The purifying trend in the chromosomal integron in Vibrio cholerae strains during the seventh pandemic.

Chromosomal integron (CI) arrays in Vibrio spp. are generally large and display great variation. Here we determined the sequence of CI array in a toxigenic O139 Vibriocholerae strain and compared it with the arrays from the genome of different O1 biotypes available in GenBank. Then PCR scanning was used to determine the CI array variations in 83 epidemic O139 strains and subsequently these variations were compared with that found in toxigenic O1 El Tor strains in our previous work. Few differences were observed in the cohort of toxigenic O139 strains compared to the toxigenic O1 El Tor strains. On the basis of CI arrays, the toxigenic O1 El Tor and O139 strains isolated concurrently in recent years appear to be more similar to each other than to the O1 strains isolated in previous decades, suggesting a closer evolutionary relationship between them. Comparison of CI arrays in toxigenic O1 El Tor and O139 V. cholerae strains isolated between 1961 and 2009 revealed a purifying trend in the CI arrays in the chronological order during the seventh pandemic.