Influence of inoculum size on comparative susceptibilities of penicillinase-positive and -negative Neisseria gonorrhoeae to 31 antimicrobial agents

Abstract

The effects of two inoculum sizes (10(4) and 10(6) CFU) on the MICs of 20 beta-lactam antibiotics, 4 aminoglycosides, and 7 other antimicrobial agents were compared for 102 unselected strains of Neisseria gonorrhoeae (26 penicillinase positive and 76 penicillinase negative), with three replicates for each test. The method was agar plate dilution on Mueller-Hinton agar supplemented with 1% hemoglobin and 1% IsoVitaleX. For penicillinase-positive strains, a large inoculum (10(6) CFU) increased the MIC greater than or equal to 16-fold for benzylpenicillin, piperacillin, azlocillin, and mezlocillin and increased the MIC greater than or equal to 8-fold for ampicillin, cefoperazone, ceftazidime, cefonicid, and cefamandole. The inoculum size had little or no influence on MICs of any antimicrobial agent for penicillinase-negative strains. For penicillinase-positive strains, the most active antibiotics (MIC, less than or equal to 0.001 microgram/ml) were the new cephalosporins: cefotaxime, ceftazidime, ceftizoxime, ceftriaxone, and cefmenoxime. For penicillinase-negative strains, the most active antibiotics were piperacillin, ceftriaxone, ceftizoxime, and cefmenoxime (MIC, less than or equal to 0.001 microgram/ml), closely followed by ceftazidime, moxalactam, azlocillin, mezlocillin, and cefuroxime.