Genetic diversity of Listeria monocytogenes isolated from ready-to-eat food products in retail in Poland.

The study describes the characterization of Listeria monocytogenes isolated from the general 2017-2019 national official control and monitoring sampling program. A total of 60,928 of ready-to-eat (RTE) food products were collected in retail in Poland, while the number of L. monocytogenes contaminated samples was 67 (0.1%). The majority of the strains belonged to molecular serotype IVb followed by IIa, frequently associated with human listeriosis. Furthermore, 61.2% of the isolates were resistant at least to one of the tested antimicrobials: penicillin, ampicillin, meropenem, erythromycin, sulfamethoxazole-trimethoprim, amoxicillin-clavulanic acid, ciprofloxacin, chloramphenicol, gentamicin, vancomycin, tetracycline and rifampicin. Virulence genes inlA, inlC, inlJ and lmo2672 were detected in all of the isolates. In our study the llsX gene (encoding LLS) exhibited 11.6% positivity. The 32 strains were grouped into 12 clonal complexes (CCs) which belong to the major clones that are in circulation in Europe. Among them, seven strains with the cgMLST close relatedness (CC2) were isolated from diverse food sectors, underlining a large circulation of this clone in Poland, most likely from multiple introduction sources. Additionally, two RTE strains CC6 and one CC37 were identified as closely related by cgMLST to two publicly available genomes of clinical strains isolated in Poland in 2012-2013. These results indicate the large strain circulation and point to RTE food products as a potential source of human listeriosis. The present study provided data to capture the contamination status of L. monocytogenes in foods at the retail level in Poland and assess the potential risk of this pathogen for human safety.

Silent Genes: Antimicrobial Resistance and Antibiotic Production.

Silent genes are DNA sequences that are generally not expressed or expressed at a very low level. These genes become active as a result of mutation, recombination, or insertion. Silent genes can also be activated in laboratory conditions using pleiotropic, targeted genome-wide, or biosynthetic gene cluster approaches. Like every other gene, silent genes can spread through horizontal gene transfer. Most studies have focused on strains with phenotypic resistance, which is the most common subject. However, to fully understand the mechanism behind the spreading of antibiotic resistance, it is reasonable to study the whole resistome, including silent genes.

Incidence and genetic variability of Listeria monocytogenes isolated from vegetables in Poland.

The aim of the present study is to investigate the prevalence and genetic diversity of Listeria monocytogenes in various fresh and frozen vegetable products available in Poland. The samples were collected at retail market within the framework of national official control and monitoring program. In the years 2016-2019 a total of 49 samples out of 8712 collected vegetable samples were positive for L. monocytogenes. Our findings demonstrated that the occurrence of L. monocytogenes in various vegetable products was generally low, on average only 0.56% in the studied years. All isolates were susceptible to 11 antimicrobial agents: penicillin, ampicillin, meropenem, erythromycin, sulfamethoxazole-trimethoprim, amoxicillin-clavulanic acid, ciprofloxacin, chloramphenicol, gentamicin, vancomycin, and tetracycline. All of them harbored virulence-associated genes (inlA, inlC, and lmo2672), 82% harbored inlJ gene and few of them (22%) also possessed the llsX gene. The majority of collected isolates (65%) belonged to molecular serogroup 1/2a-3a, followed by 4ab-4b-4d-4e (33%), and only one to serogroup 1/2b-3b-7 (2%). Isolates yielded 18 different restriction profiles, revealing a large cluster of contamination linked to frozen corn (21 strains) and distributed in 3 pulsotypes. MLST analysis classified selected isolates into nine clonal complexes (CCs). The obtained results contribute to characterizing the diversity of L. monocytogenes isolated from various vegetable products in Poland and their impact on food safety and public health.

Biofilm-Forming Ability of Pathogenic Bacteria Isolated from Retail Food in Poland.

ABSTRACT: Biofilms have a significant impact on food safety in the food industry. Many foodborne outbreaks have been associated with pathogenic bacterial strains that can form a biofilm. The present study was conducted under the Official Control and Monitoring Program in Poland to examine the ability of pathogenic bacteria collected from retail food samples to form biofilms. Biofilm formation was assessed by qualitative detection of extracellular polymeric substances on Congo red agar, by adherence to glass with the tube method, by the crystal violet biofilm (CV) assay, and by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay. A total of 40 isolates from food samples (10 strains each of Listeria monocytogenes, Staphylococcus aureus, Escherichia coli, and Bacillus cereus) were examined. The strains were classified as adherent, slightly adherent, or nonadherent; biofilm production was classified as weak (WBP), moderate (MBP), or strong (SBP); and metabolic activity was classified as weak (WMA), moderate (MMA), or high (HMA). The incubation conditions and time influenced the amount of biofilm formed as well as did the growth medium. In the test tubes with Luria-Bertani broth (LBB), 22.5% of the strains were adherent and 77.5% were slightly adherent. Stronger adhesion was obtained in brain heart infusion (BHI) with 2% sucrose; 60% of the isolates were classified as adherent. With the CV assay with LBB, SBP was noted for 7.5% of the strains after 24 h of incubation and for 37.5% of the strains after 48 h. In BHI plus 2% sucrose, SBP was noted for 42.5 and 37.6% of the strains after 24 and 48 h, respectively. With the MTT assay with LBB, HMA was found for 15% of the strains after 24 h of incubation and for 25% of the strains after 48 h. In BHI plus 2% sucrose, 70 and 85% of the incubated strains were classified as HMA after 24 and 48 h, respectively.

Occurrence and Characteristics of Listeria monocytogenes in Ready-to-Eat Meat Products in Poland.

ABSTRACT: Listeria monocytogenes is a potential hazard for food safety and therefore for public health. The aim of the present study was to determine the prevalence and characteristics of L. monocytogenes in Polish ready-to-eat (RTE) meat products for retail sale. Among the 184,439 food samples collected within the framework of a national official control and monitoring program, only 0.3% were positive for L. monocytogenes. A significant group of products that did not meet the criteria were RTE meat products. This group accounted for 40% of all noncompliant samples. Seventy L. monocytogenes isolates from these RTE meat products (meat, sausages, and delicatessen products with meat) were examined. The majority of the tested isolates (51%) belonged to serogroup 1/2a-3a followed by 1/2c-3c (21%), 1/2b-3b-7 (14%), and 4ab-4b-4d-4e (13%). Serogroup 4a-4c was not present among the tested isolates. All L. monocytogenes isolates harbored the virulence-associated genes inlA, inlC, inlJ, and lmo2672. The llsX marker was detected in 12 (17%) of the 70 isolates. Ampicillin resistance was the most common resistance phenotype and was identified in 83% of the L. monocytogenes isolates. A low incidence of resistance to amoxicillin-clavulanic acid (6% of isolates) was also detected. All L. monocytogenes isolates were susceptible to chloramphenicol, gentamicin, ciprofloxacin, meropenem, sulfamethoxazole-trimethoprim, tetracycline, and erythromycin. This work provides useful information regarding contamination of RTE meat products with L. monocytogenes, which may have implications for food safety risks.