocytogenes isolates. At a concentration of 5 μg/mL of L. plantarum CFS, none of the L. monocytogenes isolates exhibited an inhibition zone. However, an inhibition zone was observed against L. monocytogenes strains isolated from pasteurized milk and pregnant women's stools when using a concentration of 10 μg/mL. Transmission electron microscopy (TEM) revealed that L. plantarum CFS induced morphological and intracellular structural changes in L. monocytogenes. In conclusion, this study identified virulent MDR L. monocytogenes isolates with strong biofilm-forming abilities in food products in Egypt, posing significant risks to food safety. Monitoring the prevalence and antimicrobial resistance profile of L. monocytogenes in dairy and meat products is crucial to enhance their safety. Although L. plantarum CFS showed potential antibacterial and anti-biofilm effects against L. monocytogenes isolates, further research is needed to explore its full probiotic potential.

Survival and predictive modeling of Listeria monocytogenes under simulated human gastric conditions in the presence of bovine milk products

Listeria monocytogenes is an opportunistic foodborne pathogen which has been implicated in many outbreaks of foodborne diseases. This study evaluated the effects of gastric acidity and gastric digestion time of adults, L. monocytogenes strain and food type on the survival of L. monocytogenes under simulated stomach conditions of adults in in vitro gastric models with dynamic pH changes occurring throughout the exposure. Individual strains as well as a cocktail of L. monocytogenes, inoculated at 8 log CFU/mL in filtered bovine milk products, 0 % milk, 2 % milk, 2 % chocolate milk and 3.25 % milk, were introduced to the gastric models for 2 h. The survival of L. monocytogenes depended on a combination of factors, including gastric acidity and gastric digestion time of adults, L. monocytogenes strain, food type and recovery method (P < 0.05). The survival rates of L. monocytogenes inoculated in 2 % milk after a 2-h exposure to simulated gastric fluids with pH values of 1.5, 2.0 and 3.0 were 0.003 to 0.040 %, 22.7 to 43.4 % and 16.6 to 27.2 %, respectively. Fluid milk containing a higher milk fat content (3.25 % vs 0 % milk) protected L. monocytogenes from being inactivated when they were exposed to the human stomach model with a gastric acidity of pH 2.0. Compared to 0 % and 3.25 % milk, L. monocytogenes survived the best in 2 % chocolate milk, which appears to be due to the presence of milk fat (2 %) and the additional nutrients that are present in chocolate milk. A predictive mathematical model was developed that captured the population of the strains of L. monocytogenes under the in vitro conditions. This study advances our understanding of the behaviour of L. monocytogenes under various human gastric conditions and provides key parameters that can affect the survival of L. monocytogenes in the stomachs of adults. The mathematical models developed in this study can be used as a supplementary tool to help predict the survival of L. monocytogenes under similar scenarios and for relevant risk-assessment studies.

Exploring the Diversity of Biofilm Formation by the Food Spoiler Brochothrix thermosphacta

Brochothrix thermosphacta is considered as a major spoiler of meat and seafood products. This study explores the biofilm formation ability and the biofilm structural diversity of 30 multi-origin B. thermosphacta strains using a set of complementary biofilm assays (biofilm ring test, crystal violet staining, and confocal laser scanning microscopy). Two major groups corresponding to low and high biofilm producers were identified. High biofilm producers presented flat architectures characterized by high surface coverage, high cell biovolume, and high surface area.

Photoactivated Carbon Dots for Inactivation of Foodborne Pathogens Listeria and Salmonella

Foodborne pathogens have long been recognized as major challenges for the food industry and repeatedly implicated in food product recalls and outbreaks of foodborne diseases. This study demonstrated the application of a recently discovered class of visible-light-activated carbon-based nanoparticles, namely, carbon dots (CDots), for photodynamic inactivation of foodborne pathogens. The results demonstrated that CDots were highly effective in the photoinactivation of Listeria monocytogenes in suspensions and on stainless steel surfaces. However, it was much less effective for Salmonella cells, but treatments with higher CDot concentrations and longer times were still able to inactivate Salmonella cells. The mechanistic implications of the observed different antibacterial effects on the two types of cells were assessed, and the associated generation of intracellular reactive oxygen species (ROS), the resulting lipid peroxidation, and the leakage of nucleic acid and proteins from the treated cells were analyzed, with the results collectively suggesting CDots as a class of promising photodynamic inactivation agents for foodborne pathogens. IMPORTANCE Foodborne infectious diseases have long been recognized as major challenges in public health. Contaminations of food processing facilities and equipment with foodborne pathogens occur often. There is a critical need for new tools/approaches to control the pathogens and prevent such contaminations in food processing facilities and other settings. This study reports a newly established antimicrobial nanomaterials platform, CDots coupled with visible/natural light, for effective and efficient inactivation of representative foodborne bacterial pathogens. The study will contribute to promoting the practical application of CDots as a new class of promising nanomaterial-based photodynamic inactivation agents for foodborne pathogens.

Distribution and phenotypic and genotypic characterization of Listeria monocytogenes isolated from food, Colombia, 2010-2018

Introduction: Listeria monocytogenes is a food-borne pathogen that may cause infections in humans such as meningitis, meningoencephalitis, and septicemia, as well as abortions. By serological typing 13 serotypes have been identified of which 4b is responsible for most of the outbreaks in the world. Objective: To determine the frequency and distribution of serotypes and molecular subtypes of L. monocytogenes isolated in Colombia from food from 2010 to 2018. Materials and methods: We conducted a retrospective and descriptive study based on the analysis of 2,420 isolates confirmed as L. monocytogenes and other species using biochemical and serological tests, and pulsed-field gel electrophoresis (PFGE) for molecular subtyping. Results: Of the 2,420 isolates received, 2,326 were confirmed as L. monocytogenes. The serotypes found were 4b (52%), 4d-4e (14.5%), 1/2a (11%), 1/2c (9.4%), 1/2b (9%), and 3a, 3b, 3c, 4c, 4d, 4e and 7 (less than 2%). The isolates came from Bogotá (43%), Antioquia (25%), Valle (10%), Nariño (9%), and other departments (7%). The genotypic characterization grouped the isolates in 167 PFGE patterns. The most frequent patterns were identified in various dairy and meat products, and in prepared foods. Conclusion: A 96.1% of the isolates corresponded to L. monocytogenes showing good agreement between isolates and identification. Serotype 4b, highly virulent, was the most frequent. The molecular analysis showed the possible dissemination and permanence over time of several serotypes, which highlights the importance of including this pathogen in epidemiological food surveillance programs.

Conditions of In Vitro Biofilm Formation by Serogroups of Listeria monocytogenes Isolated from Hass Avocados Sold at Markets in Mexico

Listeria monocytogenes is an important pathogen that has been implicated in foodborne illnesses and the recall of products such as fruit and vegetables. This study determines the prevalence of virulence-associated genes and serogroups and evaluates the effects of different growth media and environmental conditions on biofilm formation by L. monocytogenes. Eighteen L. monocytogenes isolates from Hass avocados sold at markets in Guadalajara, Mexico, were characterized by virulence-associated genes and serogroup detection with PCR. All isolates harbored 88.8% actA, 88.8% plcA, 83.3% mpl, 77.7% inlB, 77.7% hly, 66.6% prfA, 55.5% plcB, and 33.3% inlA. The results showed that 38.8% of isolates harbored virulence genes belonging to Listeria pathogenicity island 1 (LIPI-1). PCR revealed that the most prevalent serogroup was serogroup III (1/2b, 3b, and 7 (n = 18, 66.65%)), followed by serogroup IV (4b, 4d-4e (n = 5, 27.7%)) and serogroup I (1/2a-3a (n = 1, 5.5%)). The assessment of the ability to develop biofilms using a crystal violet staining method revealed that L. monocytogenes responded to supplement medium TSBA, 1/10 diluted TSBA, and TSB in comparison with 1/10 diluted TSB (p < 0.05) on polystyrene at 240 h (p < 0.05). In particular, the biofilm formation by L. monocytogenes (7.78 ± 0.03-8.82 ± 0.03 log₁₀ CFU/cm²) was significantly different in terms of TSBA on polypropylene type B (PP) (p < 0.05). In addition, visualization by epifluorescence microscopy, scanning electron microscopy (SEM), and treatment (DNase I and proteinase K) revealed the metabolically active cells and extracellular polymeric substances of biofilms on PP. L. monocytogenes has the ability to develop biofilms that harbor virulence-associated genes, which represent a serious threat to human health and food safety.

Growth and Survival of Attached Listeria on Lettuce and Stainless Steel Varies by Strain and Surface Type

ABSTRACT

The foodborne pathogen Listeria monocytogenes lives as a saprophyte in nature and can adhere to and grow on surfaces as diverse as leaves, sediment, and stainless steel. To discern the mechanisms used by L. monocytogenes for attachment and growth on various surfaces, we studied interactions between the pathogen on lettuce and stainless steel. A panel of 24 strains (23 L. monocytogenes and 1 Listeria innocua) were screened for attachment and growth on lettuce at 4 and 25°C and on stainless steel at 10 and 37°C. Overnight growth of attached cells resulted in a 0- to 3-log increase on lettuce, depending on the strain and the temperature. Among the worst-performing strains on lettuce were two from a large cantaloupe outbreak, indicating that factors important for interactions with cantaloupe may be different from those required on lettuce tissue. Strains that grew the best on lettuce belonged to serotypes 1/2a, 1/2b, and 4b and were from cheese, potatoes, and water-sediment near produce fields. Confocal microscopy of L. monocytogenes tagged with constitutively expressed green fluorescent protein indicated associations with the cut edges and veins of lettuce leaves. On stainless steel coupons, there was a 5- to 7-log increase at 10°C after 7 days and a 4- to 7-log increase at 37°C after 40 h. Statistically, surface growth on stainless steel was better for serotype 1/2a than for serotype 4b strains, even though certain serotype 4b strains grew well on the coupons. The latter included strains that originated from produce and water-sediment. Some strains were fit in both environments, whereas others showed variability between the two different surfaces. Further analysis of these strains should reveal molecular factors needed for adherence and surface growth of L. monocytogenes on different biotic and abiotic surfaces.

HIGHLIGHTS

Benzalkonium chloride and heavy metal resistance profiles of Listeria monocytogenes strains isolated from fish, fish products and food-producing factories in Poland

Phenotypic and genotypic resistance to benzalkonium chloride (BC), cadmium and arsenic was tested (by susceptibility assays and molecular methods) in 287 Listeria monocytogenes strains isolated from fish and fish products, and food-producing factories in Poland. Overall, 40% of the isolates were resistant to BC, 56% to cadmium and 41% to arsenic (57% displayed resistance to more than one of the tested compounds). Among BC-resistant isolates, the most commonly detected resistance determinant was the qacH gene (83%). Three distinct types of cadA gene determining resistance to cadmium were detected, with the cadA1 variant predominant (88%), while most arsenic-resistant isolates (86%) harbored the arsA gene associated with a Tn554-like transposon (one strain harbored two copies of arsA in different arsenic resistance cassettes). 53% of all tested isolates contained plasmids (from 4 kb to > 90 kb in size), which were classified into 11 groups (p1-p11) based on their restriction patterns. Interestingly, 12 isolates harbored the small mobilizable pLMST6-like plasmid pLIS3 encoding multidrug efflux pump EmrC. Clustering analysis of PFGE patterns revealed that these isolates represent several diverse bacterial populations, which strongly suggests mobility of the pLMST6-like plasmids among L. monocytogenes strains and their role in dissemination of BC resistance.

Biofilm-isolated Listeria monocytogenes exhibits reduced systemic dissemination at the early (12-24 h) stage of infection in a mouse model

Environmental cues promote microbial biofilm formation and physiological and genetic heterogeneity. In food production facilities, biofilms produced by pathogens are a major source for food contamination; however, the pathogenesis of biofilm-isolated sessile cells is not well understood. We investigated the pathogenesis of sessile Listeria monocytogenes (Lm) using cell culture and mouse models. Lm sessile cells express reduced levels of the lap, inlA, hly, prfA, and sigB and show reduced adhesion, invasion, translocation, and cytotoxicity in the cell culture model than the planktonic cells. Oral challenge of C57BL/6 mice with food, clinical, or murinized-InlA (InlAm) strains reveals that at 12 and 24 h post-infection (hpi), Lm burdens are lower in tissues of mice infected with sessile cells than those infected with planktonic cells. However, these differences are negligible at 48 hpi. Besides, the expressions of inlA and lap mRNA in sessile Lm from intestinal content are about 6.0- and 280-fold higher than the sessle inoculum, respectively, suggesting sessile Lm can still upregulate virulence genes shortly after ingestion (12 h). Similarly, exposure to simulated gastric fluid (SGF, pH 3) and intestinal fluid (SIF, pH 7) for 13 h shows equal reduction in sessile and planktonic cell counts, but induces LAP and InlA expression and pathogenic phenotypes. Our data show that the virulence of biofilm-isolated Lm is temporarily attenuated and can be upregulated in mice during the early stage (12-24 hpi) but fully restored at a later stage (48 hpi) of infection. Our study further demonstrates that in vitro cell culture assay is unreliable; therefore, an animal model is essential for studying the pathogenesis of biofilm-isolated bacteria.

Analysis of virulence genes and molecular typing of Listeria monocytogenes isolates from human, food, and livestock from 2008 to 2016 in Iran

The frequency of Listeria monocytogenes isolates collected from a total of 1150 samples including food (n = 300), livestock (n = 50), and human clinical (n = 800) was evaluated during 2008-2016. Antimicrobial resistance patterns, virulence factors, and molecular characteristics of these isolates were analyzed using disk diffusion method, sequencing, serotyping, and pulsed-field gel electrophoresis (PFGE). The analysis of 44 L. monocytogenes isolates showed that 72.7% (32 of 44) of all the isolates belonged to Serotype 1/2c, and 15.9% (7 of 44) belonged to Serotype 3c. All 44 isolates were resistant to one or more antimicrobial agents with the most frequent resistance to penicillin (75%) and tetracycline (47.7%). Of the 44 L. monocytogenes strains, 100, 69.2, and 62.5% of livestock, human, and food strains were resistant to penicillin, respectively. Using pulsed-field gel electrophoresis (PFGE) technique, the isolates' genetic diversity was determined, and 28 PFGE patterns with 8 common (CT) and 20 single types (ST) were identified. This study highlights the high prevalence of Serotype 1/2c in clinical and livestock samples, while different serotypes were observed in food samples. The presence of rare serotypes such as 4c, belonging to the Lineage III, as well as 4e and 1/2c which are infrequent in Iran indicates that paying attention to uncommon serotypes, especially 1/2c, during the listeriosis outbreaks is necessary.

Propidium monoazide for viable Salmonella enterica detection by PCR and LAMP assays in comparison to RNA-based RT-PCR, RT-LAMP, and culture-based assays

Rapid and sensitive detection of live/infectious foodborne pathogens is urgently needed in order to prevent outbreaks and food recalls. This study aimed to (1) evaluate the incorporation of propidium monoazide (PMA) into PCR or LAMP assays to selectively detect viable Salmonella Enteritidis following sublethal heat or UV treatment, and autoclave sterilization; and (2) compare the detection of PMA-PCR and PMA-LAMP to DNA-based PCR and LAMP (without PMA), RNA-based RT-PCR and RT-LAMP, and culture-based methods. Nucleic acids (DNA or RNA) from 1-mL S. Enteritidis samples were used for PCR, RT-PCR, LAMP, and RT-LAMP assays. Serially diluted samples were plated on Xylose Lysine Tergitol-4 agar for cultural enumeration. Comparable detection of overnight cultured S. Enteritidis was obtained by PMA-PCR, PCR, and RT-PCR, though 1 to 2 log less sensitive than cultural assays. PMA-LAMP and RT-LAMP showed similar detection of overnight cultures, being 1 to 2 log less sensitive than the LAMP assay, and ∼4 log less than culture-based detection. Autoclaved S. Enteritidis did not test positive by RNA-based methods or PMA-PCR, but PMA-LAMP showed detection of 1 log CFU/mL. PMA-PCR and RT-PCR showed comparable detection of sublethal heat-treated cells to cultural assays, while PMA-LAMP showed 1 to 2 log less detection. Our results suggest that PMA-PCR and PMA-LAMP assays are not suitable for selective viable cell detection after UV treatment. While PMA-LAMP assay needs optimization, PMA-PCR shows promise for live/viable S. Enteritidis detection. PMA-PCR shows potential for routine testing in the food industry with results within 1-day, albeit depending on the inactivation method employed.

The use of a CDC biofilm reactor to grow multi-strain Listeria monocytogenes biofilm

Listeria monocytogenes is one of the most concerning pathogens for the food industry due to its ability to form biofilms, particularly in difficult-to-clean sites of processing facilities. There is a current industry-wide lack of data to refer to when selecting a strategy to control L. monocytogenes biofilms in the food premises. Many strategies have been developed to study biofilm formation of bacteria; however, few have targeted L. monocytogenes biofilms under dynamic conditions. This study addresses the biofilm formation ability of L. monocytogenes on stainless steel and polycarbonate under dynamic conditions using TSBYE or BHI as media culture at 30 °C or 37 °C. Higher cell counts were recovered at 30 °C in TSBYE on polycarbonate while lower counts were obtained at 37 °C in BHI on stainless steel (P < 0.05). Nonetheless, all factors (temperature, media and material) were statistically significant (P < 0.05) and an interaction between temperature and media was observed (P < 0.05). To our knowledge, this work represents an initial framework to develop L. monocytogenes biofilms under different dynamic conditions. The use of CDC Biofilm Reactor is not widely used yet in the food industry and represent a novel approach to help sanitary control strategies implementation.

Stress Response of Vibrio parahaemolyticus and Listeria monocytogenes Biofilms to Different Modified Atmospheres

The sessile biofilms of Vibrio parahaemolyticus and Listeria monocytogenes have increasingly become a critical threat in seafood safety. This study aimed to evaluate the effects of modified atmospheres on the formation ability of V. parahaemolyticus and L. monocytogenes biofilms. The stress responses of bacterial biofilm formation to modified atmospheres including anaerobiosis (20% carbon dioxide, 80% nitrogen), micro-aerobiosis (20% oxygen, 80% nitrogen), and aerobiosis (60% oxygen, 40% nitrogen) were illuminated by determining the live cells, chemical composition analysis, textural parameter changes, expression of regulatory genes, etc. Results showed that the biofilm formation ability of V. parahaemolyticus was efficiently decreased, supported by the fact that the modified atmospheres significantly reduced the key chemical composition [extracellular DNA (eDNA) and extracellular proteins] of the extracellular polymeric substance (EPS) and negatively altered the textural parameters (biovolume, thickness, and bio-roughness) of biofilms during the physiological conversion from anaerobiosis to aerobiosis, while the modified atmosphere treatment increased the key chemical composition of EPS and the textural parameters of L. monocytogenes biofilms from anaerobiosis to aerobiosis. Meanwhile, the expression of biofilm formation genes (luxS, aphA, mshA, oxyR, and opaR), EPS production genes (cpsA, cpsC, and cpsR), and virulence genes (vopS, vopD1, vcrD1, vopP2β, and vcrD2β) of V. parahaemolyticus was downregulated. For the L. monocytogenes cells, the expression of biofilm formation genes (flgA, flgU, and degU), EPS production genes (Imo2554, Imo2504, inlA, rmlB), and virulence genes (vopS, vopD1, vcrD1, vopP2β, and vcrD2β) was upregulated during the physiological conversion. All these results indicated that the modified atmospheres possessed significantly different regulation on the biofilm formation of Gram-negative V. parahaemolyticus and Gram-positive L. monocytogenes, which will provide a novel insight to unlock the efficient control of Gram-negative and Gram-positive bacteria in modified-atmosphere packaged food.

Comprehensive proteomic analysis and pathogenic role of membrane vesicles of Listeria monocytogenes serotype 4b reveals proteins associated with virulence and their possible interaction with host

Membrane vesicles (MVs) are produced by various Gram positive and Gram negative pathogenic bacteria and play an important role in virulence. In this study, the membrane vesicles (MVs) of L. monocytogenes were isolated from the culture supernatant. High-resolution electron microscopy and dynamic light scattering analysis revealed that L. monocytogenes MVs are spherical with a diameter of 200 to 300 nm in size. Further, comprehensive proteomic analyses of MVs and whole cells of L. monocytogenes were performed using LC/MS/MS. A total of 1355 and 312 proteins were identified in the L. monocytogenes cells and MVs, respectively. We identified that 296 proteins are found in both whole cells, and MV proteome and 16 proteins were identified only in the MVs. Also, we have identified the virulence factors such as listeriolysin O (LLO), internalin B (InlB), autolysin, p60, NLP/P60 family protein, UPF0356 protein, and PLC-A in MVs. Computational prediction of host-MV interactions revealed a total of 1841 possible interactions with the host involving 99 MV proteins and 1513 host proteins. We elucidated the possible pathway that mediates internalization of L. monocytogenes MV to host cells and the subsequent pathogenesis mechanisms. The in vitro infection assays showed that the purified MVs could induce cytotoxicity in Caco-2 cells. Using endocytosis inhibitors, we demonstrated that MVs are internalized via actin-mediated endocytosis. These results suggest that L. monocytogenes MVs can interact with host cell and contribute to the pathogenesis of L. monocytogenes during infection.

Influence of some parameters on the ability of Listeria monocytogenes, Listeria innocua, and Escherichia coli to form biofilms

AIM

The present study was conducted to evaluate the capacity of Listeria monocytogenes (L.m), Listeria innocua (L.i), and Escherichia coli to form biofilms on polystyrene support under different parameters by performing crystal violet (CV) staining technique.

MATERIALS AND METHODS

Different suspensions were prepared with single strains and with multiple combinations of strains including two serogroups of L.m (IIa and IIb), L.i, and E. coli strains at different microbial load. Selected strains and combinations were grown in biofilms for 6 days attached to polystyrene microplates under aerobic and microaerophilic conditions. The evaluation of the power of adhesion and biofilm formation was determined by CV staining followed by the measurement of optical density at 24 h, 72 h, and 6 days incubation time with and without renewal of the culture medium.

RESULTS

All the strains tested, presented more or less adhesion power depending on the variation of the studied parameters as well as the ability to form multispecies biofilms. Their development is more important by renewing the culture medium and increasing the initial load of bacteria. The ability to adhere and form biofilms differs from one serogroup to another within the same species. In bacterial combination, strains and species of bacteria adopt different behaviors.

CONCLUSION

The ability to form biofilms is a key factor in the persistence of tested strains in the environment. Our study showed that L.m, L.i, and E. coli could adhere to polystyrene and form biofilms under different conditions. More researches are necessary to understand the mechanisms of biofilm formation and the influence of different parameters in their development.

Pathogenicity, genotyping and antibacterial susceptibility of the Listeria spp. recovered from stray dogs

The present study aimed to determine the prevalence of Listeria spp. in stray dogs in the Kayseri province of Turkey. In addition, serotyping, genotyping and virulence gene analysis of the isolated Listeria spp. were performed and their pathogenicity and antibacterial susceptibility were investigated. The study included 80 rectal swaps taken from 80 stray dogs of different ages and gender that were sheltered in the Kayseri Municipal Dog Shelter. Listeria selective broth and Listeria selective agar were used for the isolation of Listeria spp. and the isolates were identified using a Microbact 12L (Oxoid, England) identification test kit. 16S rDNA sequencing and species-specific polymerase chain reaction (PCR) were performed for molecular identification of the isolates, multiplex PCR and a serological test were performed for serotyping, and PCR was used for virulence gene analysis. For determining the pathogenicity of L. monocytogenes and L. innocua isolates, a total of 100 mice (50 pregnant and 50 non-pregnant) were used. The mice were infected intraperitoneally; the inoculation dose was 1 × 10⁹ CFU/mL and 0.2 mL was used for each animal. Tissue samples obtained from infected mice were processed for the re-isolation of the Listeria spp. and then stained with hematoxylin eosin and Brown-Brenn Gram stain. The antibiotic susceptibilities of the isolates were determined by the disk diffusion method. Listeria spp. were isolated from 5 (6.25%) of the 80 fecal samples. While 1 of the isolates was identified as L. monocytogenes, 4 of them were identified as L. innocua. Serotyping by serological and molecular methods revealed the isolate of L. monocytogenes to be serotype 1/2a. According to the phylogenetic trees, L. innocua and L. monocytogenes strains were clustered in different groups. The L. monocytogenes isolate was positive for all virulence genes tested. All L. innocua isolates were positive for the plcB gene. While all L. innocua isolates were negative for the lin1068 gene, 3 L. innocua isolates were found to be positive for the lin0558 gene. In mice infected with L. monocytogenes, pathological findings were observed in the uterus, intestines, pancreas, and heart. In mice infected with L. innocua, pathological findings were observed in the stomach, intestines and spleen. L. monocytogenes- or L. innocua-related infections or other inflammatory reactions were not observed in the brains of infected animals. On histopathological examination with Gram stain, an abundance of Listeria spp. was observed in the lesions of the liver, spleen, uterus, and kidney. Moreover, while abortion was observed in all animals infected with L. monocytogenes, it was not observed in any of the animals infected with L. innocua. Antibiotic susceptibility testing revealed that all 5 isolates were sensitive to ampicillin, amoxicillin/clavulanic acid, erythromycin, gentamicin, penicillin G, and trimethoprim-sulfamethoxazole and were resistant to nalidixic acid, streptomycin, and cefuroxime sodium. Considering also the pathogenicity of the isolated microorganisms, it can be suggested that stray dogs as carriers of Listeria spp. are a significant risk to public health. As L. innocua isolates, which are considered apathogenic, led to the occurrence of lesions similar to those caused by L. monocytogenes, detailed studies on the pathogenesis of L. innocua infections caused by L. innocua isolates recovered from various sources are required.

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

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

Listeria monocytogenes contamination of ready-to-eat foods and the risk for human health in the EU

Food safety criteria for Listeria monocytogenes in ready-to-eat (RTE) foods have been applied from 2006 onwards (Commission Regulation (EC) 2073/2005). Still, human invasive listeriosis was reported to increase over the period 2009-2013 in the European Union and European Economic Area (EU/EEA). Time series analysis for the 2008-2015 period in the EU/EEA indicated an increasing trend of the monthly notified incidence rate of confirmed human invasive listeriosis of the over 75 age groups and female age group between 25 and 44 years old (probably related to pregnancies). A conceptual model was used to identify factors in the food chain as potential drivers for L. monocytogenes contamination of RTE foods and listeriosis. Factors were related to the host (i. population size of the elderly and/or susceptible people; ii. underlying condition rate), the food (iii. L. monocytogenes prevalence in RTE food at retail; iv. L. monocytogenes concentration in RTE food at retail; v. storage conditions after retail; vi. consumption), the national surveillance systems (vii. improved surveillance), and/or the bacterium (viii. virulence). Factors considered likely to be responsible for the increasing trend in cases are the increased population size of the elderly and susceptible population except for the 25-44 female age group. For the increased incidence rates and cases, the likely factor is the increased proportion of susceptible persons in the age groups over 45 years old for both genders. Quantitative modelling suggests that more than 90% of invasive listeriosis is caused by ingestion of RTE food containing > 2,000 colony forming units (CFU)/g, and that one-third of cases are due to growth in the consumer phase. Awareness should be increased among stakeholders, especially in relation to susceptible risk groups. Innovative methodologies including whole genome sequencing (WGS) for strain identification and monitoring of trends are recommended.

Growth, viability and architecture of biofilms of Listeria monocytogenes formed on abiotic surfaces

The pathogenic bacterium Listeria monocytogenes can persist in food processing plants for many years, even when appropriate hygienic measures are in place, with potential for contaminating ready-to-eat products and, its ability to form biofilms on abiotic surfaces certainly contributes for the environmental persistence. In this research, L. monocytogenes was grown in biofilms up 8 days attached to stainless steel and glass surfaces, contributing for advancing the knowledge on architecture of mature biofilms, since many literature studies carried out on this topic considered only early stages of cell adhesion. In this study, biofilm populations of two strains of L. monocytogenes (serotypes 1/2a and 4b) on stainless steel coupons and glass were examined using regular fluorescence microscopy, confocal laser scanning microscopy and classic culture method. The biofilms formed were not very dense and microscopic observations revealed uneven biofilm structures, with presence of exopolymeric matrix surrounding single cells, small aggregates and microcolonies, in a honeycomb-like arrangement. Moreover, planktonic population of L. monocytogenes (present in broth media covering the abiotic surface) remained stable throughout the incubation time, which indicates an efficient dispersal mechanism, since the culture medium was replaced daily. In conclusion, even if these strains of L. monocytogenes were not able to form thick multilayer biofilms, it was noticeable their high persistence on abiotic surfaces, reinforcing the need to focus on measures to avoid biofilm formation, instead of trying to eradicate mature biofilms.

Survival and transfer efficacy of mixed strain Salmonella enterica ser. Typhimurium from beef burgers to abiotic surfaces and determination of individual strain contribution

The aim of the study was to evaluate the survival and transfer efficacy of 3 Salmonella Typhimurium strains from beef burgers to abiotic surfaces and determine the individual strain distribution. S. Typhimurium population on beef burgers during incubation remained constant at initial levels of contamination approximately 3 and 5 log CFU/g. Additionally, the survival of pathogens on soiled HDPE surfaces was significant during incubation at both initial inocula, while ca 1.5 log CFU/cm² reduction was observed at 168h. The log transformed transfer rate (log₁₀Tr) was -1.86±0.23 and -1.75±0.40 for high and low inoculum. The level of initial contamination did not have any statistical important impact on bacterial transfer (P>0.05). In addition, the results regarding the strain contribution revealed rather random individual proportion of each strain, recovered from HDPE, SS surfaces and beef burgers. However, the dominance of each strain was strongly dependent on surface at low inoculum and time in case of high inoculum. This observed strain variability during survival and transfer of S. Typhimurium might be of great importance in order to understand and consequently limit the possibility of cross contamination during food processing in a common household.

Novel Cadmium Resistance Determinant in Listeria monocytogenes

Listeria monocytogenes is a foodborne pathogen that can cause severe disease (listeriosis) in susceptible individuals. It is ubiquitous in the environment and often exhibits resistance to heavy metals. One of the determinants that enables Listeria to tolerate exposure to cadmium is the cadAC efflux system, with CadA being a P-type ATPase. Three different cadA genes (designated cadA1 to cadA3) were previously characterized in L. monocytogenes A novel putative cadmium resistance gene (cadA4) was recently identified through whole-genome sequencing, but experimental confirmation for its involvement in cadmium resistance is lacking. In this study, we characterized cadA4 in L. monocytogenes strain F8027, a cadmium-resistant strain of serotype 4b. By screening a mariner-based transposon library of this strain, we identified a mutant with reduced tolerance to cadmium and that harbored a single transposon insertion in cadA4 The tolerance to cadmium was restored by genetic complementation with the cadmium resistance cassette (cadA4C), and enhanced cadmium tolerance was conferred to two unrelated cadmium-sensitive strains via heterologous complementation with cadA4C Cadmium exposure induced cadA4 expression, even at noninhibitory levels. Virulence assessments in the Galleria mellonella model suggested that a functional cadA4 suppressed virulence, potentially promoting commensal colonization of the insect larvae. Biofilm assays suggested that cadA4 inactivation reduced biofilm formation. These data not only confirm cadA4 as a novel cadmium resistance determinant in L. monocytogenes but also provide evidence for roles in virulence and biofilm formation.IMPORTANCEListeria monocytogenes is an intracellular foodborne pathogen causing the disease listeriosis, which is responsible for numerous hospitalizations and deaths every year. Among the adaptations that enable the survival of Listeria in the environment are the abilities to persist in biofilms, grow in the cold, and tolerate toxic compounds, such as heavy metals. Here, we characterized a novel determinant that was recently identified on a larger mobile genetic island through whole-genome sequencing. This gene (cadA4) was found to be responsible for cadmium detoxification and to be a divergent member of the Cad family of cadmium efflux pumps. Virulence assessments in a Galleria mellonella model suggested that cadA4 may suppress virulence. Additionally, cadA4 may be involved in the ability of Listeria to form biofilms. Beyond the role in cadmium detoxification, the involvement of cadA4 in other cellular functions potentially explains its retention and wide distribution in L. monocytogenes.

Recombinant Expression of a Putative Amidase Cloned from the Genome of Listeria monocytogenes that Lyses the Bacterium and its Monolayer in Conjunction with a Protease

Listeria monocytogenes is a Gram-positive, non-spore forming, catalase-positive rod that is a major bacterial food-borne disease agent associated with uncooked meats, including poultry, uncooked vegetables, soft cheeses, and unpasteurized milk. The bacterium may be carried by animals without signs of disease, can replicate at refrigeration temperatures, and is frequently associated with biofilms. There is a need to discover innovative pathogen intervention technologies for this bacterium. Consequently, bioinformatic analyses were used to identify genes encoding lytic protein sequences in the genomes of L. monocytogenes isolates. PCR primers were designed that amplified nucleotide sequences of a putative N-acetylmuramoyl-L-alanine amidase gene from L. monocytogenes strain 4b. The resultant amplification product was cloned into an expression vector, propagated in Escherichia coli Rosetta strains, and the recombinant protein was purified to homogeneity. Gene and protein sequencing confirmed that the predicted and chemically determined amino acid sequence of the recombinant protein designated PlyLM was a putative N-acetylmuramoyl-L-alanine amidase. The recombinant lytic protein was capable of lysing both the parental L. monocytogenes strain as well as other strains of the bacterium in spot and MIC/MIB assays, but was not active against other bacteria beyond the genus. A microtiter plate assay was utilized to assay for the ability of the recombinant lysin protein to potentially aid with digestion of a L. monocytogenes biofilm. Protease or lysozyme digestion alone did not significantly reduce the L. monocytogenes biofilm. Although the recombinant protein alone reduced the biofilm by only 20%, complete digestion of the bacterial monolayer was accomplished in conjunction with a protease.

Probabilistic Models to Predict Listeria monocytogenes Growth at Low Concentrations of NaNO2 and NaCl in Frankfurters

This study developed probabilistic models to describe Listeria monocytogenes growth responses in meat products with low concentrations of NaNO₂ and NaCl. A five-strain mixture of L. monocytogenes was inoculated in NBYE (nutrient broth plus 0.6% yeast extract) supplemented with NaNO₂ (0-141 ppm) and NaCl (0-1.75%). The inoculated samples were then stored under aerobic and anaerobic conditions at 4, 7, 10, 12, and 15℃ for up to 60 d. Growth response data [growth (1) or no growth (0)] for each combination were determined by turbidity. The growth response data were analyzed using logistic regression to predict the growth probability of L. monocytogenes as a function of NaNO₂ and NaCl. The model performance was validated with the observed growth responses. The effect of an obvious NaNO₂ and NaCl combination was not observed under aerobic storage condition, but the antimicrobial effect of NaNO₂ on the inhibition of L. monocytogenes growth generally increased as NaCl concentration increased under anaerobic condition, especially at 7-10℃. A single application of NaNO₂ or NaCl significantly (p<0.05) inhibited L. monocytogenes growth at 4-15℃, but the combination of NaNO₂ or NaCl more effectively (p<0.05) inhibited L. monocytogenes growth than single application of either compound under anaerobic condition. Validation results showed 92% agreement between predicted and observed growth response data. These results indicate that the developed model is useful in predicting L. monocytogenes growth response at low concentrations of NaNO₂ and NaCl, and the antilisterial effect of NaNO₂ increased by NaCl under anaerobic condition.

Highly Invasive Listeria monocytogenes Strains Have Growth and Invasion Advantages in Strain Competition

Multiple Listeria monocytogenes strains can be present in the same food sample; moreover, infection with more than one L. monocytogenes strain can also occur. In this study we investigated the impact of strain competition on the growth and in vitro virulence potential of L. monocytogenes. We identified two strong competitor strains, whose growth was not (or only slightly) influenced by the presence of other strains and two weak competitor strains, which were outcompeted by other strains. Cell contact was essential for growth inhibition. In vitro virulence assays using human intestinal epithelial Caco2 cells showed a correlation between the invasion efficiency and growth inhibition: the strong growth competitor strains showed high invasiveness. Moreover, invasion efficiency of the highly invasive strain was further increased in certain combinations by the presence of a low invasive strain. In all tested combinations, the less invasive strain was outcompeted by the higher invasive strain. Studying the effect of cell contact on in vitro virulence competition revealed a complex pattern in which the observed effects depended only partially on cell-contact suggesting that competition occurs at two different levels: i) during co-cultivation prior to infection, which might influence the expression of virulence factors, and ii) during infection, when bacterial cells compete for the host cell. In conclusion, we show that growth of L. monocytogenes can be inhibited by strains of the same species leading potentially to biased recovery during enrichment procedures. Furthermore, the presence of more than one L. monocytogenes strain in food can lead to increased infection rates due to synergistic effects on the virulence potential.

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.

Intra- and inter-species interactions within biofilms of important foodborne bacterial pathogens

A community-based sessile life style is the normal mode of growth and survival for many bacterial species. Under such conditions, cell-to-cell interactions are inevitable and ultimately lead to the establishment of dense, complex and highly structured biofilm populations encapsulated in a self-produced extracellular matrix and capable of coordinated and collective behavior. Remarkably, in food processing environments, a variety of different bacteria may attach to surfaces, survive, grow, and form biofilms. Salmonella enterica, Listeria monocytogenes, Escherichia coli, and Staphylococcus aureus are important bacterial pathogens commonly implicated in outbreaks of foodborne diseases, while all are known to be able to create biofilms on both abiotic and biotic surfaces. Particularly challenging is the attempt to understand the complexity of inter-bacterial interactions that can be encountered in such unwanted consortia, such as competitive and cooperative ones, together with their impact on the final outcome of these communities (e.g., maturation, physiology, antimicrobial resistance, virulence, dispersal). In this review, up-to-date data on both the intra- and inter-species interactions encountered in biofilms of these pathogens are presented. A better understanding of these interactions, both at molecular and biophysical levels, could lead to novel intervention strategies for controlling pathogenic biofilm formation in food processing environments and thus improve food safety.

Identification of high-risk Listeria monocytogenes serotypes in lineage I (serotype 1/2a, 1/2c, 3a and 3c) using multiplex PCR

AIMS

Using molecular subtyping techniques, Listeria monocytogenes is divided into three major phylogenetic lineages, and a multiplex PCR method can differentiate five L. monocytogenes subgroups: 1/2a-3a, 1/2c-3c, 1/2b-3b-7, 4b-4d-4e and 4a-4c. In this study, we conducted genome comparisons and evaluated serotype-associated genes for their utility as a multiplex PCR-based method for distinguishing high-risk serotypes 1/2a and 1/2c in lineage I from low-risk serotypes 3a and 3c.

METHODS AND RESULTS

Primer sets were developed that are specific for serotype 1/2c (LMOSLCC2372_0308) and serotype 3a (LMLG_0742). These primers were then tested in a multiplex format with primers specific for serotype 1/2a (flaA) to separate serotypes 1/2a, 1/2c, 3a and 3c using 25 strains of lineage I L. monocytogenes.

CONCLUSIONS

Here, for the first time, we report primers specific for L. monocytogenes serotype 1/2c and serotype 3a, and we demonstrate a multiplex PCR method for separating the four serotypes of lineage I L. monocytogenes.

SIGNIFICANCE AND IMPACT OF THE STUDY

The described multiplex PCR assay consistently showed successful separation of 1/2a and 1/2c strains from 3a and 3c strains. PCR is routinely performed in many diagnostic and epidemiologic investigations for L. monocytogenes, and these primers should increase the feasibility and accessibility of L. monocytogenes serotyping.

Ingested Salmonella enterica, Cronobacter sakazakii, Escherichia coli O157:H7, and Listeria monocytogenes: transmission dynamics from adult house flies to their eggs and first filial (F1) generation adults

BACKGROUND

The mechanical transmission of pathogenic bacteria by synanthropic filth flies is widely recognized. While many studies report the fate and the temporospatial distribution of ingested foodborne bacteria by filth flies, there is little evidence about the transmission dynamics of ingested foodborne bacteria by adult house flies (Musca domestica) to their progeny. In this study, we fed parental house fly adults with food contaminated with low, medium, and high concentrations of Salmonella enterica, Cronobacter sakazakii, Escherichia coli O157:H7, and Listeria monocytogenes and evaluated the probability of transmission of these pathogens to house fly eggs and the surface and the alimentary canal of their first filial (F1) generation adults.

RESULTS

All foodborne pathogens were present in samples containing pooled house fly eggs. The probability of transmission was higher after parental house flies ingested food containing medium bacterial loads. Cronobacter sakazakii was 16, 6, and 3 times more likely to be transmitted to house fly eggs than S. enterica, E. coli O157:H7, and L. monocytogenes, respectively. Only S. enterica and C. sakazakii were transmitted to F1 generation adults and their presence was 2.4 times more likely on their body surfaces than in their alimentary canals. The highest probabilities of finding S. enterica (60 %) and C. sakazakii (28 %) on newly emerged F1 adults were observed after parental house flies ingested food containing medium and high levels of these pathogens, respectively.

CONCLUSION

Our study demonstrates that adult house flies that fed from food contaminated with various levels of foodborne bacteria were able to transmit those pathogens to their eggs and some were further transmitted to newly emerged F1 generation adults, enhancing the vector potential of these insects. Understanding the type of associations that synanthropic filth flies establish with foodborne pathogens will help to elucidate transmission mechanisms and possible ways to mitigate the spread of foodborne pathogens.

Multiplex detection and genotyping of pathogenic bacteria on paper-based biosensor with a novel universal primer mediated asymmetric PCR

Traditionary multiplex asymmetric polymerase chain reaction (PCR) can be applied to detect multiplex target organisms simultaneously, but complex optimizations of primer concentrations and staggered additions of primers are required to achieve equal amplification of multiplex genes. To overcome this shortcoming, we propose a novel method based on multiplex asymmetric PCR and paper-based nucleic acid diagnostics (PBNAD). In the asymmetric PCR, a universal primer was introduced to break the bottlenecks of low sensitivity and self-inhibition among different sets of primers. Amplification using the novel multiplex asymmetric PCR boosted the quantity of single-stranded amplicons, and the amplified products contained the same sequence at the 5' end. Therefore, only one gold nanoparticle-based signal probe was needed for the simultaneous detection of three genes using the PBNAD platform, and the detection signals could be observed with the naked eye. With this highly efficient, novel multiplex asymmetric PCR, as little as 1 pg/μL genomic DNA can be detected. This method can also be applied to genotyping for reliable epidemiological investigations. This proof-of-concept study highlights the potential of the PBNAD platform for cost- and labor-effective applications in the detection of pathogenic bacteria.

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.

Co-culture with Listeria monocytogenes within a dual-species biofilm community strongly increases resistance of Pseudomonas putida to benzalkonium chloride

Biofilm formation is a phenomenon occurring almost wherever microorganisms and surfaces exist in close proximity. This study aimed to evaluate the possible influence of bacterial interactions on the ability of Listeria monocytogenes and Pseudomonas putida to develop a dual-species biofilm community on stainless steel (SS), as well as on the subsequent resistance of their sessile cells to benzalkonium chloride (BC) used in inadequate (sub-lethal) concentration (50 ppm). The possible progressive adaptability of mixed-culture biofilms to BC was also investigated. To accomplish these, 3 strains per species were left to develop mixed-culture biofilms on SS coupons, incubated in daily renewable growth medium for a total period of 10 days, under either mono- or dual-species conditions. Each day, biofilm cells were exposed to disinfection treatment. Results revealed that the simultaneous presence of L. monocytogenes strongly increased the resistance of P. putida biofilm cells to BC, while culture conditions (mono-/dual-species) did not seem to significantly influence the resistance of L. monocytogenes biofilm cells. BC mainly killed L. monocytogenes cells when this was applied against the dual-species sessile community during the whole incubation period, despite the fact that from the 2nd day this community was mainly composed (>90%) of P. putida cells. No obvious adaptation to BC was observed in either L. monocytogenes or P. putida biofilm cells. Pulsed field gel electrophoresis (PFGE) analysis showed that the different strains behaved differently with regard to biofilm formation and antimicrobial resistance. Such knowledge on the physiological behavior of mixed-culture biofilms could provide the information necessary to control their formation.

Novel epidemic clones of Listeria monocytogenes, United States, 2011

We identified a novel serotype 1/2a outbreak strain and 2 novel epidemic clones of Listeria monocytogenes while investigating a foodborne outbreak of listeriosis associated with consumption of cantaloupe during 2011 in the United States. Comparative analyses of strains worldwide are essential to identification of novel outbreak strains and epidemic clones.

Attachment and biofilm formation by foodborne bacteria in meat processing environments: causes, implications, role of bacterial interactions and control by alternative novel methods

Attachment of potential spoilage and pathogenic bacteria to food contact surfaces and the subsequent biofilm formation represent serious challenges to the meat industry, since these may lead to cross-contamination of the products, resulting in lowered-shelf life and transmission of diseases. In meat processing environments, microorganisms are sometimes associated to surfaces in complex multispecies communities, while bacterial interactions have been shown to play a key role in cell attachment and detachment from biofilms, as well as in the resistance of biofilm community members against antimicrobial treatments. Disinfection of food contact surfaces in such environments is a challenging task, aggravated by the great antimicrobial resistance of biofilm associated bacteria. In recent years, several alternative novel methods, such as essential oils and bacteriophages, have been successfully tested as an alternative means for the disinfection of microbial-contaminated food contact surfaces. In this review, all these aspects of biofilm formation in meat processing environments are discussed from a microbial meat-quality and safety perspective.

Tarr C, Gerner-Smidt P, Datta AR. Genomic characterization of Listeria monocytogenes strains involved in a multistate listeriosis outbreak associated with cantaloupe in US

A multistate listeriosis outbreak associated with cantaloupe consumption was reported in the United States in September, 2011. The outbreak investigation recorded a total of 146 invasive illnesses, 30 deaths and one miscarriage. Subtyping of the outbreak associated clinical, food and environmental isolates revealed two serotypes (1/2a and 1/2b) and four pulsed-field gel electrophoresis two-enzyme pattern combinations I, II, III, and IV, including one rarely seen before this outbreak. A DNA-microarray, Listeria GeneChip®, developed by FDA from 24 Listeria monocytogenes genome sequences, was used to further characterize a representative sample of the outbreak isolates. The microarray data (in the form of present or absent calls of specific DNA sequences) separated the isolates into two distinct groups as per their serotypes. The gene content of the outbreak-associated isolates was distinct from that of the previously-reported outbreak strains belonging to the same serotypes. Although the 1/2b outbreak associated isolates are closely related to each other, the 1/2a isolates could be further divided into two distinct genomic groups, one represented by pattern combination I strains and the other represented by highly similar pattern combinations III and IV strains. Gene content analysis of these groups revealed unique genomic sequences associated with these two 1/2a genovars. This work underscores the utility of multiple approaches, such as serotyping, PFGE and DNA microarray analysis to characterize the composition of complex polyclonal listeriosis outbreaks.