Salmonella paratyphi C: genetic divergence from Salmonella choleraesuis and pathogenic convergence with Salmonella typhi.
BACKGROUND
Although over 1400 Salmonella serovars cause usually self-limited gastroenteritis in humans, a few, e.g., Salmonella typhi and S. paratyphi C, cause typhoid, a potentially fatal systemic infection. It is not known whether the typhoid agents have evolved from a common ancestor (by divergent processes) or acquired similar pathogenic traits independently (by convergent processes). Comparison of different typhoid agents with non-typhoidal Salmonella lineages will provide excellent models for studies on how similar pathogens might have evolved.


RESULTS
We sequenced a strain of S. paratyphi C, RKS4594, and compared it with previously sequenced Salmonella strains. RKS4594 contains a chromosome of 4,833,080 bp and a plasmid of 55,414 bp. We predicted 4,640 intact coding sequences (4,578 in the chromosome and 62 in the plasmid) and 152 pseudogenes (149 in the chromosome and 3 in the plasmid). RKS4594 shares as many as 4346 of the 4,640 genes with a strain of S. choleraesuis, which is primarily a swine pathogen, but only 4008 genes with another human-adapted typhoid agent, S. typhi. Comparison of 3691 genes shared by all six sequenced Salmonella strains placed S. paratyphi C and S. choleraesuis together at one end, and S. typhi at the opposite end, of the phylogenetic tree, demonstrating separate ancestries of the human-adapted typhoid agents. S. paratyphi C seemed to have suffered enormous selection pressures during its adaptation to man as suggested by the differential nucleotide substitutions and different sets of pseudogenes, between S. paratyphi C and S. choleraesuis.


CONCLUSIONS
S. paratyphi C does not share a common ancestor with other human-adapted typhoid agents, supporting the convergent evolution model of the typhoid agents. S. paratyphi C has diverged from a common ancestor with S. choleraesuis by accumulating genomic novelty during adaptation to man.
Liu
Wei-Qiao
WQ
Genomics Research Center, Harbin Medical University, Harbin, China.
Feng
Ye
Y
Wang
Yan
Y
Zou
Qing-Hua
QH
Chen
Fang
F
Guo
Ji-Tao
JT
Peng
Yi-Hong
YH
Jin
Yan
Y
Li
Yong-Guo
YG
Hu
Song-Nian
SN
Johnston
Randal N
RN
Liu
Gui-Rong
GR
Liu
Shu-Lin
SL
D000818:Animals; D005798:Genes, Bacterial / Q000235:genetics*; D016680:Genome, Bacterial; D006801:Humans; D010802:Phylogeny; D012475:Salmonella / Q000235:genetics*; D012476:Salmonella arizonae / Q000235:genetics; D012479:Salmonella paratyphi C / Q000235:genetics; D012485:Salmonella typhi / Q000235:genetics; D012641:Selection, Genetic; D017422:Sequence Analysis, DNA; D013045:Species Specificity; D013552:Swine