The in vitro activity of 8 cephalosporins – cephalothin, cefamandole, cefoxitin, cefuroxime, cefotaxime, cefoperazone, moxalactam and ceftriaxone (Rocephin®) was studied on 109 strains of Neisseria gonorrhoeae and 60 strains of Neisseria meningitidis, isolated from pathological material. Determination of the MICs of these antibiotics by an agar dilution method shows that on N. gonorrhoeae ceftriaxone (geometric mean of the MICs: 0.0008 μg/ml) is the most active, followed by cefotaxime (0.001 μg/ml); cefoperazone (0.008 μg/ml) and moxalactam (0.01 μg/ml) are one-tenth as active; then come cefuroxime (0.05 μg/ml), cefamandole (0.10 μg/ml) and finally cephalothin (0.26 μg/ml) and cefoxitin (0.26 μg/ml). The least susceptibility to penicillin and, to a greater degree, the production of β-lactamase (8 strains) affect the level of susceptibility to these cephalosporins, but the MICs always remain relatively low.

The MICs and MBCs of mecillinam, ticarcillin, mezlocillin, azlocillin and piperacillin were determined by the microdilution method in liquid medium using 700 strains of gram-negative bacilli and enterococci isolated from pathological sources and classified as a function of their sensitivity to ampicillin and carbenicillin. The ampicillin and carbenicillin-sensitive strains were generally sensitive to the other penicillins, although there were differences in activity. The ampicillin and carbenicillin-resistant strains of Escherichia coli that produce a TEM-type penicillinase were sensitive to mecillinam. Mezlocillin, piperacillin and azlocillin had MICs of between 32 and 64 mg/l for 40% of these strains. The Klebsiella strains, whose broad-spectrum penicillinase deactivates ampicillin and carbenicillin, remained sensitive to mecillinam. Mezlocillin, azlocillin and piperacillin had MICs of less than 8 mg/l for 50% of these strains. The carbenicillin-resistant strains of Enterobacter and Citrobacter were also resistant to the other penicillins. Piperacillin and mezlocillin displayed some activity against certain strains of carbenicillin-resistant Serratia, Proteus and Acinetobacter. Azlocillin, piperacillin and, to a lesser degree, mezlocillin were active against the strains of Pseudomonas, for which carbenicillin had an MIC of about 512 mg/l. Ampicillin, mezlocillin and azlocillin showed the best activity against the enterococci, against which mecillinam was inactive. The MBC of these antibiotics is greatly influenced by the density of the bacterial inoculum.

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