Isolation of lactic acid bacteria producing organic acids and possessing anti-Aeromonas hydrophila activity from gastrointestinal tract of Nile tilapia (Oreochromis niloticus) (Thai)

Abstract

In this study lactic acid bacteria (LAB) that produced organic acids and possessed anti-Aeromonas hydrophila activity, which might be used as a probiotic, were isolated and the biological properties of these bacteria studied. The results show that the quantity of LAB depends on the age and environment of the fish. Out of 30 LAB isolates, 3 isolates (FNE-1, FNE-2 and FNE-3) exhibited anti-Aeromonas hydrophila activity with % inhibition ratios of 20.0%, 15.7% and 15.7%, respectively. All three LAB isolates were gram positive cocci, arranged in two or more cells, produced catalase, grew in culture medium of pH 9 and resisted 10% NaCl. In addition, biochemical study by Api 20E assay showed that FNE-2 and FNE-3 were similar in giving positive results to ADH, TDA, VP, GLU, MAN, INO, SOR, RHA, SAC, MEL, AMY and ARA, while FNE- 1 differed from FNE-2 and FNE-3 only in yielding negative results on ADH and AMY. The results clearly identify that these LAB are Pediococcus sp. The bacterial growth and pH changes in a time course of 72 h were also studied. The results show a pH reduction from 4.8 to 3.2 during 0-32 h of bacterial culture, which gradually increased to 5.4 during 48-72h. The LAB culture, pH adjusted to 6.5, exhibited no anti- A. hydrophila activity, which suggests that the acidic environment produced by LAB is one of the mechanisms for inhibiting A. hydrophila growth.