Anti-Aeromonas hydrophila activity and characterisation of novel probiotic strains of Bacillus subtilis isolated from the gastrointestinal tract of giant freshwater prawns

The antagonistic activity of two Bacillus strains isolated from the gastrointestinal tract of giant freshwater prawns against Aeromonas hydrophila was evaluated in vitro. The characterisation of the novel probiotic strains of these bacilli was also performed. Bacillus subtilis P33 and 72 were found to have high inhibition activities against the growth of A. hydrophila by two assay methods: paper disc and well diffusion. Probiotic properties, namely acid and bile salt tolerance, autoaggregation, coaggregation, hydrophobicity and adhesion to Caco-2 cells, were further analysed. Survival rates in model gastrointestinal tract condition, viz. pH 2.5 for 3 h and 0.3% bile salt for 24 h, were shown to be more than 95% and 90% respectively. The ability of B. subtilis strains of P33 and P72 to adhere to epithelial cells of the host animal was measured by percentage autoaggregation (35.7 and 42.2%), coaggregation (11.1 and 11.6%), hydrophobicity in n-hexadencane (25.6 and 30.0%), xylene (32.2 and 36.1%), toluene (30.3 and 31.6%), and adhesion to Caco-2 cells (4.21 and 3.23 log cfu/ml respectively). These results indicate that both strains of B. subtilis P33 and P72 can be considered to be good novel probiotic candidates for use in the prawn aquaculture industry.