Predatory Bacteria as Natural Modulators of Vibrio parahaemolyticus and Vibrio vulnificus in Seawater and Oysters

Abstract

ABSTRACTThis study shows that naturally occurringVibriopredatory bacteria (VPB) exert a major role in controlling pathogenic vibrios in seawater and shellfish. The growth and persistence ofVibrio parahaemolyticusandVibrio vulnificuswere assessed in natural seawater and in the Eastern oyster,Crassostrea virginica. The pathogens examined wereV. vulnificusstrain VV1003,V. parahaemolyticusO1:KUT (KUT stands for K untypeable), andV. parahaemolyticusO3:K6 and corresponding O3:K6 mutants deficient in thetoxRSvirulence regulatory gene or therpoSalternative stress response sigma factor gene. Vibrios were selected for streptomycin resistance, which facilitated their enumeration. In natural seawater, oysters bioconcentrated eachVibriostrain for 24 h at 22°C; however, counts rapidly declined to near negligible levels by 72 h. In natural seawater with or without oysters, vibrios decreased more than 3 log units to near negligible levels within 72 h. NeithertoxRSnorrpoShad a significant effect onVibriolevels. In autoclaved seawater,V. parahaemolyticusO3:K6 counts increased 1,000-fold over 72 h. Failure of the vibrios to persist in natural seawater and oysters led to screening of the water samples for VPB on lawns ofV. parahaemolyticusO3:K6 host cells. Many VPB, includingBdellovibrioand like organisms (BALOs;Bdellovibrio bacteriovorusandBacteriovorax stolpii) andMicavibrio aeruginosavorus-like predators, were detected by plaque assay and electron microscopic analysis of plaque-purified isolates from Atlantic, Gulf Coast, and Hawaiian seawater. WhenV. parahaemolyticusO3:K6 was added to natural seawater containing trace amounts of VPB,Vibriocounts diminished 3 log units to nondetectable levels, while VPB increased 3 log units within 48 h. We propose a new paradigm that VPB are important modulators of pathogenic vibrios in seawater and oysters.