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

Abstract

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 lm fraction) and associated with microphytoplankton ([20 lm) or zooplankton ([60 lm), in the Karnaphuli estuary of Bangladesh during pre- and post-monsoon seasons. Cultivable Vibrio populations were *102 –104 colony forming units (CFU) ml-1 in the high saline zone (19–23 practical salinity unit, PSU) and declined in freshwater (\101 CFU ml-1 ). Culture independent detection of toxigenic V. cholerae O1 and O139 serogroups revealed a higher abundance of ‘‘free-living’’ (104 –105 cells l-1 ) than those attached to plankton (101 –103 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 lm 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 lm) fraction requires validation of the concept of the ‘‘free living’’ state of V. cholerae in aquatic habitats.