Responses of oyster Crassostrea virginica hemocytes to environmental and clinical isolates of Vibrio parahaemolyticus

Ingestion of bacteria by oysters Crassostrea virginica and bactericidal activity of oyster hemocytes were studied using 4 environmental isolates (shellfish) and 3 clinical isolates (fecal) of Vibrio parahaemolyticus. Clinical isolates (2030, 2062, 2107) were obtained from the feces of patients with gastroenteritis who became ill during the 1998 food poisoning outbreak traced to consumption of raw oysters from Galveston Bay, Texas. This outbreak was the first reported occurrence in the United States of the virulent serotype O3:K6. Environmental isolates were from oysters (1094, 1100), crab (1163) and sardines (ATCC 17802). All isolates possessed the thermolabile direct hemolysin (tlh) gene, whereas only the clinical isolates possessed the thermostable direct hemolysin (tdh) gene, a virulence determinant. On average, environmental isolates were more susceptible than clinical isolates to killing by oyster hemocytes, as determined by an in vitro dye reduction assay. Isolate 2062 was the most susceptible of the clinical isolates; it lacked identifiable capsular material present in the other clinical isolates and displayed the most diffuse colony morphology on nutrient agar plates. When oysters were exposed in vivo to mixtures of a clinical (2030) and an environmental (1163) isolate, more clinical than environmental isolates were found in the tissues and hemolymph.