Effects of three veterinary antibiotics and their binary mixtures on growth of Pseudokirchneriella subcapitata

CARUSSO, SOFIA; JUAREZ, ANGELA BEATRIZ; MORETTON, JUAN; MAGDALENO, ANAHI

Curitiba

Congreso; XIV Congresso Brasileiro de Ecotoxicologia; 2016

ECOTOX

Large percentage of antibiotics administered to animals (30-90%) is excreted without being metabolized, and is dispersed with the manure used for soil amendment, eventually reaching surface waters. Since several classes of antibiotics can enter and remain simultaneously in the aquatic ecosystems, the non-target organisms may be exposed to synergistic or antagonistic interactions of these compounds. The objective of this study was to evaluate the toxicity of individual and binary mixtures of three veterinary antimicrobials commonly used in beef cattle production (chlortetracycline, oxytetracycline and enrofloxacin) to the green alga Pseudokirchneriella subcapitata.The three antibiotics analyzed showed toxicity to P. subcapitata. The EC50 values obtained for the antibiotics CTC, OTC and ENF were 2.30 (0.15-3.10), 1.70 (0.20-4.00) and 2.50 (1.70-7.70) mg/L, respectively. Then, OTC was the most toxic antibiotic tested. Similar results were obtained by van der Grinten et al. (2010) with values of 0.34 (0.32?0.36) mg/L. According to the CA model, the two compounds of a binary mixture have a similar site and mode of action within an organism, which means that the effect of a mixture remains constant if one component is exchanged by an amount of the other component that causes an equal effect. On the other hand, the IA model assumes that the substances act independently and have different sites of actions. This concept implies that the effect of a single substance should not be changed in the presence of another substance (Altenburger et al. 2004). Experimental results showed that only the mixture CTC+OTC was more toxic than the antibiotics acting individually at low concentrations. On the other hand, experimental results obtained for the mixtures CTC+ENF and OTC+ENF were lower toxic than the respective antibiotics acting individually. Then, the last two mixtures showed antagonism. In the first mixture, the total inhibitory effect could be explained by CTC, and in the second mixture, the total inhibitory effect could be explained by OTC. Only the additive CA model predicted the algal growth inhibition in the mixture CTC+OTC, whereas this model underestimated the results obtained in the other two mixtures. On the other hand, the IA non additive model overestimated the three mixtures tested.In the presented study the antimicrobials chlortetracycline (CTC), oxytetracycline (OTC) and enrofloxacin (ENF) were tested individually or in binary mixtures on toxicity to the green algae Pseudokirchneriella subcapitata. According to the EC50 values, OTC was the most toxic antibiotic tested. Only the mixture CTC+OTC showed synergism at low concentrations, whose results could be predicted by the CA model. The IA model did not predict the effect of compounds acting in binary form.