a Department of Pathology, King Fahad National Guard Hospital, PO Box 22490, Riyadh 11426, Saudi Arabia b Department of Infection Control, King Fahad National Guard Hospital, PO Box 22490, Riyadh 11426, Saudi Arabia
Sir,
Reduced susceptibility to penicillin amongst specific serotypic clusters of Streptococcus pneumoniaeisolates has been described previously. 1 Regional differences in susceptibility patterns have also been recognized. 2 In the present study, we investigated the relationship between serotype and antibiotic susceptibility for pneumococci isolated between 1994 and 1996 in the central and western regions of Saudi Arabia.
One hundred and thirty-one randomly selected, non-replicate clinical isolates of S. pneumoniaewere identified by standard laboratory procedures. MICs of penicillin, cefotaxime, imipenem, trimethoprim/sulphamethoxazole, erythromycin, ciprofloxacin, vancomycin and chloramphenicol for the strains were determined by the Etest method (AB Biodisk, Solna, Sweden). The medium used was thymidine-free Mueller- Hinton agar supplemented with 5% lysed sheep blood and the turbidity of the inoculum was equivalent to that of a 0.5 McFarland standard. Susceptibility categories were assigned according to MIC breakpoints recommended by the National Committee for Clinical Laboratory Standards. 3 Serotyping was performed by the Quellung reaction with pneumococcal antisera by Marguerite Lovgren at the National Centre for Streptococcus, Edmonton, Alberta, Canada; the Danish nomenclatural system was used. 2
Of the 131 isolates, 111 were typable and 48 of these exhibited reduced susceptibility to penicillin (37 intermediate susceptibility and 11 resistance). As the purpose of the study was to attempt to correlate the serotype and the antibiotic profile of strains exhibiting reduced susceptibility to penicillin, only data relating to these 48 strains were analysed. Of the 37 (33.3%) strains exhibiting intermediate susceptibility, the predominant serotypes were 14 (eight (21.6%) strains) and 23F (seven (18.9%) strains), whereas seven of the 11 (63.6%) that were resistant to penicillin belonged to serotype 9V. The antibiotic susceptibilities of the pneumococci exhibiting reduced susceptibility to penicillin are summarized in the Table. According to criteria proposed by Butler et al., 4 38 (34.2%) of the 111 typable isolates were categorized as multiresistant, i.e. resistant to two or more of the six major antibiotic groups tested (data not shown); the predominant serotypes were 9V, 14 and 23F. No correlation between penicillin-resistant strains belonging to serotype 9V and either temporal and/or geographic clustering was detected.
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The emergence of 9V as the predominant serotype amongst our penicillin-resistant strains is a recent observation and parallels the increase in the incidence of this serotype noted in other surveys. 4 Periodic surveillance of pneumococcal antibiotic/serotype relationships serves a useful purpose in terms of facilitating local therapeutic strategies and formulating vaccines that are relevant to the local population.
Notes
References
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