Comparative inhibitory and bactericidal activities of moxalactam and seven other antibiotics against 520 clinical isolates including 65 multi-drug resistant strains

The in-vitro antibacterial activity of moxalactam, cephalothin, cefamandole, cefoxitin, ticarcillin, gentamicin, tobramycin, and amikacin was tested for two inocula against 520 clinical isolates with a microdilution technique. Moxalactam was the most effective agent against Enterobacteriaceae with a geometric mean (Gm) of the minimum inhibitory concentration (MIC) ≤ mg/l. Moxalactam was slightly more active than ticarcillin against Pseudomonas aeruginosa and less active than ticarcillin against Acinetobacier spp. For Staphylococcus aureus, Staph. epidermidis and streptococci the Gm MIC of moxalactam was higher than for the other cephalosporins and ticarcillin. Of 58 aminoglycoside-resistant isolates only one strain was resistant to moxalactam. Isolates resistant to either cefamandole or cefoxitin were inhibited by moxalactam (MIC90 ≤2 mg/l); however, resistance to both of these cephalosporins was associated with an increase in the MIC for moxalactam to > l6·0 mg/l. With a two log increase in the inoculum a sharp increase in the MIC of cefamandole occurred for four genera, for ticarcillin for two and cephalothin, amikacin and moxalactam for only one genus. Minimum bactericidal concentrations (MBCs) were the same as the 16–18 h MlCs and 36–38 h MICs for 85·9% and 94·0% of the isolates respectively. Quantitative loop and replicator subcultures for MBCs were within one dilution for 97% of the isolates. Todd Hewitt broth provided for the best antimicrobial susceptibility testing of streptococci since it supported the growth of these organisms better than the three other media used.