Evolution of Vibrio cholerae
Changes in the populations of V. cholerae O1 are highly dynamic and unusual among bacterial pathogens. A significant hallmark of V. cholerae is that the previously prevalent strains disappear globally when new variant strains emerge. Seven cholera pandemics have been recognized since the early 19th century. The sixth and seventh (current) cholera pandemics are attributed to the classical and El Tor biotype strains, respectively. El Tor strains primarily contain a satellite phage, RS1, at the CTX phage integration site of chromosome 1 and thus contain various arrays of RS1 and CTX, whereas no elements are integrated on chromosome 2. The seventh cholera pandemic remains ongoing, lasting for more than 50 years, which is much longer compared with earlier cholera pandemics. However, the El Tor strains in the seventh pandemic have undergone several changes that are subtle but considered to be significant with regard to the worldwide spread of the disease. Based on their genome analysis and the CTX phage, the seventh pandemic strains have been categorized into three waves of global dissemination.