Screening of lactobacilli from fish intestines to select a probiotic for warm freshwater fish

The aim of this study was to select a Lactobacillus with probiotic abilities suitable for in vivo studies in farmed freshwater fish. Fifty-five Lactobacillus isolated from the intestines of freshwater fish were screened for inhibition of fish and human pathogenic bacteria in vitro; and some selected strains by absence of production of biogenic amines and resistance to gastric and intestinal fluids (GIF) in a simulation model. A strain was studied in co-cultures with a pathogen in fish feed extract. Selected strains were tentatively identified as Lactobacillus plantarum 44a, whose a mechanism of inhibition was based on acid production, and L. brevis 18f which was detected as a high H 2 O 2 producer, because its supernatant adjusted to pH 6 strongly inhibited Aeromonas hydrophila; this activity was not observed when supernatant was treated with catalase. In the exposure of cells to GIF, L. plantarum 44a survived better than the other strains to pH 2, 2.5 and 3 and pepsin. L. brevis 18f had a very low survival in GIF. L. plantarum 44a co-cultured with A. hydrophila in fish feed extract with an initial ratio 10 3 : 10 7 and 10 7 : 10 3 respectively, started killing the pathogen when pH was around 5.5. L. plantarum 44a has potential applications as probiotic in freshwater fish. L. brevis 18f as a H 2 O 2 producer may have application as a possible fish pathogen antagonist in the upper gastrointestinal tract, the skin, the gills and eggs where oxygen is available.