Responses of oysters and their hemocytes to clinical and environmental isolates of Vibrio parahaemolyticus

Interactions of Vibrio parahaemolyticus with oysters and oyster hemocytes were studied using three environmental isolates (1094, 1163 and ATCC 17802) and three clinical isolates (2030, 2062, 2107). Clinical isolates were from patients who became ill during the June 1998 food poisoning outbreak involving oysters from Galveston Bay in Texas. Environmental isolates were from oysters, crabs or sardines. All V. parahaemolyticus isolates possessed the thermolabile direct hemolysin (tlh) gene; only the clinical isolates had the thermostable direct hemolysin (tdh) gene (a putative virulence determinant). The capacity of oyster hemocytes to kill each V. parahaemolyticus isolate was examined in vitro using a novel dye reduction assay. Differences in killing by oyster hemocytes existed between and among environmental and clinical isolates. On average, environmental isolates were more susceptible to hemocyte killing than clinical isolates. Clinical isolate 2062 was more susceptible to killing by oyster hemocytes than the other two clinical isolates (2030, 2107) and displayed the most diffuse colony morphology on nutrient agar plates. Also, unlike the other two isolates, it lacked identifiable Alcian Blue stabilized capsular material that appears as irregularly distributed, spike-like, electron-dense deposits often observed spanning gaps between cells. Additional experiments showed that when oysters were exposed to mixtures of a clinical (2030) and an environmental (1163) isolate, higher numbers of the clinical isolate were found in tissue and hemolymph. The significance of this research is that differences in V. parahaemolyticus isolates are described that influence ways in which these bacterial pathogens interact with oysters