Diversidade de microbiota Gram negativa em sistemas de cultivo de tilápia do Nilo (Oreochromis niloticus)

Abstract

The aim of this work was to evaluate the diversity of Gram-negative microbiota in three different systems of Nile tilapia (Oreochromis niloticus). The three culture systems were characterized by: system 1, concrete pond and commercial pelleted feed; system 2 land pond and commercial pelleted feed; and system 3, land pond and animal manure fertilization. The strains were previously isolated and stoked in freezing medium under low temperature (-70ºC). One hundred eighty eight gram-negative strains were selected for bacterial identification. The strains were submitted to gram stain, catalise, oxidase, oxidation-fermentation test (O-F) and characterized by bacterial identification kits Bac-Tray I and II (Laborclin, Brazil), API 20E and API 20 NE (Biomerieux, France). One strain of each bacterial genus and species were selected for antibacterial susceptibility testing. Antimicrobials resistance patterns were determined by an agar disk diffusion method (CLSI, 2006). Disks containing the following agents were used: ampicilin, cefuroxime, chloramphenicol, florfenicol, eritromycin, gentamicin, nitrofurantoin, norfloxacin, sulfonamide and tetracycline. A total of 12 different bacterial families were identified. Aeromonadaceae (50%), Enterobacteriaceae (24%) and Pseudomonadaceae (15%) were the most frequent bacterial families in all culture systems analyzed. Aeromonas hydrophila was the predominant bacterial species isolated in the three culture systems. It was observed a higher diversity of gram-negative bacteria than previous studies. The three different culture systems showed similar patterns of gram-negative microbiota. Eritromycin and ampicilin were the antibiotics with higher levels of resistant bacteria. In contrast norfloxacin and florfenicol showed low frequencies of resistant bacteria. Ninery five percent of the strains showed MAR (Multiple antibiotic resistance) index ≥ 0,2 being characterized as multiresistant. The three different culture systems analyzed show high levels of antibiotic resistant bacterial populations might be a risk source for human and animals.