Development of in vitro antimicrobial resistance in bacteria exposed to residue level exposures of antimicrobial drugs, pesticides and veterinary drugs.

Ten ppb or less of antibacterial drugs, pesticides and veterinary drugs could increase antibacterial resistance in bacteria. The minimum inhibitory concentration (MIC) was the indicator of resistance and Staphylococcus aureus ATCC 9144 the indicator organism. Seventeen compounds used in human, veterinary medicine, crop production, and found in the environment, were studied singly, and as combinations. Single compounds decreased MIC values in 5.3% of possibilities; showed no changes in 74.4% of values; increased values in 21.8%; 16.5% of MICs increased 4- to 8-fold; 4.7% of changes were >8-fold. For two-compound mixtures 5.1% of MICs showed no changes; 34.3% increased 4-8 times; 60.6% increased >8-fold. For six-compound combinations: 5.9% of MICs showed no changes; 15.6% increased 4- to 8-fold; 78.5% increased >8-fold.