Aquatic disease outbreaks particularly from bacterial pathogens, represent a major barrier to more efficient fish production. Therefore, novel and innovative disease treatment and prevention strategies without dependence on antibiotics are essential for aquaculture to sustain increased production. Poly-β-hydroxybutyrate (PHB), a biopolymer synthesized by specific gram-negative and gram- positive bacteria, is one such compound with potential immunostimulatory capabilities, as seen in preliminary in vitro assays of the current study . For the in vivo feeding trial , PHB-synthesizing bacteria, Zobellella denitrificans , were produced on-site at the Texas A&M Aquacultural Research and Teaching Facility and then supplemented to a basal diet (36% crude protein and 6% c rude lipid) to produce five isonitrogenous and isolipidic experimental diets containing PHB in stepwise increments (0.125, 0.25, 0.5, 1.0, and 2.0% of dry-diet weight) . Two experimental control diets of similar proximate composition also were utilized; one practical diet (Basal) contained no supplementation of PHB-producing bacteria while a nother control contained 0.5% supplementation of a commercial purified PHB product to compare with the PHB produced by the bacteria and included the bacterial cell wall .
Groups of 15 juvenile Nile tilapia (~1.3 g/ fish initial weight) were stocked into 28, 38-L aquaria fashioned as a recirculating aquaculture system with quadruplicate aquaria randomly assigned to each dietary treatment and fed to apparent satiation for 8 weeks . Remarkably, Nile tilapia exhibited significant (P<0.05) dose-dependent linear and quadratic relationships of percentage weight gain (Figure 1) , as well as feed efficiency, protein conversion efficiency, and hepatosomatic index . Alternativ ely, intraperitoneal fat ratio and muscle yield ratio did not exhibit significant (P>0.05) linear and quadratic relationship s. Whole-body proximate composition (dry matter, crude protein, lipid, and ash) did not exhibit any significant relationship based on supplementation of graded doses of PHB . Immunological assays, namely intra- and extra- cellular superoxide anion production of head-kidney- derived macrophages exhibited significant (P<0.05) linear and quadratic relationships when graded doses of PHB (0.0, 0.5, 1.0, 2.0, 4.0 and 8.0 mM ) we re added to the cell culture media. However, oxidative radical species production of whole blood was not significant (P>0.05) . A disease challenge exposing experimental tilapia to Streptococcus iniae is currently being investigated. The current study indicates that supplementation of PHB-producing bacteria in the diets of juvenile Nile tilapia offers significant growth and immunological benefits when compared to a practical reference diet.