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 fermentation-negative 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.