Molecular phylogenetic analysis ofVibrio cholerae O1 El Tor strains isolated before, during and after the O139 outbreak based on the intergenomic heterogeneity of the 16S-23S rRNA intergenic spacer regions

We have cloned, sequenced and analysed all the five classes of the intergenic (16S-23S rRNA) spacer region (ISR) associated with the eightrrn operons (rrna-rrnh) ofVibrio cholerae serogroup O1 El Tor strains isolated before, during and after the O139 outbreak. ISR classes ‘a’ and ‘g’ were found to be invariant, ISR-B (ISRb and ISRe) exhibited very little variation, whereas ISR-C (ISRc, ISRd, and ISRf) and ISRh showed the maximum variation. Phylogenetic analysis conducted with all three ISR classes (ISR-B, ISR-C and ISRh) showed that the pre-O139 serogroup and post-O139 serogroup O1 El Tor strains arose out of two independent clones, which was congruent with the observation made by earlier workers suggesting that analyses of ISR-C and ISR-h, instead of all five ISR classes, could be successfully used to study phylogeny in this organism.