Natural Atypical Listeria innocua Strains with Listeria monocytogenes Pathogenicity Island 1 Genes

Abstract

Identification of bona fide Listeria isolates into the six species of the genus normally requires only a few tests. Aberrant isolates do occur, but even then only one or two extra confirmatory tests are generally needed for identification to species level. We have discovered a hemolytic-positive, rhamnose and xylose fermentationnegative Listeria strain with surprising recalcitrance to identification to the species level due to contradictory results in standard confirmatory tests. The issue had to be resolved by using total DNA-DNA hybridization testing and then confirmed by further specific PCR-based tests including a Listeria microarray assay. The results show that this isolate is indeed a novel one. Its discovery provides the first fully documented instance of a hemolytic Listeria innocua strain. This species, by definition, is typically nonhemolytic. The L. innocua isolate contains all the members of the PrfA-regulated virulence gene cluster (Listeria pathogenicity island 1) of L. monocytogenes. It is avirulent in the mouse pathogenicity test. Avirulence is likely at least partly due to the absence of the L. monocytogenes-specific allele of iap, as well as the absence of inlA, inlB, inlC, and daaA. At least two of the virulence cluster genes, hly and plcA, which encode the L. monocytogenes hemolysin (listeriolysin O) and inositol-specific phospholipase C, respectively, are phenotypically expressed in this L. innocua strain. The detection by PCR assays of specific L. innocua genes (lin0198, lin0372, lin0419, lin0558, lin1068, lin1073, lin1074, lin2454, and lin2693) and noncoding intergenic regions (lin0454-lin0455 and nadA-lin2134) in the strain is consistent with its L. innocua DNA-DNA hybridization identity. Additional distinctly different hemolytic L. innocua strains were also studied.

A Population Genetics-Based and Phylogenetic Approach to Understanding the Evolution of Virulence in the Genus Listeria

“…londoniensis. The phylogenetic placement of atypical hemolytic L. innocua isolates in the L. innocua clade was confirmed by our analysis and is consistent with previous studies (31,68).…”

“…Importantly, the presence or absence of the prfA cluster and virulence characteristics can also be used to classify Listeria species and clades into three groups, including (i) species that do contain the prfA virulence cluster and are known pathogens, like L. monocytogenes and L. ivanovii, (ii) species that lack the prfA virulence cluster and are nonpathogenic (L. marthii and L. welshimeri), and (iii) species in which the presence of the prfA virulence cluster varies by strain. The last group contains L. seeligeri, which is nonpathogenic, although the majority of strains in the population contain the prfA virulence cluster (69), and L. innocua, which is also nonpathogenic, and although most strains lack the prfA virulence cluster, a small proportion of strains do carry this cluster (31,68). The facts that the genus Listeria contains closely related nonpathogenic and pathogenic species and that strains with and without the prfA cluster within the same species make this genus an interesting model system for studies on the evolution of pathogenicity in opportunistic environmental pathogens.…”

Investigation of Specific Substitutions in Virulence Genes Characterizing Phenotypic Groups of Low-Virulence Field Strains of Listeria monocytogenes

“…The common evolution of some low-virulence strains is reinforced by analysis of the DNA macrorestriction profiles, which are different from those of the 623 virulent L. monocytogenes strains tested (A. Kerouanton, personal communication). It now seems important to investigate whether these substitutions are due to a common ancestry or to horizontal transfer, as is the case for the natural atypical L. innocua (21).…”

Detection of Listeria Spp. During Production and Ripening of Petrovská Klobása/Detekcija Listeria Spp. U Toku Proizvodnje I Zrenja Petrovačke Kobasice

“…To this conclusion we reached after PCR , based on L. monocytogenes specifi c hlyA encoding listeriolysin ( Figure 1) and after sequencing of the obtained hlyA DNA fragment. Literature data indicate this aberrant strain is exceptional in that it contains the pathogenicity island [22] and a homologue of a surface protein, internalin inlA [23]. L. innocua strain FSL J1-023 is hemolytic-positive, rhamnose and xylose fermentationnegative Listeria strain which gives contradictory results in standard confi rmatory tests [22].…”

“…Literature data indicate this aberrant strain is exceptional in that it contains the pathogenicity island [22] and a homologue of a surface protein, internalin inlA [23]. L. innocua strain FSL J1-023 is hemolytic-positive, rhamnose and xylose fermentationnegative Listeria strain which gives contradictory results in standard confi rmatory tests [22]. The genomic sequence of this strain is of great importance because it will help us to understand the overall importance of homologue recombinations in the evolution of L. innocua and L. monocytogenes.…”