Background: Salmonella enterica consists of over 2500 serovars and displays dichotomy in disease manifestations and host range. Except for the enrichment of pseudogenes in genomes for human-restricted serovars, no hallmark has been identified to distinguish those with host-generalist serovars. The serovar Sendai is rare and human-restricted. Notably, it exhibits an O, H antigen formula as the host-generalist serovar Miami.
Results: We sequenced the complete genomes of the two serovars Sendai and Miami. Analysis at both nucleotide identity and gene content level demonstrates the same high degree of similarity between Sendai and Paratyphi A, but their distinct CRISPR spacers suggests a recent divergence history. A frameshift mutation occurred in rfbE for the entire lineage of Paratyphi A but not in Sendai, which may explain their distinct O antigens. The nucleotide sequence of Miami's fliC is nearly identical to Sendai's. The incongruent phylogeny of this gene with that of the adjacent genes suggests a recombination event responsible for Sendai and Miami possessing the same H antigen. Sendai's even greater number of pseudogenes than that of Paratyphi A and Typhi indicates its undergoing continued genomic degradation. The phylogenetically distinct human-restricted serovars/strains share pseudogenes with the same inactivation mutations, therefore suggesting that recombination may have occurred and have been facilitated by their overlap in niches.
Conclusions: Analysis of Sendai's genome and comparison with others reflect the finer evolutionary signatures of Salmonella in the process of niches changing from facultative to obligate parasite.
Keywords: CRISPR; Host restriction; Pseudogene; Recombination; Serovar.