Impact of Environmental pH Change on Survival and Growth of Vibrio parahaemolyticus

Vibrio parahaemolyticus is a foodborne pathogen recognized as the leading cause of acute gastroenteritis linked to seafood consumption, particular raw oysters, with symptoms of nausea, vomiting, abdominal cramps and diarrhea. The distribution of V. parahaemolyticus in the marine environments is known to relate to water temperature. However, impact of pH changes on the occurrence of V. parahaemolyticus in the environments 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) with pH value adjusted to 5.5. Five environmental isolates and 5 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. Inoculated TSB-salt was incubated at 35-37°C for 8 hr and populations of V. parahaemolyticus were determined by the 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. The populations increased by 5.4 to 6.6 log cfu/ml after 8 hr at 35-37°C. Growth of V. parahaemolyticus in TSB 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 that can be used to develop post-harvest processes for decontaminating V. parahaemolyticus in raw oysters.