Preliminary Studies on Siderophore Production and Probiotic Effect of Bacteria Associated with the Guppy, Poecilia reticulata Peters, 1859

To study the possible use of probiotics in aquaculture, we evaluated the in vitro antagonism of antibacterial strains isolated from the guppy, Poecilia reticulata Peters, 1859 against the fish-pathogenic bacteria such as Aeromonas hydrophila, Vibrio cholerae, Flavobacterium, Acinetobacter and Alcaligenes. As iron is important in virulence and bacterial interactions, the effect of four isolates, namely, MBTU_PB1, MBTU_PB2, MBTU_PB3 and MBTU_PB4 was studied under iron-rich and iron-limited conditions which established the presence of a siderophore-based ironsequestration mechanism in the isolates from guppy. All the four isolates produced siderophores in the presence of an iron-chelating compound, 2, 2’ dipyridyl. The ability to synthesize siderophores would undoubtedly be an advantage for survival, growth and pathogenicity of the isolates. All the investigated pathogenic strains displayed growth along each of the four selected strains under both iron-sufficient and iron-limited conditions. Both FeCl3 and tetrazolium tests produced positive results for all the four selected strains indicating the presence of hydroxamate siderophores. On basis of quantitative assay, the isolate MBTU_PB2 produces maximum siderophore (2.71 mg. mL^-1) followed by MBTU_PB3 (2.23 mg.mL^-1), MBTU_PB4 (2.20 mg.mL^-1) and MBTU_PB1 (1.26 mg.mL^-1). These siderophore producing isolates could find potential applications, such as in the medical, fish farming and food microbiology industries.