Survival and Growth of Clinical and Environmental isolates of Vibrio parahaemolyticus at various pH values
Vibrio parahaemolyticus is a foodborne pathogen recognized as the leading cause of acute gastroenteritis linked to seafood consumption, particular raw oysters. The distribution of V. parahaemolyticus in the marine environments is known to relate to water temperature. However, other environmental effects like pH is not clear. This study investigated survival and growth of environmental isolates and clinical strains of V. parahaemolyticus at various pH values and oyster movement in artificial seawater (ASW, 30 ppt) when pH maintained at 5.5. Five environmental isolates and five clinical strains of V. parahaemolyticus were each cultured in trypticase soy broth (TSB) containing 2% NaCl (TSB-salt) of pH 5.5, 7.3 or 9.0 at levels of 2-3 log cfu/ml. The inoculated TSB-salt was incubated at 35-37°C and the population was checked every hour until 8 hours by pour-plating method using trypticase soy agar containing 2% salt with incubation at 35-37°C for 24 hr. Movement of oysters in ASW (pH 5.5) at room temperature was monitored by a Gape Ometer and recorded every 5 min for up to 24 h. V. parahaemolyticus grew well in TSB at pH 7.3 and 9.0 with the populations increase by 5.4 to 6.6 log cfu/ml after 8 hr at 35-37°C. Growth of V. parahaemolyticus in TSB-salt at pH 5.5 was retarded with smaller increases in populations (1.2 to 5.2 log cfu/ml) were observed after 8 hr. Nonpathogenic strains were found to be less sensitive than pathogenic strains to pH 5.5. Oyster movement was observed while being held in ASW of pH 5.5. Understanding effects of pH on growth of V. parahaemolyticus and oyster gapping provide new information for developing post-harvest processes to reduce V. parahaemolyticus in raw oysters.