Survey of antimicrobial activity of four commonly used third generation cephalosporins tested against recent bacterial isolates from ten American medical centers, and assessment of disk diffusion test performance. AST Surveillance Group.

Over 2,000 clinical isolates from ten American medical centers were tested for susceptibility to cefotaxine, ceftriaxone, ceftizoxime, and ceftazidime by both broth microdilution and disk diffusion methods. Typically resistant (e.g. enterococci) and highly susceptible (e.g. streptococci) isolates showed no change in susceptibility patterns compared to previous surveys. Pseudomonas aeruginosa and Stenotrophomonas maltophilia exhibited significant decreases in susceptibility to these cephalosporins. Among Escherichia coli and Klebsiella spp isolates, two to three percent were resistant to one of the four drugs whereas they were very susceptible to the other three. Of the four antibiotics, the cefotaxime disk diffusion test results correlated best with the microdilution results. With the other three drugs the disk diffusion test yielded 1 to 9% more susceptible test results and 1 to 20% fewer resistant test results than broth microdilution when testing gram negative bacilli. The clinical significance of such discrepancies is not known, but the impact on antibiotic susceptibility surveys and antibiogram comparisons could be significant.