Prevalence and macrolide resistance phenotypes and genotypes among clinical isolates of Streptococcus pneumoniae collected in Sofia, Bulgaria from 2001 to 2005

A total of 328 clinical isolates of Streptococcus pneumoniae were analyzed to determine the rate of macrolide and penicillin resistance as well as macrolide resistance phenotypes and genotypes. Erythromycin resistance was found in 81 pneumococcal isolates (24.7%) and 10.7% of isolates were clindamycin resistant. The prevalence of penicillin G-intermediate (minimum inhibitory concentrations, MICs, 0.125 to 1 mu g/ml) and penicillin-resistant (MICs, >= 2 mu g/ml) S. pneumoniae isolates was 25.6% and 13.7%, respectively. The rate of ceftriaxone-intermediate and ceftriaxone-resistant strains was 2.7% and 1.2%, respectively. Among erythromycin-resistant S. pneumoniae isolates, strains harboring mef(A) genes (n=42; 51.8%) were found to be predominant over strains with erm(B) genes (n=34; 42.0%). One (1.2%) isolate carried both erm(B) and mef(A), while 4 (4.9%) isolates carried L4 protein mutations. By using the erythromycin, clindamycin and rokitamycin triple-disk test, 42 strains were assigned to the M phenotype of macrolide resistance, 31 isolates were assigned to the partially inducible (iMcLS) phenotype, 4 were assigned to the constitutive (cMLS) phenotype. Four strains with L4 gene showed a rare phenotype with the triple-disk test. Serotyping of S. pneumoniae isolates suggested that serotype (or serogroup) 14, 6 and 19 were predominant (81.5%) among erythromycin-resistant strains. Among mef(A) positive isolates serotype 14 was predominant, among erm(B) positive isolates serogroups 6 and 19 were the most prevalent.