The contamination of seafood by bacteria of fecal origin, especially Escherichia coli, is a widely documented sanitary problem. The objective of the present study was to isolate E. coli strains from the gills, muscle, and body surface of farmed Nile tilapias (Oreochromis niloticus) fresh-marketed in supermarkets in Fortaleza (Ceará, Brazil), to determine their susceptibility to antibiotics of different families (amikacin, gentamicin, imipenem, cephalothin, cefotaxime, ciprofloxacin, aztreonam, ampicillin, nalidixic acid, tetracycline, and sulfametoxazol-trimetoprim), and to determine the nature of resistance by plasmid curing. Forty-four strains (body surface = 25, gills = 15, muscle = 4) were isolated, all of which were susceptible to amikacin, aztreonam, cefotaxime, ciprofloxacin, gentamicin, and imipenem. Gill and body surface samples yielded 11 isolates resistant to ampicillin, tetracycline, and sulfametoxazol-trimetoprim, 4 of which of plasmidial nature. The multiple antibiotic resistance index was higher for strains isolated from body surface than from gills. The overall high antibiotic susceptibility of E. coli strains isolated from fresh-marketed tilapia was satisfactory, although the occasional finding of plasmidial resistance points to the need for close microbiological surveillance of the farming, handling, and marketing conditions of aquaculture products. 1. Introduction The bacterium Escherichia coli is widely used as indicator of the bacteriological condition of food and environments due to its almost exclusively fecal origin [1]. The presence of E. coli in fresh-marketed seafood indicates recent contamination and is usually attributed to infected handlers or storage on contaminated ice [2]. The intensification of production and the consequent increase in stocking density have made fish farming more vulnerable to disease [3, 4]. The indiscriminate use of antibiotics to treat infections and promote growth has been shown to be inefficient in the long run and to put selective pressure on bacterial populations favoring the development of resistant strains potentially hazardous to public health [5–7]. In fact, bacterial strains resistant to different families of antibiotics have been isolated from environmental samples by a number of researchers [8–10]. Foodborne strains resistant to antibiotics pose a risk to consumers’ health and favor the transference of the phenotype to humans through the food chain [11–13]. There are no reports of illnesses caused by E. coli in farmed fish, but resistant strains may be selected due to the presence of
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