Determination of body surface bacterial flora in grow-out (300-500 g) rainbow trout in Amol city farms

Bacterial microflora in water and fish body surface has an important role for fish bacterial diseases outbreak and are known as secondary bacterial infections. These bacteria can be pathogenic and develop high mortality in stress conditions. In this study, samples were collected from body surface and gills of 70 rainbow trout (300-500 g) in seven fish farms on Haraz road between Larijan and Poloor regions by sterile swabs. Water resource was Haraz River. Skin and gill swabs were transferred to the lab in capped tubes containing sterile PBS (Phosphate Buffer solution), individually and were cultured in TSA (Tryptic Soy Agar), Plate Count Agar and MacConkey Agar media. Each sample were cultured in two TSA medium plates for conserving in aerobic and anaerobic conditions in 25°C, and one MacConkey Agar plate to reassure of gram negative bacteria absence. Plate count agar culturing was applied for mesophilic and psychrophilic bacteria isolation in 13 and 37° C. Nine bacterial species were isolated from gills and fish body surface (including: E.coli, Staph. areus, Staph. saprophyticus, Staph. epidermidis, Bacillus cereus, Aeromonas hydrophila, Proteus mirabilis, Pasteurella multocida, Psuedomonas aeruginosa). E. coli were identified in all samples. There was no significant difference between gills and skin in all facilities (P>0.05). Among the facilities, there was no significant difference between isolated bacterial species (P>0.05). Except E.coli that was identified in all farms, and irregularity frequency for P. aeruginosa in farms, in other cases, bacterial frequency increased up to downstream fish farms. Isolation of such bacteria may be an indicator for urban and home waste water contamination.