Effects of the Monomeric Components of Poly-hydroxybutyrate-co-hydroxyhexanoate on the Growth of Vibrio penaeicida In Vitro and on the Survival of Infected Kuruma Shrimp (Marsupenaeus japonicus)

SIMPLE SUMMARY: Outbreaks of bacterial disease in shrimp aquaculture are major causes of economic and production losses. It is, therefore, important to prevent such outbreaks, preferably without the use of antibiotics to avoid encouraging the development of antibiotic-resistant bacterial strains. Several hydroxyalkanoic acids are known to inhibit the growth of pathogenic Vibrio species. Poly-hydroxybutyrate-co-hydroxyhexanoate is a biodegradable, water-insoluble polymer comprising two hydroxyalkanoic acids: 3-hydroxybutyrate and 3-hydroxyhexanoate. Here, we evaluated the inhibitory activities of 3-hydroxybutyrate and 3-hydroxyhexanoate on the growth of the shrimp-pathogenic bacterium Vibrio penaeicida in vitro, and the effect of adding poly-hydroxybutyrate-co-hydroxyhexanoate to the diet on the survival of infected kuruma shrimp (Marsupenaeus japonicus). We found that the activities of 3-hydroxybutyrate and 3-hydroxyhexanoate were pH dependent and that they inhibited bacterial growth at pH close to that found in the shrimp gut (pH 5.9–6.7). Moreover, in shrimp infected with V. penaeicida, survival rates were significantly increased in individuals reared on feed containing poly-hydroxybutyrate-co-hydroxyhexanoate compared with those reared on standard diet, without any negative effects on shrimp growth. These findings suggest that supplementation of standard diet with poly-hydroxybutyrate-co-hydroxyhexanoate could protect aquaculture shrimp from infection by V. penaeicida, which is expected to increase production and reduce overall operational costs. ABSTRACT: 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 Vibrio penaeicida. In vitro experiments revealed that 3HH showed greater growth inhibitory activity than 3HB against V. penaeicida. 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 V. penaeicida. In vivo bacterial challenge experiments revealed that survival rates in kuruma shrimp (Marsupenaeus japonicus) infected by V. penaeicida were significantly increased in shrimp reared on feed containing PHBH (0.1%–5% w/w 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 V. penaeicida, thereby increasing shrimp aquaculture productivity.