This study investigated the interaction of three commonly used pesticides, carbaryl, captan, and malathion, with combinations of antibiotics occurring commonly in milk, all at levels below established tolerances. The modality of measurement was the MIC; the assay organism was Staphylococcus aureus ATCC 9144. Acetone alone or individual pesticides in acetone caused no increase in the baseline MIC of any of the marker antibiotics. For single antibiotics, (ampicillin, dihydrostreptomycin, erythromycin, neomycin, oxytetracycline, and sulfamethazine) 7.1 % of the combination possibilities showed increased MICs. The three pesticides together resulted in an increased MIC in 4.8% of the combinations. Varying combinations of three of the aforelisted antibiotics showed increases in the baseline MIC in 18.5% of the possibilities. Combinations of three antibiotics and the three pesticides showed increased MICs in 24.4% of the possibilities. There appears to be an additive effect upon the development of antibiotic resistance of S. aureus cells between the three pesticides and the antibiotics in the combinations studied.
Keywords: Antibiotics; MIC assay; pesticide-antibiotic interactions; pesticides; residues.