Characterization of virulence properties of Listeria monocytogenes serotype 4b strains of different origins

Small molecules targeting LapB protein prevent Listeria attachment to catfish muscle

Listeria monocytogenes is a Gram-positive foodborne pathogen and the causative agent of listeriosis. L. monocytogenes lapB gene encodes a cell wall surface anchor protein, and mutation of this gene causes Listeria attenuation in mice. In this work, the potential role of Listeria LapB protein in catfish fillet attachment was investigated. To achieve this, boron-based small molecules designed to interfere with the active site of the L. monocytogenes LapB protein were developed, and their ability to prevent L. monocytogenes attachment to fish fillet was tested. Results indicated that seven out of nine different small molecules were effective in reducing the Listeria attachment to catfish fillets. Of these, three small molecules (SM3, SM5, and SM7) were highly effective in blocking Listeria attachment to catfish fillets. This study suggests an alternative strategy for reduction of L. monocytogenes contamination in fresh and frozen fish products.

Listeria monocytogenes Strains Underrepresented during Selective Enrichment with an ISO Method Might Dominate during Passage through Simulated Gastric Fluid and In Vitro Infection of Caco-2 Cells

Various Listeria monocytogenes strains may contaminate a single food product, potentially resulting in simultaneous exposure of consumers to multiple strains. However, due to bias in strain recovery, L. monocytogenes strains isolated from foods by selective enrichment (SE) might not always represent those that can better survive the immune system of a patient. We investigated the effect of cocultivation in tryptic soy broth with 0.6% yeast extract (TSB-Y) at 10°C for 8 days on (i) the detection of L. monocytogenes strains during SE with the ISO 11290-1:1996/Amd 1:2004 protocol and (ii) the in vitro virulence of strains toward the Caco-2 human colon epithelial cancer cell line following exposure to simulated gastric fluid (SGF; pH 2.0)-HCl (37°C). We determined whether the strains which were favored by SE would be effective competitors under the conditions of challenges related to gastrointestinal passage of the pathogen. Interstrain competition of L. monocytogenes in TSB-Y determined the relative population of each strain at the beginning of SE. This in turn impacted the outcome of SE (i.e., favoring survival of competitors with better fitness) and the levels exposed subsequently to SGF. However, strong growth competitors could be outcompeted after SGF exposure and infection of Caco-2 cells by strains outgrown in TSB-Y and underdetected (or even missed) during enrichment. Our data demonstrate a preferential selection of certain L. monocytogenes strains during enrichments, often not reflecting a selective advantage of strains during infection. These findings highlight a noteworthy scenario associated with the difficulty of matching the source of infection (food) with the L. monocytogenes isolate appearing to be the causative agent during listeriosis outbreak investigations.IMPORTANCE This report is relevant to understanding the processes involved in selection and prevalence of certain L. monocytogenes strains in different environments (i.e., foods or sites of humans exposed to the pathogen). It highlights the occurrence of multiple strains in the same food as an important aspect contributing to mismatches between clinical isolates and infection sources during listeriosis outbreak investigations.

Enhanced surveillance of invasive listeriosis in the Lombardy region, Italy, in the years 2006-2010 reveals major clones and an increase in serotype 1/2a

BACKGROUND

Invasive listeriosis is a rare, life-threatening foodborne disease. Lombardy, an Italian region accounting for 16% of the total population, reported 55% of all listeriosis cases in the years 2006-2010. The aim of our study was to provide a snapshot of listeriosis epidemiology in this region after the implementation of a voluntary laboratory-based surveillance system.

METHODS

We characterized by serotyping, pulsed-field gel electrophoresis, multilocus sequence typing and detection of epidemic clone markers, 134 isolates from 132 listeriosis cases, including 15 pregnancy-related cases, occurring in the years 2006-2010 in Lombardy. Demographic and clinical characteristics of cases have also been described.

RESULTS

The mean age of non pregnancy-associated cases was 64.7 years, with 55.9% of cases being older than 65 years. Cases having no underlying medical conditions accounted for 11.6%. The all-cause fatality rate of 83 cases with a known survival outcome was 25.3%.Serotypes 1/2a and 4b comprised 52.2% and 38.8% of isolates, respectively. Seventy-three AscI pulsotypes and 25 sequence types assigned to 23 clonal complexes were recognized. Moreover, 53 (39.5%) isolates tested positive for the epidemic clone markers. Twelve molecular subtype clusters including at least three isolates were detected, with cluster 11 (1/2a/ST38) including 31 isolates identified during the entire study period. No outbreaks were notified to public health authorities during this period.

CONCLUSIONS

The findings of our study proved that epidemiology of listeriosis in Lombardy is characterized by a high prevalence of major clones and the increasing role of serotype 1/2a. Molecular subtyping is an essential tool in the epidemiology and surveillance of listeriosis. Rapid molecular cluster detection could alert about putative outbreaks, thus increasing the chance of detecting and inactivating routes of transmission.

Listeria monocytogenes ArcA contributes to acid tolerance

The foodborne pathogen Listeria monocytogenes is able to colonize the human and animal intestinal tracts and subsequently crosses the intestinal barrier, causing systemic infection. For successful establishment of infection, L. monocytogenes must survive and adapt to the low pH environment of the stomach. Gene sequence analysis indicates that lmo0043, an orthologue of arcA, encodes a protein containing conserved motifs and critical active amino acids characteristic of arginine deiminase that mediates an arginine deimination reaction. We attempted to characterize the role of ArcA in acid tolerance in vitro and in mice models. Transcription of arcA was significantly increased in L. monocytogenes culture subjected to acid stress at pH 4.8, as compared with that at pH 7.0. Deletion of arcA impaired growth of L. monocytogenes under mild acidic conditions at pH 5.5, and reduced its survival in synthetic human gastric fluid at pH 2.5 and in the murine stomach. Bacterial load in the spleen of mice intraperitoneally inoculated with an arcA deletion mutant was significantly lower than that of the wild-type strain. These phenotypic changes were recoverable by genetic complementation. Thus, we conclude that L. monocytogenes arcA not only mediates acid tolerance in vitro but also participates in gastric survival and virulence in mice.