Comparative activity of six beta-lactam antibiotics against strains of Haemophilus influenzae, Neisseria meningitidis and Streptococcus pneumoniae.

The minimum inhibitory concentrations (MIC) of penicillin G, ampicillin, mezlocillin, azlocillin, cephalothin and cefoxitin were determined for 47 strains of Haemophilus influenzae, 68 strains of Neisseria meningitidis and 45 strains of Streptococcus pneumoniae. These strains were isolated during the past three years from patients with acute bacterial meningitis. Three strains of H. influenzae were ampicillin-resistant while no pneumococcus or meningococcus strain was penicillin-resistant. Mezlocillin was the most potent antibiotic against the Haemophilus and pneumococcus strains, followed closely by azlocillin. Mezlocillin inhibited 77.7% of the meningococci strains tested at a concentration of 0.03 mg/l. Penicillin G was the most effective of the drugs against these strains. It inhibited 100% at a concentration of 0.5 mg/l. The cephalosporins were the least active of the six beta-lactam antibiotics tested.