Early warning systems for cholera: Using satellites to improve public health

Vibrio cholerae, the causative agent of cholera, is a bacterium autochthonous to the aquatic environment, and a serious public health threat. Other Vibrio spp. also pose a threat to human health. V. cholerae serogroup O1 is responsible for the previous two cholera pandemics, in which classical and El Tor biotypes were dominant in the 6th and the current 7th pandemics, respectively. Vibrio researchers continually face newly emerging and re-emerging pathogenic clones carrying various combinations of phenotypic and genotypic properties, which significantly hampered control of cholera and other Vibrio diseases. To elucidate evolutionary mechanisms governing genetic diversity of pandemic V. cholerae and related Vibrio species, the genome sequences of 23 V. cholerae strains isolated from a variety of sources over the past 98 years and 12 other Vibrio species were compared. The genome-based phylogeny revealed 12 distinct V. cholerae phyletic lineages, of which one comprises both O1 classical and El Tor biotypes. All 7th pandemic clones share nearly identical gene content. The transition from 6th to 7th pandemic strains is defined as a "shift" between pathogenic clones belonging to the same O1 serogroup, but from significantly different phyletic lineages. In contrast, transition among clones during the present 7th pandemic period is characterized as a "drift" between clones, differentiated mainly by varying composition of laterally-transferred genomic islands, resulting in emergence of variants, exemplified by V. cholerae O139 and V. cholerae O1 El Tor hybrid clones. Based on the comparative genomics it is concluded that V. cholerae undergoes extensive genetic recombination via lateral gene transfer, and, therefore, genome assortment, not serogroup, should be used to define pathogenic V. cholerae clones. Similar observations have been made with respect to other Vibrio spp., notably Vibrio parahaemolyticus.