Methodological implications of testing anaerobe susceptibility to cephalosporins (cefazolin, cefamandole, cefoxitin)

By simultaneously performing broth dilution, agar dilution, and agar diffusion tests with Bacteroidaceae and Peptococcaceae, the influence of methodology upon the outcome of susceptibility testing to cefazolin, cefamandole and cefoxitin was studied. With beta-lactamase positive and negative Bacteroidaceae, the results of the broth dilution and agar dilution tests were in good agreement for cefoxitin. However, when tested with cefaxolin and cefamandole, beta-lactamase positive Bacteroides strains had mostly high broth dilution MICs and relatively low agar dilution MICs. The statistical analysis of the relationship between zone size and broth dilution or agar dilution MICs frequently showed lack of stochastic linearity or relatively low correlation coefficients.