Journal of Advances in Biology & Biotechnology

This study investigated culturable bacteria from the gastrointestinal tract of the freshwater ornamental swordtail, Xiphophorus helleri, to identify isolates with preliminary characteristics relevant to probiotic screening. Five fish were obtained from a local aquarium source and acclimatised before intestinal sampling. The gastrointestinal tracts were aseptically removed, homogenised in sterile normal saline and serially diluted. Samples were spread on de Man, Rogosa and Sharpe agar and incubated anaerobically. Eleven bacterial strains were initially recovered, and nine morphologically distinct isolates—CX1, CX2, CX4, CX9, CX11, CX13(a), CX13(b), CX14 and CX19—were selected for further examination. The isolates were evaluated by colony morphology, Gram staining and biochemical tests, including pH tolerance, catalase, oxidase, nitrate reduction and methyl red reactions. Most isolates were Gram-negative, whereas CX4 was Gram-positive. All isolates tolerated pH 8, and most showed growth between pH 6 and 8. Catalase activity was detected in CX1, CX4 and CX11; only CX4 was oxidase-positive; and nitrate reduction was observed in CX4 and CX13(a). All isolates were methyl-red-negative. Based on the observed morphological and biochemical characteristics, the isolates were assigned to Enterobacter sp., Enterococcus faecalis and Bacillus subtilis. These findings provide a preliminary description of culturable gut bacteria from X. helleri. Further molecular identification, safety assessment and in vivo validation are required before any isolate can be considered suitable for probiotic application.