Surveillance of Streptococcus pneumoniae serotypes in Riyadh and their susceptibility to penicillin and other commonly prescribed antibiotics.

A total of 358 recent distinct isolates of Streptococcus pneumoniae were recovered from clinical specimens of patients in various hospitals in Riyadh, Saudi Arabia. The commonest serotypes were Groups C, F, B and A. Using specific monosera for typing it was found that serotype 14 was the commonest followed by serotypes 3, 7, 1, 2, 19 and 8 respectively. The minimal inhibitory concentrations (MIC) of penicillin determined by an agar dilution method, showed that 81% were susceptible (MIC less than 0.1 mg/L), 18% were relatively resistant (MIC = 0.1-1 mg/L) and 1% showed increased resistance (MIC greater than or equal to 1.0 mg/L). The use of a 1 microgram oxacillin disc distinguished between susceptible and relatively penicillin resistant pneumococci more reliably than did the use of a penicillin disc (1 or 10 micrograms). Resistance of S. pneumoniae to tetracycline, co-trimoxazole, chloramphenicol, and erythromycin were 70%, 43%, 12% and 4% respectively. All isolates were susceptible to oral cephalosporins (cefadroxil, cephalexin, cefaclor, and cefuroxime axetil) with an MIC range of less than or equal to 0.03-2 mg/L. The selection of antimicrobial therapy and the efficiency of vaccines depend on the knowledge of the local isolates of S. pneumoniae. Clinical isolates should be routinely screened to detect susceptibility to penicillin. The relatively high incidence of resistance to multiple antibiotics indicates the need to perform antibiotic susceptibility testing in order to avoid possible therapeutic failure.