Ceftriaxone: a summary of in vitro antibacterial qualities including recommendations for susceptibility tests with 30-micrograms disks.

The new aminothiazoyl-cephalosporin, ceftriaxone (Ro 13-9904), was found to have excellent inhibitory activity against the Enterobacteriaceae (minimum inhibitory concentration needed to inhibit 50% of isolates (MIC50) less than or equal to 0.004-0.5 microgram/ml, Haemophilus influenzae (MIC50 less than or equal to 0.004 micrograms/ml), Neisseria species (MIC50 less than or equal to 0.001 microgram/ml), pneumococci (MIC50 0.25 micrograms/ml), Staphylococcus aureus (MIC50 2.0 micrograms/ml), and Streptococcus pyogenes (MIC50 0.015 micrograms/ml). Ceftriaxone was less effective against Acinetobacter species, Pseudomonas aeruginosa, and other Pseudomonas species (MIC50 8.0-16 micrograms/ml). Methicillin-resistant S. aureus and enterococci were not significantly inhibited by ceftriaxone. Ceftriaxone was very resistant to beta-lactamase hydrolysis, although the type IV cephalosporinase minimally destroyed the compound at 16.4-19.9% of the rates for cephaloridine. Type I cephalosporinases were inhibited by ceftriaxone and related enzyme-stable cephalosporins. Based on analysis of disk-MIC regression statistics, tentative recommendations for the disk test of the National Committee for Clinical Laboratory Standards are 21 mm or more = susceptible, 14-20 mm = moderately susceptible, and 13 mm or less = resistant. These criteria produce interpretive accuracy of more than 92%, with very rare major errors. Ceftriaxone was comparable to cefotaxime in spectrum and activity, thus allowing the use of the "spectrum-class" concept (for example, cefotaxime tests in vitro to predict ceftriaxone susceptibility, and vice versa).