ABSTRACT
The recent expansion of the seventh cholera pandemic into South America emphasizes the need for a safe, long-lasting, protective, and nonreactogenic vaccine for this disease. Since the predominant Vibrio cholerae O1 strains in the world today are of the El Tor biotype, a bivalent vaccine containing both classical and El Tor biotypes may be desirable. We have constructed a new oral vaccine candidate, V. cholerae CVD110 El Tor, Ogawa, from which all toxin genes so far identified in V. cholerae have been deleted. Three of these genes, those encoding cholera toxin (ctx), zonula occludens toxin (zot), and accessory cholera enterotoxin (ace), are located on a 4.5-kb virulence cassette flanked by repetitive sequences (RS1 elements). Homologous recombination between these RS1 elements resulted in the deletion of this virulence cassette to yield V. cholerae CVD109. Insertion of genes encoding mercury resistance (mer) and the cholera toxin B subunit (ctxB) into the hemolysin locus (hlyA) produced CVD110. This insertion serves three purpose. (i) It genetically tags the vaccine strain so as to distinguish it from wild-type V. cholerae O1. (ii) It produces cholera toxin B subunit in order to elicit antitoxic immunity. (iii) It inactivates the hemolysin gene, rendering the strain nonhemolytic on sheep erythrocyte plates. Supernatants from V. cholerae CVD110 cultures are nonreactogenic when assayed in Ussing chambers.
