Abstract
Only Vibrio cholerae strains of serotype O1 are known to cause epidemics, while non-O1 strains are associated with sporadic cases of cholera. It was therefore unexpected that the recent cholera epidemic in Asia was caused by a non-O1 strain with the serotype O139. We provide evidence that O139 arose from a strain closely related to the causative agent of the present cholera pandemic, V. cholerae O1 El Tor, by acquisition of novel DNA which was inserted into, and replaced part of, the O antigen gene cluster of the recipient strain. Part of the novel DNA was sequenced and two open reading frames (otnA and otnB) were observed, the products of which showed homology to proteins involved in capsule and O antigen synthesis, respectively. This suggests that the otnAB DNA determines the distinct antigenic properties of the O139 cell surface. The otnAB DNA was not detected in O1 strains, but was present in two non-O1 V. cholerae strains with serotypes O69 and O141. In the O69 and O139 strains the otnAB genes were located proximate to the putative insertion sequence (IS) element rfbQRS, which is associated with O antigen synthesis genes in O1 strains, and may have played a role in the insertion of the otnAB DNA in the recipient chromosome. Our results suggest that the O139 strain arose by horizontal gene transfer between a non-O1 and an O1 strain. The acquired DNA has altered the antigenic properties of the recipient O1 strain, providing a selective advantage in a region where a large part of the population is immune to O1 strains.(ABSTRACT TRUNCATED AT 250 WORDS)