In vitro Assessment of the Probiotic Characteristics of Three Bacillus Species from the Gut of Nile Tilapia, Oreochromis niloticus

Probiotics used in aquaculture are mostly from non-fish sources, as a result ineffective in eliciting the desired effects in aquatic animals. In this study, threeBacillusspecies were isolated from the digestive tract of freshwater fishOreochromis niloticusand characterised based on their morphological, biochemical and evolutionary relationships. Their probiotic potentials were evaluated based on their ability to tolerate low pH, bile salt concentration, high temperatures, adhesion ability (auto-aggregation and hydrophobicity), haemolytic activity and antimicrobial activity including biosafety assay. ThreeBacillusstrains identified asBacillus velezensisTPS3N (MK130897),Bacillus subtilisTPS4 (MK130899) andBacillus amyloliquefaciensTPS17 (MK130898) were designated as TPS3N, TPS4 and TPS17, respectively. TPS3N and TPS17 were alpha-haemolytic, while TPS4 was gamma-haemolytic. The three isolates had higher viability ability after exposure to higher temperatures (80 degrees C, 90 degrees C and 100 degrees C) and were resistant to low pH (1) and bile salt concentration (0.5%) as well as high cell surface hydrophobicity and auto-aggregation. The three isolates were compatible with one another and thus can be used in consortia. These strains were susceptible to gentamicin, cephalexin, ampicillin, ceftriaxone, kanamycin, amikacin, penicillin, cefoperazone, chloramphenicol, erythromycin, tetracycline, doxycycline, ciprofloxacin, clindamycin (except TPS4) and furazolidone (except TPS17). The antimicrobial assessment showed that among the three isolates, TPS3N and TPS17 exhibited good antimicrobial activity against the three fish pathogens (Streptococcus agalactiae,Aeromonas hydrophila,Vibrio harveyi), while TPS4 was effective againstStreptococcus agalactiaeonly. The results of this work suggest thatBacillusstrains TPS3N, TPS4 and TPS17 could be considered as potential probiotics in tilapia aquaculture.