First clinical isolate of Streptococcus pneumoniae exhibiting high-level resistance to fluoroquinolones in Taiwan

Sir, The increasing resistance to fluoroquinolones among Streptococcus pneumoniae has been noted worldwide. Bacterial resistance to fluoroquinolones originates from point mutations at the highly conserved amino acid residues in the quinolone resistance-determining regions (QRDRs) of the gyrA subunit of DNA gyrase and the parC subunit of DNA topoisomerase IV.1–3 In addition, an active efflux mechanism may contribute to fluoroquinolone resistance in pneumococci, particularly to ciprofloxacin.4–6 So far, the incidence of quinolone resistance in pneumococci is still low in Taiwan; however, here we report the first clinical isolate of S. pneumoniae exhibiting a high level of resistance to quinolones in Taiwan. The clinical isolate S. pneumoniae SP39 was recovered from a sputum specimen of a 65-year-old patient at Chang Gung Memorial Hospital (Lin-Kou, Taiwan) in 1999. The isolate belonged to serotype 23F and was resistant to penicillin (MIC 4 mg/L), macrolides (azithromycin MIC > 256 mg/L and clarithromycin MIC > 256 mg/L), clindamycin (MIC 32 mg/L), co-trimoxazole (MIC 32 mg/L) and tetracycline (MIC 16 mg/L), but susceptible to chloramphenicol (MIC 0.25 mg/L) and vancomycin (MIC 0.25 mg/L). The susceptibilities to various antimicrobial agents of the isolate were determined by broth dilution method recommended by the NCCLS. Ciprofloxacin and moxifloxacin were obtained from Bayer Corporation, West Haven, CT, USA. Ofloxacin and reserpine were obtained from Sigma Chemical Co., St Louis, MO, USA. Trovafloxacin was obtained from Pfizer Pharmaceuticals, New York, NY, USA. Medium containing reserpine (10 mg/L) was prepared freshly before use, and was used immediately. The reference strain S. pneumoniae ATCC 49619 was used as a control strain. The isolate SP39 exhibited high-level resistance to ciprofloxacin (MIC 32 mg/L), ofloxacin (32 mg/L), trovafloxacin (4 mg/L) and moxifloxacin (2 mg/L) in the absence of reserpine. The ciprofloxacin MIC for the isolate was significantly reduced (16-fold, from 32 to 2 mg/L) in the presence of reserpine, while other fluoroquilonones were not affected. This phenotype is suggestive of an efflux mechanism of resistance. The isolate was further characterized with respect to mutations within the QRDRs of the gyrA, gyrB, parC and parE by PCR and sequencing. The sequence data showed that the SP39 isolate bore amino acid substitutions: Ser-81→Phe (TCC to TTC) in GyrA and Ser-79→Phe (TCT to TTT) in ParC. These mutations have been previously reported in other studies and have been shown to greatly affect the activities of fluoroquinolones. Another substitution outside the QRDR region of ParC was also observed (Lys-137→Asn) (AAG to AAT). Several silent mutations were observed in the genes of gyrA, parC and paE of SP39. For unknown reasons, these changes were mostly C to T. No mutations were detected in GyrB. The combination of double mutations in ParC and GyrA, and non-gyrase targets such as overexpression of a putative efflux mechanism, may contribute the high-level fluoroquinolone resistance in isolate SP39. The emergence of fluoroquinolone resistance in S. pneumoniae is of great concern, especially in Taiwan where resistance to penicillin, extended-spectrum cephalosporins, and other agents is remarkably high. In summary, this is the first report from Taiwan of an S. pneumoniae isolate with high-level resistance to fluoroquinolones. Continued surveillance of the susceptibility of S. pneumoniae isolates to fluoroquinolones and a better understanding of resistance mechanism will be crucial for the effective management of resistant strains.