Staphylococcus aureus ATCC 9144 was exposed to “safe levels” of ampicillin, dihydrostreptomycin, erythromycin, neomycin, oxytetracycline, and sulfamethazine, singly and in combinations of three drugs, for a 14-d interval, transferring daily. After the 14-d exposure interval, the minimum inhibitory concentration (MIC) of the organism for each treatment was determined for nine antibiotics/antimicrobials. There was one increase in the MIC out of 81 possibilities for the control or an incidence of 1.23%. There were four increases in the MIC in the single drug exposures for 54 possible increases in the MIC or an incidence of 7.40%. For the 18 combinations of three drugs, there were 55 increases in the MIC for 162 possible increases or 33.95%. This was an increase of 27.6 times over the control incidence and 4.6 times over the single drug incidence. The single drug incidence is 6.0 times the control incidence. These data indicate that the “safe levels” of antibiotics/antimicrobials that can appear in milk have the strong potential to select for antibiotic/antimicrobial-resistant bacterial populations.

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