JOURNAL OF THE WORLD AQUACULTURE SOCIETY Vol. 43, No. 5 October, 2012 Effect of Seawater on the Activity of Antibiotics Against Vibrios Isolated from the Hemolymph of Cultured Pacific White Shrimp Renata Albuquerque Costa 1 Department of Fishing Engineering, Federal University of Cear´ a, CAPES, Fortaleza, Cear´ a, Brazil Lorena Pontes Colares Department of Fishing Engineering, Federal University of Cear´ a, Fortaleza, Cear´ a, Brazil Rayza Ara ´ ujo Lima Department of Fishing Engineering, Federal University of Cear´ a, CNPq, Fortaleza, Cear´ a, Brazil Regine Helena Silva dos Fernandes Vieira and Oscarina Viana de Sousa Sea Sciences Institute, Federal University of Cear´ a, Fortaleza, Cear´ a, Brazil Abstract. – The purpose of this study was to eval- uate the effect of seawater (SW) on the activity of antibiotics belonging to 10 families (aminoglycosides, aminopenicillins, carbapenems, β-lactams, chlorampheni- cols, monobactams, nitrofurans, quinolones, sulfonamides, and tetracyclines) against Vibrio strains isolated from hemolymph of Pacific white shrimp, Litopenaeus vannamei, farmed in Northeastern Brazil and standard strain Vib- rio cholera ATCC 19582. Susceptibility of the strains to antibiotics was determined by disk diffusion method and the minimum inhibitory concentration was determined by macrodilution method. The media Mueller – Hinton agar and broth used in the above methods were diluted in distilled water (control, 1% NaCl, pH 7.5) and SW (2.5% NaCl, pH 7.5). The antibiotics most affected by dilution in SW were tetracycline, penicillin, cephalothin, aztreonam, ampicillin, and imipenem, as indicated by a considerable increase in the number of strains classified as intermediate or resistance. Thus, in this study, the efficiency of these antibiotics on Vibrio strains was found to be reduced by contact with SW. Antibiotics are extensively used in aqua- farming. According to H¨ olmstrom et al. (2003), they are used in shrimp farming to prevent or treat disease although little has been published regarding patterns of use. The same authors interviewed 76 shrimp farmers along the coast of Thailand and found that 74% admitted to 1 Corresponding author. administering antibiotics. Gr¨ aslund and Bengts- son (2001) reported that an array of chemi- cal and biological products are used by most shrimp farmers in Southeast Asia and pointed out the risk for human health represented by improper use of antibiotics. The use of antibiotics for prophylaxis in aquaculture not only favors the selection of resistant bacteria in the pond environ- ment, thereby changing the natural micro- biota of pond water and sediments, but also increases the risk of transferring resistance genes to pathogens infecting humans and ter- restrial animals (Cabello 2006). Likewise, Le and Munekage (2004) reported high levels of drug residues (sulfametoxazol, trimetoprim, norfloxacin, and oxolinic acid) in pond water and sediments from tiger prawn farms in North- ern and Southern Vietnam due to indiscriminate use of antibiotics. In Brazil, Costa et al. (2008) detected Vibrio strains resistant to ampicillin, sulfametoxa- zol–trimetoprim, and ceftriaxone in samples of pond water and cultured Pacific white shrimp, Litopenaeus vannamei, suggesting ponds and livestock might constitute a potential source of dissemination of resistant bacteria. © Copyright by the World Aquaculture Society 2012 727