Minimal inhibition concentration (MIC) values of 100 Finnish and 100 Israeli Escherichia coli isolated from clinical bovine mastitis were determined for ampicillin, cephalexin, ceftazidime, dihydrostreptomycin, gentamicin, tetracycline, trimethoprim-sulfadiazine, and ciprofloxacin by an agar dilution method. The in vitro antimicrobial susceptibility of the E. coli isolates was high; only 27% showed resistance to one or more tested antimicrobial agents. Fifteen percent of the Israeli isolates and 14% of the Finnish isolates were resistant to tetracycline, 3 and 16% to cephalexin, 10 and 7% to ampicillin, 13 and 9% to dihydrostreptomycin, and 4 and 2% to trimethoprim-sulfadiazine. No gentamicin-, ceftazidime-, or ciprofloxacin-resistant isolates were detected. Eleven percent of all the isolates were resistant to two or more antimicrobial agents. Tetracycline was most often associated with multiresistant patterns. Most of the multiresistant isolates had very high MIC values, whereas most of those that were resistant to only one tested antibiotic had MIC values close to the susceptibility breakpoint. Antimicrobial resistance appeared to pose no problem in E. coli isolated from mastitic milk of both countries. This is probably due to the controlled use of antimicrobial agents in the treatment of dairy herds. Some differences were present in the resistance patterns, which may reflect the different use of antimicrobial agents in these two countries.
A novel approach was used to assess disk diffusion accuracy for determination of antibiotic susceptibility of various bovine mastitis pathogens (Escherichia coli, Staphylococcus aureus, Staphylococcus chromogenes, and Streptococcus dysgalactiae). MIC and disk diffusion diameters were compared for 587 bovine mastitis bacterial isolates collected in Israel and 3,186 drug-organism combinations. Results were analyzed by ROC curves, Bayesian statistics, and standard descriptive methods. Low correlation was observed between results of disk diffusion and MIC for S. dysgalactiae and all antimicrobial agents, S. aureus and erythromycin and neomycin, and E. coli and gentamicin, neomycin, and polymyxin B. On a few occasions in which correlation was satisfactory, accepted susceptibility breakpoints to some of the antimicrobial agents resulted in high discrepancies with MIC results and new breakpoints were suggested-e.g., 21 mm for S. aureus susceptibility to penicillin G instead of 29 mm recommended by the National Committee for Clinical Laboratory Standards (NCCLS) and <21 mm, resistant, 21-25 mm, intermediate, and >25 mm, susceptible for susceptibility of E. coli to trimethoprim/sulfamethoxazole. Thus, this approach enabled determination of the most accurate breakpoints that best fitted the specific prevalence of susceptibility in Israel. Thus, we suggest its adoption by microbiology diagnostic laboratories for the provision of accurate antimicrobial susceptibility results when using the disk diffusion test.
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