Low potential virulence associated with mutations in the inlA and prfA genes in Listeria monocytogenes isolated from raw retail poultry meat

Evidence of Virulent Multi-Drug Resistant and Biofilm-Forming Listeria Species Isolated from Various Sources in South Africa

Listeriosis is a foodborne disease caused by Listeria monocytogenes species and is known to cause severe complications, particularly in pregnant women, young children, the elderly, and immunocompromised individuals. The aim of this study was to investigate the presence of Listeria species in food and water using both biochemical and species-specific PCR analysis. L. monocytogenes isolates were further screened for the presence of various antibiotic resistance, virulence, and biofilm-forming determinants profiles using phenotypic and genotypic assays. A total of 207 samples (composed of meat, milk, vegetables, and water) were collected and analyzed for presence of L. monocytogenes using species specific PCR analysis. Out of 267 presumptive isolates, 53 (19.85%) were confirmed as the Listeria species, and these comprised 26 L. monocytogenes, 3 L. innocua, 2 L. welshimeri, and 1 L. thailandensis. The remaining 21 Listeria species were classified as uncultured Listeria, based on 16SrRNA sequence analysis results. A large proportion (76% to 100%) of the L. monocytogenes were resistant to erythromycin (76%), clindamycin (100%), gentamicin (100%), tetracycline (100%), novobiocin (100%), oxacillin (100%), nalidixic acid (100%), and kanamycin (100%). The isolates revealed various multi-drug resistant (MDR) phenotypes, with E-DA-GM-T-NO-OX-NA-K being the most predominant MDR phenotypes observed in the L. monocytogenes isolates. The virulence genes prfA, hlyA, actA, and plcB were detected in 100%, 68%, 56%, and 20% of the isolates, respectively. In addition, L. monocytogenes isolates were capable of forming strong biofilm at 4 °C (%) after 24 to 72 h incubation periods, moderate for 8% isolates at 48 h and 20% at 72 h (p < 0.05). Moreover, at 25 °C and 37 °C, small proportions of the isolates displayed moderate (8−20%) biofilm formation after 48 and 72 h incubation periods. Biofilm formation genes flaA and luxS were detected in 72% and 56% of the isolates, respectively. These findings suggest that proper hygiene measures must be enforced along the food chain to ensure food safety.

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.

Genomic insights into persistence of Listeria species in the food processing environment

AIMS

Listeria species may colonize and persist in food processing facilities for prolonged periods of time, despite hygiene interventions in place. To understand the genetic factors contributing to persistence of Listeria strains, this study undertook a comparative analysis of seven persistent and six presumed non-persistent strains, isolated from a single food processing environment, to identify genetic markers correlating to promoting persistence of Listeria strains, through whole genome sequence analysis.

METHODS AND RESULTS

A diverse pool of genetic markers relevant to hygiene tolerance was identified, including disinfectant resistance markers qacH, emrC and the efflux cassette bcrABC. Both persistent and presumed non-persistent cohorts encoded a range of stress resistance markers, including heavy metal resistance, oxidative and pH stress, although trends were associated with each cohort (e.g., qacH and cadA1C resistance was more frequently found in persistent isolates). Persistent isolates were more likely to contain mutations associated with attenuated virulence, including a truncated InlA. Plasmids and transposons were widespread between cohorts.

CONCLUSIONS

Results suggest that no single genetic marker identified was universally responsible for a strain's ability to persist. Persistent strains were more likely to harbour mutation associated with hypovirulence.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study provides additional insights into the distribution of genetic elements relevant to persistence across Listeria species, as well as strain virulence potential.

In Vitro Preformed Biofilms of Bacillus safensis Inhibit the Adhesion and Subsequent Development of Listeria monocytogenes on Stainless-Steel Surfaces

Listeria monocytogenes continues to be one of the most important public health challenges for the meat sector. Many attempts have been made to establish the most efficient cleaning and disinfection protocols, but there is still the need for the sector to develop plans with different lines of action. In this regard, an interesting strategy could be based on the control of this type of foodborne pathogen through the resident microbiota naturally established on the surfaces. A potential inhibitor, Bacillus safensis, was found in a previous study that screened the interaction between the resident microbiota and L. monocytogenes in an Iberian pig processing plant. The aim of the present study was to evaluate the effect of preformed biofilms of Bacillus safensis on the adhesion and implantation of 22 strains of L. monocytogenes. Mature preformed B. safensis biofilms can inhibit adhesion and the biofilm formation of multiple L. monocytogenes strains, eliminating the pathogen by a currently unidentified mechanism. Due to the non-enterotoxigenic properties of B. safensis, its presence on certain meat industry surfaces should be favored and it could represent a new way to fight against the persistence of L. monocytogenes in accordance with other bacterial inhibitors and hygiene operations.

Susceptibility (re)-testing of a large collection of Listeria monocytogenes from foods in China from 2012 to 2015 and WGS characterization of resistant isolates

OBJECTIVES

Our aim was to determine the antimicrobial susceptibilities of 2862 Listeria monocytogenes cultured from various foods in China and to use WGS to characterize the antimicrobial resistance and virulence genotypes of those expressing a resistance phenotype.

METHODS

The susceptibilities of 2862 L. monocytogenes were determined by broth microdilution. Twenty-eight L. monocytogenes were found to be resistant to one to four antibiotics. All 28 resistant isolates were subsequently sequenced using short-read high accuracy protocols. The corresponding genomes were assembled and further analysis was carried out using appropriate bioinformatics pipelines.

RESULTS

All 28 resistant L. monocytogenes were classified into five STs (ST3, ST8, ST9, ST155 and ST515). Both ST9 and ST155 were dominant and their genotypes correlated with their resistance phenotypes. All ST9 isolates were MDR and could be phylogenetically classified into two clusters. One was relatively close to clinical origins and one to food. Downstream analysis of the genetic contexts in which these resistance genotypes were found suggested that these may have been acquired from other bacteria by horizontal transfer or insertion into the chromosome. All isolates harboured Listeria pathogenicity island (LIPI)-1 and LIPI-2, and only two harboured LIPI-3.

CONCLUSIONS

This study reported on the antimicrobial susceptibilities of 2862 foodborne L. monocytogenes along with the genomic characterization of 28 resistant isolates, 11 of which expressed an MDR phenotype. These data showed that this bacterium can acquire resistance by horizontal gene transfer in and between species. This study may necessitate a re-evaluation of risk to public health, associated with this bacterial species.

Analysis of Benzalkonium Chloride Resistance and Potential Virulence of Listeria monocytogenes Isolates Obtained from Different Stages of a Poultry Production Chain in Spain

ABSTRACT

Listeria monocytogenes can survive in food production facilities and can be transmitted via contamination of food during the various stages of food production. This study was conducted to compile the results of three independent previous studies on the genetic diversity of L. monocytogenes in a poultry production company in Spain and to determine the potential virulence and sanitizer resistance of the strains by using both genotype and phenotype analyses. L. monocytogenes was detected at three production stages: a broiler abattoir, a processing plant, and retail stores marketing fresh poultry products from the same company. These three stages spanned three locations in three provinces of Spain. A set of 347 L. monocytogenes isolates representing 39 subtypes was obtained using pulsed-field gel electrophoresis (PFGE). A total of 28 subtypes (68%) had a full-length internalin A gene, and two subtypes had a phenotype with low potential for virulence because of a mutation in the prfA gene. A total of 32 subtypes (82%) were classified as benzalkonium chloride resistant (BAC-R) and contained the resistance determinant bcrABC (21 subtypes, 54%) or the resistance gene qacH (11 subtypes, 28%). A total of 13 persistent BAC-R subtypes (minimum of 3 months between the first and last sample from with the isolate was recovered) were identified at the abattoir and processing plant. The three production stages shared a unique subtype (PFGE type 1), which had the mutation in the prfA gene and the bcrABC resistance determinant. Whole genome sequencing revealed this subtype to be sequence type 31. Limited genetic diversity was noted in the isolates studied, including some subtypes that were persistent in the environment of the investigated facilities. Given the high prevalence of BAC-R subtypes, these results support the association between resistance to biocides and persistence of L. monocytogenes.

HIGHLIGHTS

Whole-Genome Sequences of Seven Listeria monocytogenes Strains from Different Stages of a Poultry Meat Production Chain

Here, we present the draft genome sequences of seven Listeria monocytogenes strains isolated during three independent studies carried out in three stages of a poultry meat production chain. The genome sequences of these strains obtained from different stages can help to understand the possible transmission of L. monocytogenes.

Here, we present the draft genome sequences of seven Listeria monocytogenes strains isolated during three independent studies carried out in three stages of a poultry meat production chain. The genome sequences of these strains obtained from different stages can help to understand the possible transmission of L. monocytogenes.

Characterization of specific alleles in InlA and PrfA of Listeria monocytogenes isolated from foods in Osaka, Japan and their ability to invade Caco-2 cells

Listeria monocytogenes expresses the surface protein internalin A (InlA), enabling the invasion of human intestinal epithelial cells to cause severe food-borne diseases. Full-length sequence analysis of inlA of 114 food isolates resulted in the detection of 29 isolates with a premature stop codon (PMSC) mutation and 6 isolates with 3-codon deletion mutations (aa 738 to 740) in inlA. The isolates with inlA PMSCs demonstrated a significantly lower level of invasion than the other food isolates in a Caco-2 cell invasion assay (P<0.01), but the isolates with the 3-codon deletion exhibited invasion comparable to the isolates with non-truncated InlA (P>0.05). According to analysis of the positive regulatory factor A (PrfA) sequences of 114 L. monocytogenes isolates, 7 isolates of serotype 1/2a from chicken samples contained a PrfA protein with a 5-nucleotide deletion from 712 to 716, including a stop codon. Although the isolates with a 5-nucleotide deletion in prfA demonstrated invasion comparable to the isolates with non-truncated InlA and PrfA after growth at 30 °C (P>0.05), they exhibited a significantly higher level of invasion than the other isolates after growth at 20 °C (P<0.01). To the authors' knowledge, this is the first report of L. monocytogenes isolates with the stop-codon deletion of PrfA.

Listeria monocytogenes serotype 1/2b and 4b isolates from human clinical cases and foods show differences in tolerance to refrigeration and salt stress

Control of Listeria monocytogenes in food processing facilities is a difficult issue because of the ability of this microorganism to form biofilms and adapt to adverse environmental conditions. Survival at high concentrations of sodium chloride and growth at refrigeration temperatures are two other important characteristics of L. monocytogenes isolates. The aim of this study was to compare the growth characteristics under stress conditions at different temperatures of L. monocytogenes serotypes responsible for the majority of clinical cases from different sources. Twenty-two L. monocytogenes isolates, 12 from clinical cases (8 serotype 4b and 4 serotype 1/2a) and 10 from food (6 serotype 4b and 4 serotype 1/2a), and an L. monocytogenes Scott A (serotype 4b) reference strain were analyzed for the ability to grow in brain heart infusion broth plus 1.9 M NaCl (11%) at 4, 10, and 25°C for 73, 42, and 15 days, respectively. The majority of L. monocytogenes strains was viable or even grew at 4°C and under the high osmotic conditions usually used to control pathogens in the food industry. At 10°C, most strains could adapt and grow; however, no significant difference (P > 0.05) was found for lag-phase duration, maximum growth rate, and maximum cell density. At 25°C, all strains were able to grow, and populations increased by up 5 log CFU/ml. Clinical strains had a significantly longer lag phase and lower maximum cell density (P < 0.05) than did food strains. Regarding virulence potential, no significant differences in hemolytic activity were found among serotypes; however, serotype 4b strains were more invasive in Caco-2 cells than were serotype 1/2a strains (P < 0.05). The global tendency of decreasing NaCl concentrations in processed foods for health reasons may facilitate L. monocytogenes survival and growth in these products. Therefore, food companies must consider additional microbial growth barriers to assure product safety.