Isolation and characterization of Bacillus spp. from aquaculture cage water and its inhibitory effect against selected Vibrio spp.

The aim of this study was to evaluate the probiotic potential of Bacillus strains with antibacterial activity against Vibrio spp. in vitro. 13 Bacillus strains were selected using Bacillus-selective agar. Among the strains elected in the preliminary screening for antagonistic effect, only the strain PJ_11 with better antibacterial ability was selected and used in further experiments. Based on the morphological and biochemical features with phylogenetic analysis of the 16S rRNA sequencing, selected strain was identified as B. pumilus. PJ_11 showed probiotic properties such as able to survive in wide range of temperatures and salinity, tolerance to 0.3% oxgal and pH 2.0, and high cell-adhesion activity. In this study, there was no special feature regarding the inhibitory effect of the isolate against Gram-positive or Gram-negative indicator bacteria. However, the inhibition effect of the culture was found to be greater than that of the supernatant. PJ_11 exhibited strong antibacterial activity against Vibrio vulnificus, Vibrio parahaemolyticus and Vibrio anguillarum that were isolated from fish (Dicentrarchus labrax) with disease symptoms. The inhibition diameter of strain PJ_11 ranged from 17.63 to 26.68 mm. In co-culture assay with the strain PJ_11 culture, the growth of V. vulnificus, V. parahaemolyticus and V. anguillarum was inhibited after 120 h with an initial level of 1.0 × 108 CFU/mL. PJ_11 had antibiotic susceptibility to commonly used antibiotics in aquaculture but only resistant to chloramphenicol. Isolate was non-hemolytic. These findings suggested that the strain PJ_11 is good probiotic candidate and may have potential applications to prevent/control vibriosis in aquaculture.