An inhibitory substance produced by Aeromonas media A199, an aquatic probiotic

Whether or not the probiotic activity of the Aeromonas media strain A199 derived from the production of an extracellular inhibitory substance was investigated. Ethyl acetate extraction of broth cultures of A199 and preparative thin layer chromatography methodologies revealed a fraction that contained inhibitory activity against bacterial and fungal indicators. Gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) analyses identified indole (2,3 benzopyrrole, henceforth referred to as T1) as the major chemical component in this fraction. The presence of inhibitory activity in broth culture extracts of A199 was found to be entirely dependent on the production of T1 by the organism. The inhibitory activity of T1 in vitro against Edwardsiella tarda, Fibrio anguillartim, Yersinia ruckeri, Aeromonas salmonicida and Lactococcus garvieae was found to be concentration dependent (300-600 mu g ml(-1)). Antifungal activity (75-300 mu g ml-1) was obtained against the vegetative stage and cysts of Saprolegnia parasitica, with cysts showing a higher susceptibility. Morphological changes observed within hyphae suggested that TI could be a potential cytoplasmic toxin. Equivalent inhibitory activity was obtained from commercial indole against the majority of indicators, but discrepancies were encountered consistently with failure to inhibit particular bacterial indicators as well as decreased antifungal activity. A comparison of the inhibitory activity of A199 and other indole producers, that included various strains of A. media and Escherichia coli, indicated that the ability of a bacterium to produce indole might not necessarily afford it with inhibitory activities. (c) 2005 Elsevier B.V. All rights reserved.