Typing ofListeria monocytogenesstrains isolated in Italy byinlA gene characterization and evaluation of a new cost-effective approach to antisera selection for serotyping

Evaluation of the Virulence Potential of Listeria monocytogenes through the Characterization of the Truncated Forms of Internalin A

Listeria monocytogenes is a widespread Gram-positive pathogenic bacterium that causes listeriosis, a rather rare but severe foodborne disease. Pregnant women, infants, the elderly, and immunocompromised individuals are considered particularly at risk. L. monocytogenes can contaminate food and food-processing environments. In particular, ready-to-eat (RTE) products are the most common source associated with listeriosis. L. monocytogenes virulence factors include internalin A (InlA), a surface protein known to facilitate bacterial uptake by human intestinal epithelial cells that express the E-cadherin receptor. Previous studies have demonstrated that the presence of premature stop codon (PMSC) mutations naturally occurring in inlA lead to the production of a truncated protein correlated with attenuate virulence. In this study, 849 L. monocytogenes isolates, collected from food, food-processing plants, and clinical cases in Italy, were typed and analyzed for the presence of PMSCs in the inlA gene using Sanger sequencing or whole-genome sequencing (WGS). PMSC mutations were found in 27% of the isolates, predominantly in those belonging to hypovirulent clones (ST9 and ST121). The presence of inlA PMSC mutations in food and environmental isolates was higher than that in clinical isolates. The results reveal the distribution of the virulence potential of L. monocytogenes circulating in Italy and could help to improve risk assessment approaches.

Newly Designed Primers for the Sequencing of the inlA Gene of Lineage I and II Listeria monocytogenes Isolates

Listeria monocytogenes is a major human foodborne pathogen responsible for listeriosis. The virulence factor Internalin A (inlA) has a key role in the invasion of L. monocytogenes into the human intestinal epithelium, and the presence of premature stop-codons (PMSC) mutations in the inlA gene sequence is correlated with attenuated virulence. The inlA sequencing process is carried out by dividing the gene into three sections which are then reassembled to obtain the full gene. The primers available however were only able to entirely amplify the lineage II isolates. In this study, we present a set of new primers which allow inlA sequencing of isolates belonging to both lineages, since lineage I isolates are the ones most frequently associated to clinical cases. Using newly designed primers, we assessed the presence of inlA PMSCs in food, food processing environments and clinical isolates.

Prevalence and antimicrobial resistance of Listeria species and molecular characterization of Listeria monocytogenes isolated from retail ready-to-eat foods

A wide variety of foods can be contaminated with Listeria species, especially L. monocytogenes. Ready-to-eat (RTE) foods are predominantly associated with human listeriosis caused by L. monocytogenes. Therefore, this study aimed to assess the presence of Listeria species in RTE foods and to characterize L. monocytogenes isolates by means of detection of virulence markers, serotypes and genetic relatedness. Of the 300 RTE food samples, 59 (19.7%) were positive for Listeria species: L. innocua (13.3%), L. monocytogenes (5%), L. welshimerii (2.3%), L. grayi subsp. murrayi (1.3%), L. grayi (1%), L. ivanovii (1%) and L. ivanovi subsp. londoniensis (0.3%). All L. monocytogenes isolates identified were positive for the actA, iap, inlA, inlB, inlC, inlJ, plcA and prfA virulence genes and biofilm. The isolates were serotyped as 1/2c (33.3%), 4b (26.7%), 1/2a (26.7%), 1/2b (6.7%) and 3c (6.7%) by the multiplex-PCR and agglutination methods. PCR-restriction fragment length polymorphism with AluI and MluCI resulted in three and two profiles, respectively. Pulsed-field gel electrophoresis differentiated the L. monocytogenes isolates into 15 ApaI and 12 AscI patterns. Antimicrobial resistance of all Listeria isolates was determined by the disk diffusion method. Most L. monocytogenes isolates were sensitive to antimicrobials used in the treatment of listeriosis. This study shows the presence of potential pathogenic and antimicrobial-resistant L. monocytogenes in RTE foods that may lead to consumer health risks.

Unraveling the emergence and population diversity of Listeria monocytogenes in a newly built meat facility through whole genome sequencing

The food processing environments of a newly opened meat processing facility were sampled in ten visits carried out during its first 1.5 years of activity and analyzed for the presence of Listeria monocytogenes. A total of 18 L. monocytogenes isolates were obtained from 229 samples, and their genomes were sequenced to perform comparative genomic analyses. An increase in the frequency of isolation of L. monocytogenes and in the diversity of sequence types (STs) detected was observed along time. Although the strains isolated belonged to six different STs (ST8, ST9, ST14, ST37, ST121 and ST155), ST9 was the most abundant (8 out of 18 strains). Low (0 and 2) single nucleotide polymorphism (SNP) distances were found between two pairs of ST9 strains isolated in both cases 3 months apart from the same processing room (Lm-1267 and Lm-1705, with a 2 SNPs distance in the core genome; Lm-1265 and Lm-1706, with a 0 SNPs distance), which suggests that these strains may be persistent L. monocytogenes strains in the food processing environment. Most strains showed an in silico attenuated virulence potential either through the truncation of InlA (in 67% of the isolates) or the absence of other virulence factors involved in cell adhesion or invasion. Twelve of the eighteen L. monocytogenes isolates contained a plasmid, which ranged in size from 4 to 87 Kb and harbored stress survival, in addition to heavy metals and biocides resistance determinants. Identical or highly similar plasmids were identified for various sets of L. monocytogenes ST9 isolates, which suggests the clonal expansion and persistence of plasmid-containing ST9 strains in the processing environments of the meat facility. Finally, the analysis of the L. monocytogenes genomes available in the NCBI database, and their associated metadata, evidenced that strains from ST9 are more frequently reported in Europe, linked to foods, particularly to meat and pork products, and less represented among clinical isolates than other L. monocytogenes STs. It also showed that the ST9 strains here isolated were more closely related to the European isolates, which clustered together and separated from ST9 North American isolates.

Retrospective Study of Listeria Monocytogenes Isolated in the Territory of Inner Eurasia from 1947 to 1999

Listeriosis is one of the most significant humans and animals foodborne infectious diseases. Here, we characterized 48 Listeria monocytogenes strains isolated in the territory of inner Eurasia during the second half of the 20th century. A total of 23 strains (52.3%) were susceptible to the nine antibiotics tested, 30.43%, 15.22%, and 8.7% were resistant penicillin G, ampicillin, and enrofloxacin, respectively. We applied the multilocus sequence typing (MLST) scheme to determine the phylogenetic positions of the strains. All but one strain belonged to the II phylogenetic lineage, and the majority of the strains belonged to one of the previously described clonal complexes (СCs). More than 60% of the strains belonged to the clonal complex CC7 that prevailed among all sources, including cattle (58%), small ruminants (64%), rodents (71%), and humans (50%). Further, CC7, CC101, and CC124 were found among human isolates. The MLST scheme was supplemented with virulence gene analysis. In total, eight inlA, six inlB, and six inlC allelic variants were found, and all but one strain carried one of the two inlE alleles. Most strains (62.5%) belonged to the same multivirulence locus sequence typing (MvLST) type, which includes CC7, inlA allele 4, inlB allele 14, inlC allele 6, and inlE allele 8.

High resolution melting analysis for the characterization of lineage II Listeria monocytogenes serovars 1/2a and 1/2c based on single nucleotide polymorphisms identification within the Listeria Pathogenicity Island-1 and inlAB operon: a novel approach for epidemiological surveillance

AIMS

A high resolution melting (HRM) assay was developed for characterizing lineage II Listeria monocytogenes based on the amplification and the melting profiles analysis of 81 fragments targeting the region from the prs to ldh loci, including the Listeria Pathogenicity Island-1 (LIPI-1) genes and the inlAB operon.

METHODS AND RESULTS

Real-time PCR and HRM protocols were standardized using 10 replicate assays from L. monocytogenes EGD-e reference strain (serovar 1/2a). Twenty wild-type isolates of serovar 1/2a and two of serovar 1/2c were tested, and differences between EGD-e strain and the wild-type isolates were defined if the melting temperature (Tm ) of an amplicon was not within the lower and the upper limits calculated from replicate testing on EGD-e. The analysis revealed 17 and 19 HRM profiles with respect to prs/LIPI-1/ldh and inlAB target regions (Simpson's Index of Diversity 0·979 and 0·983) respectively. The 1/2c cultures showed 98·1% similarity to melting characteristics with EGD-e, whilst 1/2a isolates had the greatest heterogeneity that was related to inlA, inlB and actA genes. Sequencing of amplicons generating different Tm values from EGD-e confirmed the presence of point mutations.

CONCLUSIONS

This method was useful for L. monocytogenes subtyping based on single nucleotide polymorphisms detection through the melting behaviour analysis of main virulence genes.

SIGNIFICANCE AND IMPACT OF THE STUDY

The study underlines the effectiveness of HRM in differentiating L. monocytogenes strains with high discriminatory power, thus rendering it useful for epidemiological surveillance.

Benzalkonium chloride tolerance of Listeria monocytogenes strains isolated from a meat processing facility is related to presence of plasmid-borne bcrABC cassette

Listeria monocytogenes is a serious foodborne pathogen capable of persisting in food processing environments. Tolerance to disinfectants used in industrial settings constitutes an important factor of Listeria survival. In the present study, the mechanism of tolerance to benzalkonium chloride (BAC) was investigated in 77 L. monocytogenes isolates from a meat facility. By PCR approach, the mdrL and lde chromosomal efflux pump genes were detected in all isolates. No isolate was positive for qacH and emrE genes. However, the bcrABC cassette was present in 17 isolates of serogroup IIa possessing the same AscI/ApaI pulsotype, the operon being localized on a plasmid. The significant relation of BAC tolerance with bcrABC presence was confirmed as all bcrABC positive isolates showed the highest minimal inhibitory concentration (MIC) values for BAC and increased sensitivity to BAC was observed after plasmid curing. No effect of the efflux pump inhibitor reserpine on BAC tolerance in bcrABC positive strains was observed in contrast to all bcrABC negative strains. Lower ethidium bromide efflux in bcrABC positive isolates compared to bcrABC negative and plasmid-cured L. monocytogenes isolates was observed. The expression of bcrABC genes was BAC-induced. The confirmed effect of bcrABC to increased BAC tolerance, coupled with its plasmid location, may be an important factor in potential dissemination of the biocide resistance among Listeria species. The understanding of molecular mechanisms of biocide tolerance should help to improve control measures to prevent further spread of L. monocytogenes in food production environments with frequent use of BAC.

The Continuous Challenge of Characterizing the Foodborne Pathogen Listeria monocytogenes

Listeria monocytogenes is an important foodborne pathogen commonly isolated from food processing environments and food products. This organism can multiply at refrigeration temperatures, form biofilms on different materials and under various conditions, resist a range of environmental stresses, and contaminate food products by cross-contamination. L. monocytogenes is recognized as the causative agent of listeriosis, a serious disease that affects mainly individuals from high-risk groups, such as pregnant women, newborns, the elderly, and immunocompromised individuals. Listeriosis can be considered a disease that has emerged along with changing eating habits and large-scale industrial food processing. This disease causes losses of billions of dollars every year with recalls of contaminated foods and patient medical treatment expenses. In addition to the immune status of the host and the infecting dose, the virulence potential of each strain is crucial for the development of disease symptoms. While many isolates are naturally virulent, other isolates are avirulent and unable to cause disease; this may vary according to the presence of molecular determinants associated with virulence. In the last decade, the characterization of genetic profiles through the use of molecular methods has helped track and demonstrate the genetic diversity among L. monocytogenes isolates obtained from various sources. The purposes of this review were to summarize the main methods used for isolation, identification, and typing of L. monocytogenes and also describe its most relevant virulence characteristics.

Listeria monocytogenes: Strain Heterogeneity, Methods, and Challenges of Subtyping

Listeria monocytogenes is a food-borne bacterial pathogen that is associated with 20% to 30% case fatality rate. L. monocytogenes is a genetically heterogeneous species, with a small fraction of strains (serotypes 1/2a, 1/2b, 4b) implicated in human listeriosis. Monitoring and source tracking of L. monocytogenes involve the use of subtyping methods, with the performance of genetic-based methods found to be superior to phenotypic-based ones. Various methods have been used to subtype L. monocytogenes isolates, with the pulsed-field gel electrophoresis (PFGE) being the gold standard. Although PFGE has had a massive impact on food safety through the establishment of the PulseNet, there is no doubt that whole genome sequence (WGS) typing is accurate, has a discriminatory power superior to any known method, and allows genome-wide differences between strains to be quantified through the comparison of nucleotide sequences. This review focuses on the different techniques that have been used to type L. monocytogenes strains, their performance challenges, and the tremendous impact WGS typing could have on the food safety landscape.

Evaluation of transcription levels of inlA, inlB, hly, bsh and prfA genes in Listeria monocytogenes strains using quantitative reverse-transcription PCR and ability of invasion into human CaCo-2 cells

Listeria monocytogenes virulence depends on the activity of well-characterized virulence factors. In this study, transcription levels of inlA, inlB, hly, bsh and prfA genes in L. monocytogenes strains, and the ability of invasion into CaCo-2 cells were investigated. Serotyping, multiplex-PCR for serovar identification and restriction fragment analysis of inlA were performed. Transcription levels and invasiveness were evaluated by quantitative reverse-transcription PCR and by in vitro assays, respectively. The isolates were of serovars 1/2a, 4b, 1/2c, 1/2b and 3a. Full-length inlA profiles were found for nine of ten clinical isolates, while five of seven cultures from foods showed truncated profile. The analysis of transcription levels of virulence factors encoding genes demonstrated a substantial inter-strain heterogeneity, with clinical strains showing higher levels for almost all genes than isolates from food. A correlation between transcription levels of inlA and inlB, as well as between bsh and prfA, was observed. Significant differences between clinical strains and food isolates in the invasion of CaCo-2 cells were found. Analysis of gene transcription and invasiveness of human cells suggests different virulence phenotypes among L. monocytogenes populations, and this characterization could be a useful tool for risk assessment purposes and for the development of public health strategies.

Quantification of the extracellular matrix of the Listeria monocytogenes biofilms of different phylogenic lineages with optimization of culture conditions

AIMS

The purpose of this study was to quantify the extracellular matrix of Listeria monocytogenes biofilm. A preliminary study was carried out to establish a relationship between phylogenetic lineage of 27 strains and their ability to form biofilm in various conditions.

METHODS AND RESULTS

Biofilm formation on microtitre plates of 27 strains of L. monocytogenes belonging to lineages I or II was evaluated in different conditions [two temperatures (37 and 22°C) and two media (tryptone soy broth yeast extract medium (TSBYE) and MCDB 202 defined medium)] using crystal violet assay. Lineage II strains produced significantly more biofilm than lineage I strains. In microtitre plates assay, biofilm quantities were greater in MCDB 202 vs TSBYE medium [confirmed by scanning electron microscopy (SEM) analysis] and at 37 vs 22°C. Cultivable bacteria from biofilm population on Petri dishes were enumerated in greater quantities in TSBYE than in MCDB 202 medium. The SEM investigation established that L. monocytogenes biofilms produce extracellular matrix in both media at 37°C. The amount of exopolymers in the extracellular matrix and the pH values were significantly higher in TSBYE than in MCDB 202 medium. The exception was the ScottA strain that presented similar pH values and exopolymer contents in both media. Proteins were the most abundant exopolymer components, followed by DNA and polysaccharides.

CONCLUSIONS

The interpretation of results of biofilm quantification was depending on the growth conditions, the viability of the bacteria and the analysis method. The quantities of proteins, DNA and polysaccharides were different according to the strains and the medium.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study screened the potential of a wide panel of L. monocytogenes strains to synthesize exopolymers in biofilm growing condition. The characterization of L. monocytogenes biofilm composition may help to develop new strategies to prevent the formation of biofilms and to remove the biofilms.

Antimicrobial susceptibilities of Listeria monocytogenes strains isolated from food and food processing environment in Poland

A total of 471 Listeria monocytogenes isolates from different types of food and food-related sources in Poland during 2004-2010 were examined. This number includes 200 isolates from fish, 144 from fresh and frozen vegetables, 43 ready-to-eat products (deli foods, cold cuts), 13 from dairy products, 16 from raw meats, 15 from confectionery products and 40 directly from processing plants. All isolates were subjected to serotyping and lineage assays using PCR, and antimicrobial susceptibility using E-test and a broth microdilution method. Of all isolates, 256 (54.4%), 120 (25.5%), 59 (12.5%), 36 (7.6%) were identified as serotypes 1/2a (or 3a), 1/2c (or 3c), 1/2b (or 3b or 7), and 4b (or 4d or 4e), respectively. A direct correlation between the most common serotypes and three L. monocytogenes lineages was also observed. All L. monocytogenes isolates belonged to lineages I (20.2%) and II (79.8%). All strains were sensitive to ampicillin, amoxicillin, gentamicin, erythromycin, trimethoprim, rifampicin, vancomycin, chloramphenicol and sulfamethoxazol. Two of the L. monocytogenes strains (0.42%) showed phenotypic resistance. One strain was resistant to tetracycline and minocycline due to the presence of tet(M). It did not carry gene int, which may indicate that the tet(M) gene in this strain was not integrated in the transposon Tn916-Tn1545 family. The resistance of the second strain to ciprofloxacin and norfloxacin was attributed to active efflux associated with overexpression of gene lde. Our data indicate the low prevalence of antimicrobial resistance among L. monocytogenes isolates from food and food-related sources in Poland.

Gene Scanning of an Internalin B Gene Fragment Using High-Resolution Melting Curve Analysis as a Tool for Rapid Typing of Listeria monocytogenes

The ability to accurately track Listeria monocytogenes strains involved in outbreaks is essential for control and prevention of listeriosis. Because current typing techniques are time-consuming, cost-intensive, technically demanding, and difficult to standardize, we developed a rapid and cost-effective method for typing of L. monocytogenes. In all, 172 clinical L. monocytogenes isolates and 20 isolates from culture collections were typed by high-resolution melting (HRM) curve analysis of a specific locus of the internalin B gene (inlB). All obtained HRM curve profiles were verified by sequence analysis. The 192 tested L. monocytogenes isolates yielded 15 specific HRM curve profiles. Sequence analysis revealed that these 15 HRM curve profiles correspond to 18 distinct inlB sequence types. The HRM curve profiles obtained correlated with the five phylogenetic groups I.1, I.2, II.1, II.2, and III. Thus, HRM curve analysis constitutes an inexpensive assay and represents an improvement in typing relative to classical serotyping or multiplex PCR typing protocols. This method provides a rapid and powerful screening tool for simultaneous preliminary typing of up to 384 samples in approximately 2 hours.