Bacteriophage K139 was recently characterized as a temperate phage of O1 Vibrio cholerae. In this study we have determined the phage adsorption site on the bacterial cell surface. Phage-binding studies with purified lipopolysaccharide (LPS) of different O1 serotypes and biotypes revealed that the O1 antigen serves as the phage receptor. In addition, phage-resistant O1 El Tor strains were screened by using a virulent isolate of phage K139. Analysis of the LPS of such spontaneous phage-resistant mutants revealed that most of them synthesize incomplete LPS molecules, composed of either defective O1 antigen or core oligosaccharide. By applying phage-binding studies, it was possible to distinguish between receptor mutants and mutations which probably caused abortion of later steps of phage infection. Furthermore, we investigated the genetic nature of O1-negative strains by Southern hybridization with probes specific for the O antigen biosynthesis cluster (rfb region). Two of the investigated O1 antigen-negative mutants revealed insertions of element IS1004 into the rfb gene cluster. Treating one wbeW::IS1004 serum-sensitive mutant with normal human serum, we found that several survivors showed precise excision of IS1004, restoring O antigen biosynthesis and serum resistance. Investigation of clinical isolates by screening for phage resistance and performing LPS analysis of nonlysogenic strains led to the identification of a strain with decreased O1 antigen presentation. This strain had a significant reduction in its ability to colonize the mouse small intestine.Vibrio cholerae strains from serogroups O1 and O139 are the etiologic agents of cholera, a life-threatening acute diarrhea. The O1 serogroup is divided into the main serotypes Inaba and Ogawa, and O1 is subdivided into two distinct biotypes, designated classical and El Tor (22). Lipopolysaccharide (LPS) is the major integral component of the outer membrane and chemically consists of an O antigen, a core oligosaccharide, and lipid A. The O antigen of O1 V. cholerae consists of a homopolymer of approximately 18 (132) linked linear 4-(3-deoxy-L-glycero-tetronamido)-4,6-dideoxy-D-mannose) (23, 36). The LPS also contains the carbohydrate quinovosamine, which at the present time cannot be precisely defined as a component of either the O antigen or the core oligosaccharide (45). The Ogawa and Inaba serotypes differ by the presence of a 2-Omethyl group in the nonreducing terminal carbohydrate in the Ogawa O antigen (19,21). It was shown that Ogawa and Inaba O1 LPS can interconvert and that this serotype variation is due to spontaneous mutations in the wbeT gene (47). Strains of the serogroup O139 contain only a short O antigen but, in contrast to O1 strains, are encapsulated (51). Molecular and epidemiological analyses as well as phage typing revealed that O139 strains are very similar to O1 El Tor strains (2,17,18). One characteristic difference is the replacement of the 22-kb O1 rfb region with a 35-kb DNA fragment encoding the O139 O antigen and capsule (4,5,10,48)...