Antagonism against fish pathogens by cellular components and verification of probiotic properties in autochthonous bacteria isolated from the gut of an Indian major carp,<i>Catla catla</i>(Hamilton)

Abstract

Probiotic potential of the autochthonous bacteria in catla, Catla catla has been evaluated through determination of antagonistic activity (in vitro) of the cellular components of gut bacteria against seven fish pathogens. Altogether 208 strains were isolated, inhibitory activity of the isolates was evaluated through cross-streaking and 16 primarily selected antagonistic strains were confirmed using the double-layer method. Four bacteria that showed antagonism against ≥4 pathogens were selected as putative probiotics. The intracellular, extracellular, whole-cell and heat-killed cell components exhibited bactericidal activity against the pathogens. In addition, the selected strains were capable of producing different extracellular enzymes, competent to grow in intestinal mucus and could tolerate diluted bile juice. Analysis of 16SrRNA partial gene sequence revealed that both the strains CC1FG2 and CC1FG4 were Bacillus methylotrophicus (KF559344 and KF559345), while the isolates CC1HG5 and CC2HG7 were Bacillus subtilis subsp. spizizenii (KF559346) and Enterobacter hormaechei (KF559347) respectively. Bio-safety evaluation through intra-peritoneal injection of the isolates did not induce any pathological signs or mortalities in C. catla. The study confirmed probiotic properties of autochthonous gut bacteria in C. catla and demonstrated potential for using them as bio-control agents. However, in vivo studies are essential to explore their efficacy in the commercial aquaculture.