Continuous non-therapeutic use of antibacterial drugs in feed and drug resistance of the gram-negative enteric florae of food-producing animals.

Fecal samples from Illinois farm animals and Montana range cattle were examined for the frequency of gram-negative enteric organisms resistant to oxytetracycline, dihydrostreptomycin, or ampicillin. The Illinois farm animals were continually fed rations containing various antibacterial drugs. The Montana range cattle had minimal exposure to antibacterial drugs. The majority of samples from Illinois farm animals demonstrated 10% or more antibiotic-resistant organisms. In contrast, resistant organisms could not be detected in the majority of samples from Montana range cattle. The majority of Escherichia coli isolated from Illinois farm animals was resistant to various antibacterial drugs, whereas less than 1.6% of the isolates from Montana range cattle were resistant to any one of the antibacterial drugs tested.