Mechanisms of erythromycin and trimethoprim resistance in the Alaskan Streptococcus pneumoniae serotype 6B clone.

Abstract

Sir, Clinical isolates of Streptococcus pneumoniae with reduced susceptibility to penicillin have increased worldwide in the past 20 years. Parallel to the spread of penicillin resistance has been the increase in resistance to other antibiotics. Although the mechanisms of penicillin resistance are not directly linked with other antibiotic resistance, strains resistant to penicillin are more likely to be resistant to macrolides, trimethoprim/sulfamethoxazole, tetracyclines, clindamycin and chloramphenicol. This global increase in multidrug-resistant pneumococci appears to result, in part, from the spread of individual highly resistant pneumococcal clones. We have previously described the emergence of a distinct pneumococcal serotype 6B clone with reduced susceptibility to penicillin, erythromycin and trimethoprim/ sulfamethoxazole in a remote region of Alaska. Pulsedfield gel electrophoresis typing of the 142 invasive pneumococcal serotype 6B isolates collected from 1982 to 1997, showed that 48% were genetically related, of which 37% and 40% were resistant to erythromycin and trimethoprim/sulfamethoxazole, respectively. Here we describe the mechanisms of resistance to erythromycin and trimethoprim in the Alaskan 6B clone and unrelated 6B isolates. Resistance to macrolides is generally mediated by methylation of a specific adenine residue in 23S rRNA and is associated with the erm(B) gene, which encodes an rRNA methylase and confers resistance to macrolides, lincosamides and streptogramin B antibiotics (MLSB phenotype), or by active macrolide-specific efflux from cells mediated by a membrane protein encoded by the mef(A) gene, which confers resistance to the 14and 15-membered macrolides (M phenotype). In this study, the mechanism of erythromycin resistance was determined by PCR in 43 isolates of invasive S. pneumoniae serotype 6B selected from a collection of 123 serotype 6B isolates submitted to the Arctic Investigations Program Laboratory between 1987 and 1997, as part of the state-wide surveillance programme. This sample included 30 isolates from the previously described Alaskan 6B clone and was selected to represent isolates recovered over the 11 year time period and a range of erythromycin MICs (0.03–16 mg/L) (see Table). The Table also lists the remaining 13 nonclonal serotype 6B isolates, 11 of which were resistant to erythromycin. Among the 25 Alaskan erythromycin-resistant clonal serotype 6B isolates (MICs of 2–16 mg/L), we found that the mef(A) gene was the only mechanism of macrolide resistance; of these, 92% and 100%, respectively, were also penicillinand trimethoprim-resistant (see Table). The mef(A) gene was also found in all but two of the erythromycin-resistant non-clonal serotype 6B Alaskan isolates analysed. In mating experiments, we were able to move the mef(A) gene from seven of the S. pneumoniae donors tested to the S. pneumoniae recipient at frequencies ranging from 1.0 10 to 3.3 10 per recipient, indicating that mef(A) is on a mobile element. Trimethoprim resistance in pneumococci has been reported to result from a single amino acid substitution (Ile-100 Leu) in the dihydrofolate reductase (DHFR) protein. The mechanism of resistance to trimethoprim in invasive pneumococcal isolates from Alaska was determined by sequencing the dhf gene from 19 clonal serotype 6B isolates. These 19 clonal serotype 6B isolates were also selected to represent isolates recovered over the 11 year time period and had trimethoprim/sulfamethoxazole MICs from 0.5/9.5 to 16/304 mg/L (see Table). In addition, the dhf gene from five randomly selected non-clonal serotype 6B isolates and a serotype 6B isolate (IC7), representing the Spanish/Icelandic 6B clone were also sequenced (see Table). In this study, it was found that the sequence variability among susceptible and intermediate pneumococcal dhf alleles was limited, compared with the variation seen within resistant pneumococcal dhf alleles (0.1% versus 5–11%), and that the conservative replacement of Ile-100 by leucine is associated with trimethoprim resistance (MIC 4 mg/L). Pneumococcal isolates that were intermediately resistant to trimethoprim (1–2 mg/L) did not have the isoleucine leucine substitution at residue 100. Whether Correspondence Journal of Antimicrobial Chemotherapy (2001) 48, 315–329