Effect of inoculum size and medium on activity of seven antimicrobial agents against Bacteroides fragilis strains

Antimicrobial activity of cefoxitin, cefotetan, cefotaxime, ceftizoxime, ceftazidime, imipenem, and clindamycin against four inocula of Bacteroides fragilis strains was determined on three different media. The inoculum sizes were 10(4), 10(5), 10(6), and 10(7) colony forming units (CFU) per spot. On all three media, substantial effects of inoculum size on minimal inhibitory concentrations (MIC) of cefotaxime and ceftizoxime were found: the doubled dilution differences in MICs between inocula of 10(7) and 10(4) CFU/spot were 2.2, 2.3, and 2.1 micrograms/ml of cefotaxime and 1.8, 4.4, and 4.0 micrograms/ml of ceftizoxime on Brucella base-laked blood agar, Wilkins-Chalgren agar, and a brain-heart infusion medium, respectively. An inoculum difference found on all three media with ceftazidime may also be of practical significance. There was evidence of larger differences between inocula on the Wilkins-Chalgren agar and brain-heart infusion than on the Brucella agar.