Clindamycin-resistant Streptococcus pneumoniae.

To the Editor: Antimicrobial medications classified as macrolides (e.g., erythromycin) and lincosamides (e.g., clindamycin) show strong activity against streptococci and are commonly used to treat community-acquired infections caused by Streptococcus pneumoniae. Moreover, these drugs are the recommended alternatives for patients who cannot tolerate [beta]-lactams.

Two main macrolide-resistant S. pneumoniae phenotypes have been reported (1). The first has a high level of resistance to all macrolides, lincosamides, ketolides, and streptogramins B due to ribosomal dimethylation, 23S rRNA mutations, or ribosomal protein mutations ([MLS.sub.B], [MS.sub.B], ML, [MKS.sub.B], and K phenotypes). The second is characterized by a low-level resistance (e.g., MIC 2-4 mg/L) to only 14- and 15-member ring macrolides (M phenotype) because of mef gene-mediated active drug efflux mechanism.

In January 2005, an erythromycin-susceptible but clindamycin-resistant pneumococcal strain was obtained from a conjunctival swab of a 10-month-old female outpatient attending the daycare center of the Clinic and Laboratory of Infectious Diseases, Siena University, Siena, Italy. To our Knowledge, such a phenotype has not been reported in the international literature for S. pneumoniae, although a similar phenotype of S. agalactiae was described by Malbruny et al. (2).

The S. pneumoniae isolate was identified by standard procedures (3) and confirmed by PCR for the common capsule gene cpsA (4). Serotyping, performed by Quellung reaction, showed a 35F serotype. Susceptibility testing was carried out by disk diffusion and confirmed with E-test according to Clinical and Laboratory Standards Institute standards (5,6) for penicillin, ceftriaxone, ciprofloxacin, erythromycin, clindamycin, linezolid, and quinupristin-dalfopristin. For telithromycin, because an E-test strip was unavailable, a microbroth dilution method was used.

The strain was susceptible to ceftriaxone (MIC 0.125 mg/L), ciprofloxacin (MIC 0.125 rag/L), erythromycin (MIC 0.125 mg/L), linezolid (MIC 1.5 mg/L), quinupristin/dalfopristin (MIC 0.5 mg/L), and telithromycin (MIC <0.0035 mg/L); it was not susceptible to penicillin (MIC 0.125 rag/L) and was resistant to clindamycin (MIC 1 mg/L). A triple disk-diffusion test with erythromycin, clindamycin, and josamycin was performed to test resistance inducibility. No inducible pattern was shown.

To understand the possible resistance mechanism, MICs for 2 lincosamides (clindamycin and lincomycin) were determined by using a microbroth dilution method in the presence and absence of 10 mg/L of the efflux pump inhibitor reserpine (Sigma Chemicals, St Louis, MO, USA), as described (7); S. pneumoniae ATCC 49619 and S. mitis 21A29 ([mefE.sup.+]) were used as controls (8). The MICs remained unchanged in the presence of reserpine: 1 mg/L for clindamycin and 4 mg/L for lincomycin.

The strain was screened for ermTR, ermB or mefA, and meJE determinants as described (8,9). All PCR controls gave the expected results. No PCR product was obtained for the studied isolate.

Preliminary data did not show classic macrolide resistance determinants for S. pneumoniae. Low-level lincosamide resistance suggests the presence of some efflux mechanism, even if no inhibition by reserpine was observed. Moreover, no mutations of ribosomal proteins and of known binding sites for lincosamides in rRNA (1) were shown by sequencing of L22, L4, and 23S rRNA domain II and V genes with primers described by Canu et al. (10). Although these findings are preliminary and the molecular basis for resistance is the subject of ongoing investigation, the identification of this S. pneumoniae phenotype may affect clinical management of pneumococcal infections, especially in the treatment of patients intolerant of [beta]-lactams.

Acknowledgments

We thank Elisabetta Mantengoli for useful suggestions on gene sequencing and Sanofi Aventis for providing telithromycin.

Strain serotyping was performed at Streptococcal Reference Unit of Respiratory and Systemic Infection Laboratory, Centre for Infections, Health Protection Agency, London, UK.

References

(1.) Edelstein PH. Pneumococcal resistance to macrolides, lincosamides, ketolides, and streptogramin B agents: molecular mechanisms and resistance phenotype. Clin Infect Dis. 2004;38:S322-7.

(2.) Malbruny B, Werno AM, Anderson TP, Murdroch DR, Leclercq R. A new phenotype of resistance to lincosamide and streptogramin A-type antibiotics in Streptococcus agalactiae in New Zealand. J Antimicrob Chemother. 2004;54:1040-4.

(3.) Rouff KL, Whiley RA, Beighton D. Streptococcus. In: Murray PR, Baron EJ, Jorgensen JH, Pfaller MA, Yolken RH, editors. Manual of clinical microbiology. 8th ed. Washington: American Society for Microbiology; 2003. p. 405-21.

(4.) Lawrence ER, Griffiths DB, Martin SA, George RC, Hall LM. Evaluation of semi-automated multiplex PCR assay for determination of Streptococcus pneumoniae serotypes and serogroups. J Clin Microbiol. 2003;41:601-17.

(5.) Clinical and Laboratory Standards Institute. Performance standards for antimicrobial disk susceptibility tests, 9th ed. Approved standard M2-A9. Wayne (PA): The Institute; 2006.

(6.) Clinical and Laboratory Standards Institute. Performance standards for antimicrobial susceptibility testing, 16th Informational Suppl. M100-S16. Wayne (PA): The Institute; 2006.

(7.) Brenwald NP, Martin JG, Wise R. Prevalence of a putative efflux mechanism among fluoroquinolone-resistant clinical isolates of Streptococcus pneumoniae. Antimicrob Agents Chemother. 1998;42:2032-5.

(8.) Oster P, Zanchi A, Cresti S, Lattanzi M, Montagnani F, Cellesi C, et al. Patterns of macrolide resistance determinants among community-acquired Streptococcus pneumoniae isolates over a 5-year period of decreased macrolide susceptibility rates. Antimicrob Agents Chemother. 1999;43:2510-2.

(9.) Cresti S, Lattanzi M, Zanchi A, Montagnani F, Pollini S, Cellesi C, et al. Resistance determinants and clonal diversity in group A streptococci collected during a period of increasing macrolide resistance. Antimicrob Agents Chemother. 2002;46:1816-22.

(10.) Canu A, Malbruny B, Coquemont N, Davies TA, Appelbaum PC, Leclercq R. Diversity of ribosomal mutations conferring resistance to macrolides, clindamycin, streptogramin, and telithromycin in Streptococcus pneumoniae. Antimicrob Agents Chemother. 2002;46:125-31.

Address for correspondence: Francesca Montagnani, Clinica e Laboratorio di Malattie Infettive, Dipartimento di Biologia Molecolare, Universita di Siena, Ospedale Le Scotte, Piano 0 lotto IV, Viale Bracci, 16, 53100 Siena, Italy; email: [e-mail]

Francesca Montagnani, * (1) Alessandra Zanchi, * (1) Lucia Stolzuoli, * Leonardo Croci, * and Carla Cellesi *

* Clinica e Laboratorio di Malattie Infettive, Universita degli Studi di Siena, Italy

(1) Contributed equally to this work.