Contrasting Genetic Diversity of Pathogenicity Islands 3 and 4 Harbored by Nonpathogenic spp.

Listeria monocytogenes causes the severe foodborne disease listeriosis. Several clonal groups of L. monocytogenes possess the pathogenicity islands pathogenicity island 3 (LIPI-3) and LIPI-4. Here, we investigated the prevalence and genetic diversity of LIPI-3 and LIPI-4 among 63 strains of seven nonpathogenic spp. from the natural environment, i.e., wildlife (black bears []) and surface water. Analysis of the whole-genome sequence data suggested that both islands were horizontally acquired but differed considerably in their incidence and genetic diversity. LIPI-3 was identified among half of the strains in the same genomic location as in L. monocytogenes ( hot spot) in a truncated form, with only three strains harboring full-length LIPI-3, and a highly divergent partial LIPI-3 was observed in three Listeria seeligeri strains, outside the hot spot. Premature stop codons (PMSCs) and frameshifts were frequently noted in the LIPI-3 gene encoding listeriolysin S. On the other hand, full-length LIPI-4 without any PMSCs was found in all Listeria innocua strains, in the same genomic location as L. monocytogenes and with ~85% similarity to the L. monocytogenes counterpart. Our study provides intriguing examples of genetic changes that pathogenicity islands may undergo in nonpathogenic bacterial species, potentially in response to environmental pressures that promote either maintenance or degeneration of the islands. Investigations of the roles that LIPI-3 and LIPI-4 play in nonpathogenic spp. are warranted to further understand the differential evolution of genetic elements in pathogenic versus nonpathogenic hosts of the same genus. Listeria monocytogenes is a serious foodborne pathogen that can harbor the pathogenicity islands pathogenicity island 3 (LIPI-3) and LIPI-4. Intriguingly, these have also been reported in nonpathogenic from food and farm environments, though limited information is available for strains from the natural environment. Here, we analyzed whole-genome sequence data of nonpathogenic spp. from wildlife and surface water to further elucidate the genetic diversity and evolution of LIPI-3 and LIPI-4 in . While the full-length islands were found only in , LIPI-3 was uncommon and exhibited frequent truncation and genetic diversification, while LIPI-4 was remarkable in being ubiquitous, albeit diversified from L. monocytogenes. These contrasting features demonstrate that pathogenicity islands in nonpathogenic hosts can evolve along different trajectories, leading to either degeneration or maintenance, and highlight the need to examine their physiological roles in nonpathogenic hosts.