Occurrence and distribution of plankton-associated and free-living toxigenic Vibrio cholerae in a tropical estuary of a cholera endemic zone

Cholera epidemics are thought to be influenced by changes in populations of estuarine Vibrio cholerae . We investigated the abundance and distribution of this bacterium, as “free-living” (<20 μm fraction) and associated with microphytoplankton (>20 μm) or zooplankton (>60 μm), in the Karnaphuli estuary of Bangladesh during pre- and post-monsoon seasons. Cultivable Vibrio populations were ~10 2 –10 4 colony forming units (CFU) ml −1 in the high saline zone (19–23 practical salinity unit, PSU) and declined in freshwater (<10 1 CFU ml −1 ). Culture independent detection of toxigenic V. cholerae O1 and O139 serogroups revealed a higher abundance of “free-living” (10 4 –10 5 cells l −1 ) than those attached to plankton (10 1 –10 3 cells l −1 ). However, “free-living” O1 and O139 cells were sometimes absent in the medium saline and freshwater areas (0.0–11 practical salinity unit [PSU]). In contrast, plankton samples always harbored these serogroups despite changes in salinity and other physico-chemical properties. Microphytoplankton and zooplankton were dominated by diatoms and blue-green algae, and copepods and rotifers, respectively. Toxigenic V. cholerae abundance did not correlate with plankton abundance or species but had a positive correlation with chitin in the <20 μm fraction, where suspended particulate matter (SPM), V. cholerae and chitin concentrations were highest. C:N ratios indicated that organic matter in SPM originated predominantly from plankton. The differential occurrence of “free-living” and attached V. cholerae suggests a pivotal function of plankton in V. cholerae spreading into freshwater areas. The probable association of this pathogen with organisms and particles in the nanoplankton (<20 μm) fraction requires validation of the concept of the “free living” state of V. cholerae in aquatic habitats.