Bacterial flora in the digestive tract of cultured Pacific oyster

Abstract

Seasonal surveys on aerobic bacterial flora in the digestive tract of 10 healthy Pacific oyster (Crassostrea gigas), which had been cultured in Ondo, Hiroshima Prefecture, were carried out in 4 seasons for 2 years. The number of bacteria in the digestive tract grown on ZoBell's 2216e agar was about 105 (104.3-105.6) CFU/g, and no statistically significant differences in the number were observed between seasons. The intestinal microflora was dominated by the genus Vibrio followed by genera Cytophaga, Alteromonas, Moraxella and Pseudomonas which occupied over 80% of the total flora in every season. Among Vibrio species, five major species (V. splendidus I/II, V. harveyi, V. campbellii and V. pelagius II) were predominantly isolated, especially V. harveyi being most common (48%) in the summer. This species fluctuation and a result of an in vitro growth temperature test on the summer-isolated and the winter-isolated vibrios supported the presence of seasonal changes in dominant bacterial species in the oysters. An additional survey was tried on the hemolymph of 21 healthy oysters in winter, and one species of bacterium was isolated in a pure state on ZoBell agar at 103.6 CFU/mL. This isolate was identified to the genus Vibrio and confirmed to be virulent for adult oysters by an injection challenge.