Abstract
BackgroundShigella bacteria cause dysentery, which remains a significant threat to public health. Shigella flexneri is the most common species in both developing and developed countries. Five Shigella genomes have been sequenced, revealing dynamic and diverse features. To investigate the intra-species diversity of S. flexneri genomes further, we have sequenced the complete genome of S. flexneri 5b strain 8401 (abbreviated Sf8401) and compared it with S. flexneri 2a (Sf301).ResultsThe Sf8401 chromosome is 4.5-Mb in size, a little smaller than that of Sf301, mainly because the former lacks the SHI-1 pathogenicity island (PAI). Compared with Sf301, there are 6 inversions and one translocation in Sf8401, which are probably mediated by insertion sequences (IS). There are clear differences in the known PAIs between these two genomes. The bacteriophage SfV segment remaining in SHI-O of Sf8401 is clearly larger than the remnants of bacteriophage SfII in Sf301. SHI-1 is absent from Sf8401 but a specific related protein is found next to the pheV locus. SHI-2 is involved in one intra-replichore inversion near the origin of replication, which may change the expression of iut/iuc genes. Moreover, genes related to the glycine-betaine biosynthesis pathway are present only in Sf8401 among the known Shigella genomes.ConclusionOur data show that the two S. flexneri genomes are very similar, which suggests a high level of structural and functional conservation between the two serotypes. The differences reflect different selection pressures during evolution. The ancestor of S. flexneri probably acquired SHI-1 and SHI-2 before SHI-O was integrated and the serotypes diverged. SHI-1 was subsequently deleted from the S. flexneri 5b genome by recombination, but stabilized in the S. flexneri 2a genome. These events may have contributed to the differences in pathogenicity and epidemicity between the two serotypes of S. flexneri.
Highlights
Shigella bacteria cause dysentery, which remains a significant threat to public health
We present here the complete genome sequence of S. flexneri 5b Sf8401 and a comparison with the S. flexneri 2a Sf301 genome, which reveals differences in the pathogenicity islands and chromosomal rearrangements between different serotypes of this species
As more bacterial genomes have been sequenced during recent years, the study of comparative genomics has progressed rapidly
Summary
Shigella bacteria cause dysentery, which remains a significant threat to public health. Shigella flexneri is the most common species in both developing and developed countries. Shigella species that cause bacillary dysentery or shigellosis are Gram-negative, non-sporulating, facultative anaerobes, and the disease remains a major worldwide health problem. Shigella was recognized as the etiological agent of bacillary dysentery in the 1890s. It was adopted as a genus in the 1950s and sub-divided into 4 species: S. dysenteriae, S. flexneri, S. boydii and S. sonnei [3]. According to this taxonomy,S. flexneri is classified into 6 serotypes (including 13 subtypes). Most previous work on the molecular pathogenesis of Shigella has been carried out in S. flexneri serotypes 2a and 5
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