| Summary: | Here, we investigated the inhibitory effects of the biodegradable, water-insoluble polymer poly-hydroxybutyrate-co-hydroxyhexanoate (PHBH) and its two constituent monomers, the hydroxyalkanoic acids 3-hydroxybutyrate (3HB) and 3-hydroxyhexanoate (3HH), on the growth of the shrimp-pathogenic bacterium <i>Vibrio penaeicida</i>. In vitro experiments revealed that 3HH showed greater growth inhibitory activity than 3HB against <i>V. penaeicida</i>. In addition, the activities of hydroxyalkanoic acids were pH dependent, being greater at pH 6.0 than at pH 7.0. Investigation of the pH of the shrimp gut revealed a pH range of 5.9–6.7 (mean 6.29 ± SD 0.20), indicating that the physiological environment was suitable for 3HB and 3HH to exert their inhibitory activities against <i>V. penaeicida</i>. In vivo bacterial challenge experiments revealed that survival rates in kuruma shrimp (<i>Marsupenaeus japonicus</i>) infected by <i>V. penaeicida</i> were significantly increased in shrimp reared on feed containing PHBH (0.1%–5% <i>w</i>/<i>w</i> PHBH) compared with that in shrimp reared on standard diet alone. Supplementation with PHBH had no significant effects on three shrimp growth parameters: increase in body weight, daily feeding rate, and feed conversion ratio. These results suggest that supplementation of standard diet with PHBH will increase shrimp resistance to infection by <i>V. penaeicida</i>, thereby increasing shrimp aquaculture productivity.
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