Atypical Hemolytic<i>Listeria innocua</i>Isolates Are Virulent, albeit Less than<i>Listeria monocytogenes</i>

Abstract

<i>Listeria innocua</i> is considered a nonpathogenic <i>Listeria</i> species. Natural atypical hemolytic <i>L. innocua</i> isolates have been reported but have not been characterized in detail. Here, we report the genomic and functional characterization of representative isolates from the two known natural hemolytic <i>L. innocua</i> clades. Whole-genome sequencing confirmed the presence of <i>Listeria</i> pathogenicity islands (LIPI) characteristic of <i>Listeria monocytogenes</i> species. Functional assays showed that LIPI-1 and <i>inlA</i> genes are transcribed, and the corresponding gene products are expressed and functional. Using <i>in vitro</i> and <i>in vivo</i> assays, we show that atypical hemolytic <i>L. innocua</i> is virulent, can actively cross the intestinal epithelium, and spreads systemically to the liver and spleen, albeit to a lesser degree than the reference <i>L. monocytogenes</i> EGDe strain. Although human exposure to hemolytic <i>L. innocua</i> is likely rare, these findings are important for food safety and public health. The presence of virulence traits in some <i>L. innocua</i> clades supports the existence of a common virulent ancestor of <i>L. monocytogenes</i> and <i>L. innocua</i>.