Ribotyping has been widely used to characterise the seventh
pandemic clone including South American and O139 variants which
appeared in 1991 and 1992 respectively. To reveal the molecular
basis of ribotype variation we analysed the rrn operons
and their flanking regions. All but one variation detected by
BglI, the most discriminatory enzyme, was found to be due
to changes within the rrn operons, resulting from
recombination between operons. The recombinants are detected
because of the presence of a BglI site in the 16S gene in
three of the nine rrn operons and/or changes of intergenic
spacer types of which four variants were identified. As the
frequency of rrn recombination is high, ribotyping becomes
a less useful tool for evolutionary studies and long term
monitoring of the pathogenic clones of Vibrio cholerae as
variation could undergo precise reversion by the same
recombination event.
