- Although penicillin-resistant pneumococci (PR-PnC) are recognized as an increasing problem worldwide, data on the prevalence of these strains among pediatric patients are incomplete. The present study was conducted in southern Israel (1) to investigate the frequency of PR-PnC in invasive and middle ear infections in pediatric patients and (2) to assess the impact of resistance on the potential role of the candidate conjugate vaccines in preventing childhood PR-PnC infections. A total of 120 blood or cerebrospinal fluid isolates from 1987 to 1993 and 78 ear isolates from 1992 to 1993 were serogrouped and tested for susceptibility to antibacterial agents. The prevalence of PR-PnC among invasive isolates increased from 16% in the years 1987 to 1991 to 36% in 1992 to 1993 (P = 0.019). This increase was noted mainly for intermediately resistant strains (minimal inhibitory concentration, 0.12 to 1.0 micrograms/ml) whereas the prevalence of highly resistant strains was 3 and 2% for the 2 periods, respectively. The prevalence of PR-PnC among ear isolates in 1992 to 1993 was 42%. Resistance to other antimicrobial agents (one or more of the following: tetracycline, erythromycin, clindamycin and chloramphenicol) was found in 16 (8%) isolates, and multiple resistance (resistance to > or = 3 antibacterial agents) was found in 9 (5%) isolates. Sixty-five (99%) of the 66 resistant isolates belong to Serogroups 6, 14, 19 and 23. The prevalence of these 4 serogroups rose from 37% in 1987 to 1991 to 66% in 1992 to 1993 (P = 0.043). This rise was mainly because of Serogroup 23, the prevalence of which rose from 3% in 1987 to 1991 to 23% in 1992 to 1993 (P < 0.001). Eighty-five percent of all isolates belonging to Serogroup 23 were resistant to penicillin. Because Serogroups 6, 14, 19 and 23 are among the commonest pediatric pneumococcal strains, the newly developed conjugate pneumococcal vaccines contain these 4 serogroups. The selection of antibiotic-resistant strains has thus led to a change in the spectrum of serotypes causing invasive disease and to a situation of potential increase in vaccine coverage for the proposed pneumococcal conjugate vaccines.