In vitro antibiotic susceptibility of enteric bacteria isolated from commercial broiler chickens

The in vitro activity of six different antimicrobial agents was evaluated by an agar dilution method against enteric bacteria isolated from broilers. Of 923 isolates, 796 were gram-positive bacteria and 127 were Escherichia coli (E. coli). Thiopeptin at 25 microgram/ml inhibited 75% of lactobacilli. This antibiotic at .78 microgram/ml inhibited 82% of staphylococci and more than 92% of other gram-positive organisms. Bacitracin at 100 microgram or less per ml was active against 42% of staphylococci and 30% of streptococci. At 6.25 microgram or less per ml, 70% of clostridia and 78% of lactobacilli were inhibited. The minimum inhibitory concentrations (MIC) of penicillin G and ampicillin shown as cumulative distribution curves for all isolates were very similar. Both antibiotics showed a wide range of MIC distribution. Streptomycin at 50 microgram or less per ml was active against 87% of lactobacilli and 50% of E. coli. Only 32% of staphylococci were inhibited at these concentrations. At 100 microgram or less per ml, 30% of streptococci and 12% of clostridia were inhibited. Tetracycline at 100 microgram or less per ml inhibited 63% of clostridia. Lactobacilli and E. coli were not inhibited by this antibiotic at 100 microgram or less per ml.