In vitro antibacterial activity of moxalactam, a new broad-spectrum semisynthetic antibiotic

The antibacterial activity of moxalactam was studied in vitro against 229 clinical isolates of gram-positive and gram-negative aerobic microorganisms using the agar dilution technique. Mueller-Hinton agar was used as growth medium. The results were compared to those obtained with cefamandole. All isolates of Staphylococcus aureus and Streptococcus pneumoniae were inhibited by moxalactam at a concentration of 8 microgram/ml or less. The concentrations of cefamandole with which the same effect was obtained were 0.5 microgram/ml and 2 microgram/ml respectively. Moxalactam was highly inhibitory against Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis and Proteus morganii - 90% of the strains were inhibited by 0.125 microgram/ml. Moxalactam was highly superior against Proteus rettgeri and Pseudomonas aeruginosa, which are usually resistant to cefamandole: the MIC100 and MIC90 were 0.25 microgram/ml and 8 microgram/ml respectively. High sensitivity was found in strains of Salmonella species, nine of which were Salmonella typhi: the MIC90 was < 0.063 microgram/ml versus the eightfold higher concentration of cefamandole. The broad-spectrum activity and unusual MIC patterns of moxalactam - eight or manyfold higher concentrations of cefamandole were needed to inhibit 90% of most gram-negative strains studied - make moxalactam an unusual and promising antibiotic.