In vitro antimicrobial assessment on lactic acid bacteria isolated from common freshwater fishes

Abstract

Probiotic is well-known as an effective agent to control and manage diseases in aquaculture. Unlike antibiotics and chemotherapeutic agents, probiotic does not trigger the emergence of antibiotic-/chemo-resistant bacteria. This study was aimed to isolate, identify and evaluate lactic acid bacteria from intestines of three common food fish, i.e. tilapia (Oreochromis niloticus), catfish (Clarias gariepinus) and rohu (Labeo rohita). Thirty-four lactic acid isolates were isolated and screened for inhibitory effect against fish pathogens e.g. Escherichia coli, Klebisella pneumoniae, Pseudomonas aeruginosa and Salmonella enterica. Positive antagonists were subsequently tested in haemolytic, salt tolerance and bacteriocin-like inhibitory substances (BLIS) assays. Our results showed only three isolates displayed positive inhibitory effect against all four pathogens. These three isolates were classified as γ-haemolytic bacteria. Our results revealed that bacterial isolates (T2.1.2 - Pediococcus acidilactici and T2.2.2 - Lactobacillus fermentum) isolated from O. niloticus (tilapia) showed a better adaptation in the range of 0 - 20 ppt; while, the bacteria isolated from L. rohita (R1.1.1 - P. acidilactici) could survive up to 35 ppt. These isolates were then identified based on 16S rRNA gene sequences. BLIS data revealed that both P. acidilactici and L. fermentum isolated from O. niloticus and L. rohita could suppress the growth of pathogens with cell density as low as 104 cfu/ml. Our study shows that P. acidilactici and L. fermentum have the potential to be further explored as biocontrol/probiotic agents in aquaculture.