Comparative in vitro studies of Ro 13-9904, a new cephalosporin derivative

The in vitro activity of Ro 13-9904, a new cephalosporin derivative, was compared with the activities of cephalothin, cefamandole, cefoxitin, cefotaxime, and moxalactam against 591 clinical isolates of gram-negative and gram-positive organisms. The spectra of activity and potency of Ro 13-9904 and cefotaxime were quite similar; they were the most active agents against Enterobacteriaceae, Streptococcus pyogenes, Haemophilus influenzae, Neisseria gonorrhoeae, and Neisseria meningitidis. Moxalactam was only slightly less active against these organisms. Ro 13-9904, cefotaxime, and moxalactam were approximately equal in activity against Pseudomonas aeruginosa; concentrations of 50 to 100 microgram/ml inhibited over 90% of the strains tested. Cefamandole and cephalothin were the most active drugs tested against staphylococci. Moxalactam demonstrated the highest intrinsic activity against Bacteroides fragilis; a concentration of 1.6 microgram/ml inhibited over 50% of the strains. All six of the antibiotics were essentially inactive against group D streptococci. The action of all of the antibiotics was bactericidal, with minimal bactericidal concentrations generally being no more than twofold greater than minimal inhibitory concentrations. The only exception to this was found when large inocula of Staphylococcus aureus were tested. Increased inoculum size generally sharply reduced the activity of Ro 13-9904, cefotaxime, and moxalactam against Enterobacteriaceae and P. aeruginosa.