ABSTRACT
This study scoped the isolation of aerobic actinomycetes with probiotic properties against bacterial pathogens in Niletilapia. Eleven rhizosphere soil samples were collected from the agricultural sites in three provinces (Chanthaburi,Nan, and Chachoengsao) of Thailand. A total of 157 actinomycete-like colonies were successfully isolated. Theantibacterial testing against four tested bacteria (Streptococcus agalactiae 2809, Aeromonas jandaei 1929, Aeromonasveronii 1930, and Edwardsiella ictaluri 2234) was carried out by a modified cross-streaked method. The results showedthat 108 strains possessed antibacterial activity against at least one of the bacterial pathogens. Seventeen active isolateswere identified in the actinomycetes by the analysis of the partial 16S ribosomal rRNA gene, and the phylogeneticrelationships of the isolates and their closely related strains were confirmed by the neighbor-joining method. IsolatesLNW002 and YNW004 survived in the liquid cultivation with International Streptomyces Project 2 at pH 2 and thepresence of 0.3% bile salt for 2 hours, which mimics the gastric acidity and bile salt in the gastrointestinal tract of Niletilapia in vitro. In conclusion, these strains might be further investigated for their efficacy as probiotics in Nile tilapia.This study is the first to report on anti-E. ictaluri activity in streptomycetes.