Role of integrative and conjugative elements in heavy metal resistance of Vibrio cholerae

Background: Environmental surveillance across Russian regions has documented the consistent isolation of nontoxigenic Vibrio cholerae strains and the sporadic detection of individual toxigenic Vibrio cholerae О1 El Tor strains. Heavy metals in the environment are considered among the factors that may drive the evolution of V. cholerae. Assessment of the tolerance of nontoxigenic V. cholerae О1 strains from water sources confirmed the presence of isolates resistant to a wide range of heavy metals in various combinations. Few investigations have addressed the genetic basis of heavy metal tolerance in Vibrio species, and no data are available on heavy metal resistance genes in toxigenic V. cholerae.

Aim: The work aimed to identify potential genes conferring heavy metal resistance in toxigenic V. cholerae.

Methods: Two V. cholerae O1 strains—83 and 5879—and Escherichia coli QD5003 Rif ^r were used. For in silico analysis, 1880 complete genomes of toxigenic V. cholerae belonging to different serogroups and collected over different periods were screened.

Results: Comparative bioinformatic analysis using genome subtraction identified the czcA gene (NCBI GenBank accession number MVB73536.1) in V. cholerae strain 83. Analysis of the sequenced genomes of 1880 V. cholerae strains from various serogroups and collection periods detected the czcA gene in 159 of 1125 toxigenic strains (14.1%), but in none of the 775 nontoxigenic genomes. The presence of the czcA gene in V. cholerae strain 83 as part of an integrative and conjugative element of the ICEVchBan11 type was confirmed by in silico polymerase chain reaction (PCR) using virtual primers. Among the 159 czcA-positive genomes of V. cholerae, two integrative and conjugative element types were reliably identified: 113 ICEVchBan9 and 27 ICEVchBan11. To investigate the possible biological role of czcA, the ICEVchBan11 was transferred by conjugation into V. cholerae 5879 cells. The resulting czcA-positive 5879 strain cells showed a significant increase in resistance to cadmium ions.

Conclusion: These findings suggest that the presence of the czcA gene in toxigenic V. cholerae may enhance resistance to the toxic effects of heavy metals. In particular, this may provide the bacteria with a selective advantage in aquatic environments with elevated cadmium ion concentrations.