In vitro evaluation of Ro 13-9904.

The in vitro activity of a new investigational cephalosporin, Ro 13-9904, was compared with those of four cephalosporins (cephalothin, cefamandole, cefoxitin, and moxalactam), five semisynthetic penicillins (mezlocillin, piperacillin, carbenicillin, ticarcillin, and azlocillin), and the aminoglycoside tobramycin. Ro 13-9904 inhibited 75% of all isolates of Enterobacteriaceae at a concentration of 6.25 micrograms/ml, including Enterobacter spp., Serratia marcescens, and indole-positive Proteus spp. Ro 13-9904 was only minimally active against Pseudomonas aeruginosa and enterococci. There was no significant change (greater than or equal to 2 dilutions) in drug activity when tested in various pH and media. A significant inoculum effect occurred when the size of the inoculum was increased from a concentration of 10(5) to 10(7) organisms per ml.