Abstract

This study scoped the isolation of aerobic actinomycetes with probiotics properties against bacterial pathogens in Nile tilapia. Eleven rhizosphere soil samples were collected from the agricultural sites in three provinces (Chanthaburi, Nan, and Chachoengsao) of Thailand. A total number of 157 actinomycete-like colonies were successfully isolated. The antibacterial testing against four tested bacteria (Streptococcus agalactiae 2809, Aeromonas jandaei 1929, A. veronii 1930, and Edwardsiella ictaluri 2234) were performed by modified cross-streaked method. The results showed that one hundred and eight strains possessed antibacterial activity against at least one of the bacterial pathogens. Seventeen active isolates were identified in the actinomycetes by the analysis of the partial 16S rRNA gene, and the phylogenetic relationships of the isolates and their closely related strains were confirmed by the neighbor-joining method. Isolate LNW002 and YNW004 could survive in the liquid cultivation with ISP2 at pH 2 and the presence of 0.3 % bile salt for two hours which mimics the gastric acidity and bile salt in the gastrointestinal tract of Nile tilapia in vitro. In conclusion, these strains might be further investigated for their efficacy as probiotics in Nile tilapia. This study was also the first report on anti- Edwardsiella ictaluri activity in streptomycetes.

Key words: actinomycetes; antibacterial activity; probiotics; bacterial pathogens; Nile tilapia