Plesiomonas shigelloides: A notable carrier of acquired antimicrobial resistance in small aquaculture farms

Small and low-cost family aquaculture tilapia (Oreochromis niloticus) production with regional commercial targets is currently common practice in continental production systems of many parts of the world. In the state of Rio de Janeiro, Brazil, fish are reared in natural ponds, with limited technology, using available water resources. Nutrition relies on pelleted feed, and production costs are minimized by adding untreated manure and other organic fertilizers directly into the water. The aim of this study was to describe antimicrobial resistance in gram-negative bacteria isolated from fish produced under these conditions in four watersheds of that state. Gram-negative aerobic bacteria isolated from fish bowel and gill samples were identified, and resistance phenotypes determined. Fecal and sediment samples were analysed for residual evidence of tetracycline, the most commonly used therapeutic drug in animal husbandry, and for the presence of tetracycline resistance genetic determinants. Gram-negative bacteria belonging to different genera were the majority of culturable bacteria obtained from these samples. Results revealed a high number of tetracycline, and to a lesser extent, chloramphenicol resistant P. shigelloides strains, by far the predominant isolated bacterial species. Tetracycline residues were detected in both, sediment and fish fecal samples, from all farms. Tetracycline resistance genetic determinants tet(A), tet(C), tet(D), tet(M) and tet(O) were detected in sediment and fecal samples, and tet(A), tet(D) and tet(B) were detected in P. shigelloides isolates. Chloramphenicol resistant P. shigelloides isolates carried the floR, catI and catII genetic determinants. These data suggest the putative role of P. shigelloides as a reservoir for antimicrobial resistance genes in traditional continental aquaculture reflecting the selective pressure imposed by a combination of varied and uncontrolled rearing practices.