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This study investigated the relative acute single action toxicity and mixture toxicity of four commonly used human antibiotics against Poecilia reticulata (Guppy) using laboratory bioassays. The sublethal toxic effects of the antibiotics were also assessed using biochemical endpoints. The antibiotics assessed in this study are amoxicillin, ampicillin, tetracycline and oxytetracycline. The assessment of mixture toxicity carried out with the binary mixtures of the antibiotics (1:1 v/v) showed potential mixture toxicity. The 96 hr LC50 values of Amoxicillin+Ampicillin and Tetracycline+Oxytetracycline were 236.129 mg/L and 33.286 mg/L respectively. The pattern of interaction of the antibiotics in binary mixtures assessed using the concentration addition model revealed a synergistic pattern of interaction. Results of the sublethal toxic effects using biochemical endpoints showed that there was a significant (p<0.05) decrease in antioxidant enzymes activities in group of fish exposed to the binary combinations of Amoxicillin+Ampicillin and Tetracycline+Oxytetracycline respectively, compared to control groups. A significant (p<0.05) increase in Malondialdehyde (MDA) activity was observed in group exposed to both binary mixture combinations compared to the control. Assessment of liver function enzymes revealed a significant increase (p<0.05) in enzymes activity in both groups compared to control. The results from this study has shown that the presence of these antibiotics in mixtures in aquatic environment can have potential toxicological effects on non-target aquatic organisms. Regulating the discharge of antibiotics into the environment is therefore necessary to protect aquatic life and ecosystems.
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