SCREENING OF PROBIOTIC CANDIDATES FROM THE REARING WATER OF TILAPIA Oreochromis sp. IN A FRESHWATER BIOFLOC SYSTEM
Biofloc technology (BFT) has been shown to improve aquaculture productivity and sustainability. This system applies the principle of assimilation of dissolved nitrogenous wastes that are excreted by the fish and also through the breakdown of organic nitrogen as microbial protein by heterotrophic bacteria by managing the carbon/nitrogen (C/N) ratio in the water. This heterotrophic bacterial biomass further forms macroaggregates known as biofloc that could serve as food for the fish. Within these bioflocs, potential probionts may reside. Hence, by screening and assessing the bacterial flora in the rearing water of BFT systems for probiotic candidates, it present opportunities for the development of site- or host-specific probionts in aquaculture.
Water samples were obtained from a tank culture of freshwater tilapia, Oreochromis sp reared using BFT. Bioflocs were maintained using wheat flour as carbon source and added at a carbon-to-nitrogen (C:N) ratio of 16:1. The water samples were concentrated, serially diluted and plated onto Nutrient agar (NA) and de Man, Rogosa and Sharpe (MRS) agar plates. Individual bacterial colonies were picked from both agar plates and maintained in NA plates. Five (5) bacterial colonies that showed inhibitory activity against Vibrio alginolyticus using the cross-treak method were further tested in terms of their phenotypic, biochemical and molecular characteristics.
The five isolates were Gram-negative and rod-shaped bacteria. Further tests showed that all the isolates were catalase-positive and oxidase-negative bacteria. They produced extracellular enzymes particularly protease and amylase, and to a certain extent cellulase. All the isolates can grow in broth supplemented with 2% sodium chloride (NaCl) and grew at temperature between 27-37oC. Two isolates, however, were able to grow at 4oC. Four of the isolates were able to tolerate a growth medium of pH 2 and all isolates grew at pH 8. In addition, all five isolates were able to grow in culture media supplemented with either pancreatin or pepsin. Molecular characterization of the 16s RNA of all the isolates revealed that they belong to the genus Citrobacter of the Enterobacteriaceae family. Further tests will be conducted to deternine the pathogenicity of these bacterial isolates to fish and will be assessed of their probiotic potential in vivo.