Isolation, biochemical and molecular detection of Bacillus subtilis and Bacillus licheniformis from the digestive system of rainbow trout (Oncorhynchus mykiss) and its inhibitory effect on Aeromonas hydrophila

Our study was focused on characterization of dominant Bacillus bacteria isolated from the digestive tract of Oncorhynchus mykiss. After 8 weeks, the fish fed with probiotic-free diet were dissected under strictly controlled conditions. Fish intestines were eviscerated and its content diluted to 10−4 using Ringer solution. Two isolated bacteria were Gram-positive, catalase-positive, and oxidase-negative, based on the biochemical tests. A mixed of the two strains was the most efficient isolates determined in O. mykiss. Molecular characterization using PCR-Ribotyping was carried out using sequencing the 16S rRNA gene of both strains, which isolated two bacteria, Bacillus subtilis Ahari.H1 and B. licheniformis Ahari. H2. The isolated bacteria could tolerate 0.3% bile salt and non-hemolytic in nature. They also showed resistance to pH at 2.5 and 4 for acid test and 2.3 and 7 for gastric juice, pepsin and trypsin. It is concluded that the efficiency of the isolated probiotics can be candidates as probiotic in fish culture. It was unveiled that these strains can successfully diminish Aeromonas hydrophila pathogenicity in rainbow trout.