Resurgent Vibrio cholerae O139: Rearrangement of cholera toxin genetic elements and amplification of rrn operon

The unprecedented genesis of a novel non-Ol Vibrio cholerae strain belongin g to serogroup O139, which caused an epidemic in late 1992 in the Indian su bcontinent, and its subsequent displacement by El Tor O1 vibrios after 18 m onths initiated a renewed investigation of the aspects of the organism that are related to pathogenesis. The reappearance of I;: cholerae O139 with al tered antibiotic sensitivity compared to O139 Bengal (O139B) in late 1996 h as complicated the epidemiological scenario of V. cholerae and has necessit ated an examination of possible rearrangements in the genome underlying suc h rapid changes in the phenotypic traits, With a view to investigating whet her the phenotypic changes that have occurred are associated with alteratio n in the genome, the genome of the resurgent V. cholerae O139 (O139R) strai ns were examined, Pulsed-field gel electrophoresis analysis of NotI- and Sf iI-digested genomic DNA of O139R isolates showed restriction fragment lengt h polymorphism including in the cholera toxin (CTX) genetic element locus a nd with O139B isolates. Analyses of the organization of the CTS genetic ele ments in O139R strains showed that in contrast to two copies of the element s connected by two direct-repeat sequences (RS) in most of the genomes of O 139B isolates, the genomes of all O139R strains examined, except strain AS1 92, have three such elements connected by a single RS, While the RS present in the upstream of the CTX genetic elements in the genome of O139R is of O 139B origin, the RS connecting the cores of the elements has several new re striction sites and has lost the Bg/II site which is supposed to be conserv ed in all O1 strains and O139B, The endonuclease I-CeuI, which has sites on ly in the rm operons in the genomes of all organisms examined so far, has 1 0 sites in the genomes of O139R strains, compared to 9 in the genomes of O1 39B strains. The recent isolates of V. cholerae O139 have thus gained one r m operon. This variation in the number of rrn operons within a serogroup ha s not been reported for any other organism. The results presented in this r eport suggest that like the pathogenic El Tor O1 strains, the genomes of O1 39 strains are undergoing rapid alterations.