Variations in SXT elements in epidemic Vibrio cholerae O1 El Tor strains in China.

Ruibai Wang,Junjie Yue,Dong Yu,Biao Kan

doi:10.1038/srep22733

Abstract

Vibrio cholerae O1 El Tor biotype strains are responsible for three multiyear epidemics of cholera in China during the seventh ongoing pandemic. The presence of the integrative conjugative element SXT is strongly correlated with resistance to nalidixic acid, tetracycline, and trimethoprim-sulfamethoxazole in these strains. Here, we sequenced the conserved genes of the SXT element, including eex, setR, and int, from 59 V. cholerae O1 El Tor strains and extracted and assembled the intact SXT sequences from the 11 genome sequenced strains. These elements had characteristics distinct from those of previously reported integrative conjugative elements (ICEs). They could be clearly divided into two types based on the clustering of conserved genes and gene structures of the elements, showing their possibly independent derivation and evolution. These two types were present before and after 2005, respectively, demonstrating the type substitution that occurred in 2005. Four to six antibiotic-resistant genes were found on the SXT elements, including genes resistant to tetracycline, trimethoprim-sulfamethoxazole, and multiple drugs. In summary, our findings demonstrated the roles of the SXT element in the emergence of multidrug resistance in epidemic O1 El Tor V. cholerae strains in China.

Highlights

The SXT/R391 family is characterized by a conserved site-specific integrase that mediates integration into the 5′ -end of the prfC gene of the host chromosome; this gene encodes peptide chain release factor 3, which is responsible for the integration and excision roles found in lambdoid phages[5]
We explored the types and evolution of SXT elements in Chinese V. cholerae O1 strains by clustering the backbone genes of the SXT element
Our SXT genomic structure analysis described the differences between two sequence types of SXT elements, positioned the antibiotic resistance genes on the SXT elements, and presented the key role of this ICE in the emergence and transmission of multidrug resistance in V. cholerae

Summary

Results and Discussion

Clustering of SXT elements by three conserved genes. The int gene encodes the integrase enzyme of the SXT element. Of the 70 commonly shared ORFs of the 11 SXT elements, 1–49 showed nearly 100% homology to the ORFs between PMI2423 and PMI2482 of P. mirabilis strain HI4320 (GenBank: AM942759), which carries a conjugative transposon (PMI2423–PMI2491) similar to the ICE R391 from P. rettgeri[16]. The remaining 20 ORFs shared 100% homology to nucleotide sequences of the ICEs of V. cholerae, half of which showed no homology to the P. mirabilis strain HI4320 genome This may support the recombination of the different origins of these Chinese SXT ICEs. Based on the insertion sites of the 11 SXT elements (Fig. 4), we could group these elements into two distinct types. Insertion 2 contained the largest resistance genes, including the floR gene, which confers resistance to chloramphenicol, and the dhfR gene, encoding a trimethoprim-resistant dihydrofolate reductase, and was present in eight of the Strain ICDC-VC1605 ICDC-VC1627 ICDC-VC2605 ICDC-VC1909 ICDC-VC4210 ICDC-VC1944 ICDC-VC0143 ICDC-VC0956

Guizhou Xinjiang Jiangxi Guangxi Guangdong Liaoning

Materials and Methods

Additional Information