Uptake and depuration of the opaque and translucent morphotypes Vibrio vulnificus and the effects of oyster passage on virulence

Vibrio vulnificus is an estuarine bacterium which is known to be a significant human pathogen. It occurs in high numbers in oysters and other molluscan shelfish, where it is part of the animals' normal flora. V. vulnificus occurs in two colony morphotypes; the opaque, encapsulated variety is virulent, whereas the translucent, non encapsulated variety is avirulent. Studies were carried out to determine if the opaque and translucent morphotypes were taken up by oysters (Crassostrea virginica) at different rates, and whether UV - assisted depuration proceeded at different rates. Possible interconversion of the opaque and translucent morphotypes was also examined, as was the possibility that the virulence ofVibrio vulnificus would be modified following passage through the oyster. All studies were carried out using a strain of V. vulnificus which harbors the transposon, TnphoA, which allowed the added cells of V. vulnificus to be readily differentiated from other bacteria naturally present in the oysters. Results indicated little difference in the rate of uptake or depuration of V. vulnificus by oysters (Figure 1). Uptake in either case was rapid (saturation appeared within 30-60 minutes), and depuration by the laboratory-infected oysters appeared complete within 48 hours. Conversions for opaque and translucent morphotypes in oysters were not significantly different from in vitro rates (Table 1). No conversion from the translucent (avirulent) to the opaque (virulent) form was seen. LD50 values in mice after oyster passage were unchanged (Table 2). Our results suggest that the presence of the capsule on V. vulnificus cells does not markedly contribute to its ability to be taken up or depurated by oysters, and that oyster passage neither increases nor decreases the virulence of this human pathogen.