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Background  The bacterial genus  Listeria  contains pathogenic and non-pathogenic species, including the pathogens  L. monocytogenes  and  L. ivanovii , both of which carry homologous virulence gene clusters such as the  prfA  cluster and clusters of internalin genes. Initial evidence for multiple deletions of the  prfA  cluster during the evolution of  Listeria  indicates that this genus provides an interesting model for studying the evolution of virulence and also presents practical challenges with regard to definition of pathogenic strains.    Results  To better understand genome evolution and evolution of virulence characteristics in  Listeria , we used a next generation sequencing approach to generate draft genomes for seven strains representing  Listeria  species or clades for which genome sequences were not available. Comparative analyses of these draft genomes and six publicly available genomes, which together represent the main  Listeria  species, showed evidence for (i) a pangenome with 2,032 core and 2,918 accessory genes identified to date, (ii) a critical role of gene loss events in transition of  Listeria  species from facultative pathogen to saprotroph, even though a consistent pattern of gene loss seemed to be absent, and a number of isolates representing non-pathogenic species still carried some virulence associated genes, and (iii) divergence of modern pathogenic and non-pathogenic  Listeria  species and strains, most likely circa 47 million years ago, from a pathogenic common ancestor that contained key virulence genes.    Conclusions  Genome evolution in  Listeria  involved limited gene loss and acquisition as supported by (i) a relatively high coverage of the predicted pan-genome by the observed pan-genome, (ii) conserved genome size (between 2.8 and 3.2 Mb), and (iii) a highly syntenic genome. Limited gene loss in  Listeria  did include loss of virulence associated genes, likely associated with multiple transitions to a saprotrophic lifestyle. The genus  Listeria  thus provides an example of a group of bacteria that appears to evolve through a loss of virulence rather than acquisition of virulence characteristics. While  Listeria  includes a number of species-like clades, many of these putative species include clades or strains with atypical virulence associated characteristics. This information will allow for the development of genetic and genomic criteria for pathogenic strains, including development of assays that specifically detect pathogenic  Listeria  strains.

Comparative genomics of the bacterial genus Listeria: Genome evolution is characterized by limited gene acquisition and limited gene loss