Mutations in the rfbT Gene Are Responsible for the Ogawa to Inaba Serotype Conversion in Vibrio cholerae O1

Abstract

In cultures of Vibrio cholerae strains of Ogawa serotype, variant strains which had undergone serotype conversion from Ogawa to Inaba were identified. The rfbT genes cloned from the parent strains were found to produce a 31-kDa protein in the maxicell system, and to cause serotype conversion when introduced into E. coli cells expressing Inaba serotype specificity. On the other hand, rfbT genes cloned from the variant strains neither produced the 31-kDa protein nor caused serotype conversion. Nucleotide sequence of these rfbT genes as well as those of two clinical Vibrio cholerae strains of Inaba serotype revealed that mutations causing premature termination of their rfbT genes were invariably present in strains expressing Inaba serotype specificity. The result strongly suggested that genetic alteration of the rfbT gene is responsible for serotype conversion of Vibrio cholerae O1.

Evidence for a clonally different origin of the two cholera epidemics of 2001–2002 and 1980–1987 in South Africa

“…It has been suggested that serotype switches are the result of selective pressure on the organism due to the acquisition of immunity. This was shown in previous work by Stroeher et al (1992), and has been borne out by others (Ito et al, 1993;Wachsmuth et al, 1993;Garg et al, 2000).…”

“…Other strains also showed point mutations within the wbeT gene that did not affect serotyping (Ito et al, 1993). Three further strains, two isolated from Mpumalanga (S69145/80 and S69175/80) and one from KwaZulu-Natal (K1290/01) (neighbouring provinces in South Africa), showed the same amino acid changes at position 157-160 (Fig.…”

“…This was confirmed by sequence analysis of the wbeT gene and was due to a deletional mutation at nucleotide 17 610, resulting in a premature stop codon at position 17 618. Other investigators have also observed that serotype switching may be the result of either deletional mutations or point mutations resulting in the truncation of the outer-membrane protein during translation (Stroeher et al, 1992;Ito et al, 1993;Manning et al, 1994). The reversal of this switch, Inaba to Ogawa, as occurred in the cholera epidemic in South America in the 1980s and India in the 1990s (Wachsmuth et al, 1993;Garg et al, 2000) would then require a second mutation, removing the premature stop codon from the gene.…”

“…It is due to a change in the genetic make-up of the wbeT (previously known as rfbT) gene (Reeves et al, 1996), resulting in premature truncation of the gene product (Manning et al, 1994;Stroeher et al, 1992). This genetic change may be due to either a point mutation or a deletion, both resulting in the transcription and translation of a premature stop codon (Stroeher et al, 1992;Ito et al,serotype Inaba carries antigens A and C. The third serotype, V. cholerae O1 Hikojima, expresses antigens A, B and C. Expression of antigens A and B in V. cholerae O1 Ogawa would thus change to expression of antigens A and C in V. cholerae O1 Inaba (Manning et al, 1994).…”

Lipopolysaccharides of Escherichia coli K12 Strains That Express Cloned Genes for the Ogawa and Inaba Antigens of Vibrio cholerae O1: Identification of O‐Antigenic Factors

“…Further subcloning experiments (11,25) showed that the gene designated rfbT, encoding a protein of 33 kDa, is responsible for the expression of the Ogawa 0-form specificity. Ito et al (13) reported that mutations in the rfbT gene resulted in disappearance of the ability to produce the full-length protein of 33 kDa and consequently caused the conversion of the serotype from Ogawa to Inaba. In the present study, LPS was isolated from strains of E. coli K12 in which cosmid clones derived from V cholerae 01 NIH 41 (Ogawa) and NIH 35A3 (Inaba) had been introduced.…”