Increases in the Amounts of Vibrio spp. in Oysters upon Addition of Exogenous Bacteria

The bacterial pathogenVibrio vulnificus is found naturally in brackish coastal waters but can be greatly concentrated by filter-feeding organisms such as shellfish. Numerous experiments in which exogenousV. vulnificus cells are added to oysters in an attempt to measure uptake and depuration have been performed. In nearly all cases, results have shown that laboratory-grown bacteria are rapidly taken up by the oysters but ultimately eliminated, while naturally presentVibrio populations in oysters are resistant to depuration. In this study, oysters harvested during winter months, with low culturableVibrio concentrations, were incubated in aquaria supplemented with strains ofV. vulnificus that were either genotypically or phenotypically distinct from the background bacteria. These exogenous cells were eliminated from the oysters, as previously seen, but other vibrios already inhabiting the oysters responded to theV. vulnificus inoculum by rapidly increasing in number and maintaining a large stable population. The presence of such an oyster-adaptedVibrio population would be expected to prevent colonization by exogenousV. vulnificus cells, thus explaining the rapid depuration of these added bacteria.