The minimal inhibitory and bactericidal concentrations for 108 group B streptococcal strains were determined for two new cephalosporins, cefonicid and ceftizoxime, and compared to those of penicillin G. Penicillin G was the most active inhibitory and bactericidal agent. Ceftizoxime was a significantly more active inhibitory and bactericidal agent than cefonicid (P < 0.0005, P < 0.025, respectively).Penicillin G and ampicillin have remained the drugs of first choice in infections due to group B streptococci (GBS) because of low minimal inhibitory and bactericidal concentrations (MICs, MBCs) reported in most studies (3). However, investigators have been prompted to examine non-penicillin regimens for in vitro inhibitory and bactericidal efficacy for several reasons: (i) high morbidity and mortality rates of GBS infections, especially in neonates (2); (ii) increasing reports of relapse or recurrence of GBS infection despite penicillin therapy (4); (iii) reports of GBS strains relatively resistant or tolerant to the killing action of penicillin (1,8); and (iv) limited data on alternative (non-penicillin) bactericidal agents for GBS, especially for patients who are allergic to penicillin or who develop penicillin end-organ toxicity (e.g., nephropathy).We have examined the in vitro inhibitory and bactericidal activity of two new cephalosporins, cefonicid (first generation) and ceftizoxime (third generation), against 108 blood and spinal fluid GBS isolates and compared these activities with those of penicillin G.The bacterial strains used in this study were identified as GBS according to Lancefield's criteria (9). The GBS were isolates of the Harbor-UCLA Medical Center, the University of Minnesota Hospitals, the University of Alabama Medical Center, and the Charity Hospital of New Orleans, Louisiana, between 1976 and 1980. There were no duplicate isolates from the same patient. Aliquots of logarithmic-phase cultures of each strain in Mueller-Hinton broth (Difco Laboratories, Detroit, Mich.) were stored at -70°C until susceptibility testing was performed.A late-logarithmic-phase culture of each GBS isolate was prepared in Mueller-Hinton broth as previously described (7,8) at -2 x 105 colonyforming units per ml. Potassium penicillin G (Bristol Laboratories, Syracuse, N.Y.), and cefonicid and ceftizoxime (Smith, Kline & French Laboratories, Philadelphia, Pa.) were reconstituted to concentrations of 10,000 ,ug/ml and stored in aliquots at -70°C until the susceptibility testing was performed (within 4 weeks of freezing).MICs and MBCs of the 108 GBS isolates to penicillin, cefonicid, and ceftizoxime were determined in duplicate by the microtiter brothdilution technique; a GBS control strain with previously determined MICs and MBCs was included with each run as a check on reproducibility. Each antibiotic agent to be tested was thawed immediately before use, and serial twofold dilutions were made in Mueller-Hinton broth in the microtiter wells. For cefonicid and ceftizoxime, the final drug concentration range in the wells wa...