Molecular analysis and identification of virulence gene on pR(ST98) from multi-drug resistant Salmonella typhi.

pR(ST98) is a large and conjugative resistant plasmid (R plasmid) of 98.6 mega-dalton from multi-drug resistant Salmonella typhi (S. typhi), which was classified to incompatibility group C (Inc C). It has been found that pRST98 made its host bacteria not only antibiotic resistant but also more virulent. In this study we explored the possibility of plasmid pRST98 in S. typhi carrying the Salmonella plasmid virulence gene - spv. The plasmid pRST98 was isolated, purified and then digested by nine restriction endonucleases to make the plasmid enzyme profile. Spv-specific PCR and Southern blot were applied to identify the virulence gene on pR(ST98). The amplified spv fragments spvR and spvB were cloned into pGEM-T EASY and then the DNA sequences were analysed. The fragments of pRST98 digested by endonucleases Bgl II, Pst I and Sac II were identified, which may be useful for molecular analysis and further epidemiological surveillance of pR(ST98). The results of PCR and Southern blot showed that spv homologous genetic sequence which had been found in all pathogenesis Salmonella spp. except S. typhi was also presented on pR(ST98). The ORF of spvR and spvB of pRST98 were 894 bp and 1,776 bp, respectively. They have more than 99% homology with that of spvR and spvB on virulence plasmid in S. typhmurium. The genotype research on pR(ST98) revealed that there is a plasmid carrying genes responsible for drug resistance and virulence in S. typhi. This is the first report for such kind chimerical plasmid in S. typhi.