Naturally occurring virulence-attenuated isolates of Listeria monocytogenes capable of inducing long term protection against infection by virulent strains of homologous and heterologous serotypes

Abstract

Experimental infections of mice with strains of Listeria spp. isolated from contaminated food sources allowed discrimination of strains into those either exhibiting high, attenuated or low virulence. Compared to the highly virulent L. monocytogenes strain EGD, an attenuated strain such as L99 persisted for shorter times (5 versus 10 days) in the infected host. Using a tissue culture cell model of infection, we found that, although strain L99 was capable of accumulating actin like its virulent counterpart following invasion, it was unable to generate the polarized actin tails required for intracellular and cell-to-cell movement. Immunoblot analysis using specific antiserum to the ActA polypeptide, a molecule that is necessary for movement of the bacterium within the eucaryotic cell, indicated that a slightly truncated form of this polypeptide was produced in the L99 strain. Despite its reduced virulence, the attenuated strain L99 was just as effective in generating protection in immune mice as the highly virulent strains, albeit with a 1000-fold higher infective dose. Based on the results obtained from this study, we suggest that one of the mechanisms accounting for widespread resistance in humans to infection by Listeria may be due to asymptomatic infections by naturally occurring strains attenuated for virulence.

Genetic and phenotypic characterization of Listeria monocytogenes lineage III

“…In contrast, Wiedmann et al (1997) reported a mean plaque size for lineage III isolates (n=5) that was significantly larger than the 10403S control (126?2±6?8 %). In that study, however, one small plaque-forming isolate (L99, plaque size 53?5±7?9 %) was excluded from analysis based on its previously reported virulence-attenuated phenotype (Chakraborty et al, 1994). When isolate L99 is included, however, the mean lineage III plaque size is 114±30 %, which is not significantly different from the mean plaque size for the lineage III isolates characterized here.…”

Development and Implementation of a Multiplex Single-Nucleotide Polymorphism Genotyping Assay for Detection of Virulence-Attenuating Mutations in theListeria monocytogenesVirulence-Associated GeneinlA

“…One of the major reasons we chose the SNaPshot multiplex kit is the ease through which the assay developed in this study can be modified to accommodate the detection of additional genetic markers by simply designing new extension primers and combining them into another multiplex reaction mixture. While previous studies described the presence of virulenceattenuating mutations in other key L. monocytogenes virulenceassociated genes (i.e., actA, hly, and inlB), only a single isolate or a few isolates were reported to carry these mutations (3,20,24,32). Mutations leading to the truncation of virulence factors other than InlA were not included in the SNP genotyping assay developed in this work because these mutations do not appear to have been accumulated at the population level and have not been shown to be responsible for attenuated virulence through the characterization of isogenic mutants in appropriate animal infection models.…”

Identification, cloning, and characterization of the Ima operon, whose gene products are unique to Listeria monocytogenes

“…However, these strains showed hybridization with lmaD and lmaC probes, suggesting that they probably harbor a deletion of the lmaBA genes. Heterogeneity in the expression of virulence traits in strains belonging to serogroup 4 has repeatedly been reported in the literature (4,38). We also analyzed L. innocua 6a, a serotype in which several strains have previously been reported to harbor the lmaA gene (15).…”