Distribution and evolutionary characteristics of non-hemolytic mutant of El Tor biotype epidemic Vibrio cholerae in China

ObjectiveTo analyze the distribution and evolutionary characteristics of the non-hemolytic mutant of El Tor biotype strain of Vibrio cholerae O1 in China during the 7^th global cholera pandemic.

MethodsToxigenic El Tor biotype strains of V. cholerae in different cholera pandemic periods since 1961 were selected for hemolytic phenotype test and whole genome sequencing. The temporal distribution of the non-hemolytic mutant was displayed. The sequence variation in hlyA gene was illustrated. ML tree was constructed based on the SNPs in the non-repetitive and non-recombinant genome regions of 230 El Tor biotype strains of V. cholerae O1 isolated in China since 1961. The distribution of the non-hemolytic mutants was mapped on the ML tree.

ResultsDuring the 7^th cholera pandemic, there was an obvious sequence type conversion of the hemolysin gene hlyA in the strains detected in China after the first phase of pandemic. The non-hemolytic mutants emerged after the first phase of pandemic and became a common type in the 2^nd pandemic in 1970s. The non-hemolytic mutant dispersed in the 2^nd, 3^rd pandemic and the inter-pandemic periods. However, certain clones of non-hemolytic mutant spread for several years.

ConclusionThe evolution of hlyA gene was independent of that of the whole genome in the 7^th cholera pandemic, which might be a random mutation. The results showed the role of hemolytic phenotype in the biotyping of epidemic strains.