Antibiotic sensitivity pattern of pneumococci relatively insensitive to penicillin and cephalosporin antibiotics.

Penicillin-insensitive pneumococci, of 10 serotypes, which had been isolated in Australia and New Guinea from healthy carriers and patients with pneumococcal infections, were shown to be relatively resistant to penicillin G, penicillin V, methicillin, cloxacillin, cephaloridine and cephalothin, compared with pneumococci fully sensitive to penicillin G. Most penicillin-insensitive pneumococci either were fully sensitive to ampicillin or showed a slight decrease in sensitivity. The size of inoculum, measured in viable units, had little effect on the minimal inhibitory concentration of penicillin G; this applied to both penicillin-insensitive and penicillin-sensitive pneumococci. In bacterial tests, the minimal bactericidal concentration of penicillin G either equalled or exceeded the minimal inhibitory concentration.