Vol.57 No.3 May 2009

Resistance of Streptococcus pneumoniae to clarithromycin developing by serial exposure

Takeshi Saika¹⁾, Kaoru Matsuzaki¹⁾, Miyuki Hasegawa¹⁾, Akiko Kanayama¹⁾, Fumiaki Ikeda¹⁾, Intetsu Kobayashi²⁾, Akiyoshi Tsuji²⁾ and Noboru Yamanaka³⁾

¹⁾Chemotherapy Division, Mitsubishi Chemical Medience Co., 3-30-1 Shimura, Itabashi-ku, Tokyo, Japan
²⁾Department of Infection Control and Prevention, School of Nursing, Faculty of Medicine, Toho University
³⁾Department of Otolaryngology - Head and Neck Surgery, Wakayama Medical University

Abstract

We studied changes in susceptibility to clarithromycin(CAM) and in molecular analysis using 8 CAM-susceptible Streptococcus pneumoniae (MIC: 0.03-0.12 μg/mL) after exposure to CAM.
Test strains were cultured 10 times on Mueller Hinton agar(MHA) with 5% horse blood containing sub-MICs of CAM. Also, strains were cultured 15 times on MHA with 5% horse blood containing CAM at gradually higher concentrations, then cultured 10 times on the same agar without CAM. Using these strains, we determined susceptibility to CAM, detected mefA and ermB genes, and analyzed mutations of 23S rRNA gene, rplD and rplV (encoding L4 and L22 protein).
CAM MICs of mefA- or ermB-positive strains were increased by 10-times exposure to sub-MIC of CAM, so their MICs were 4 to 64 times higher than before. Serial exposure to high CAM concentration made 6 of 8 test strains highly resistant (MIC: ≥64 μg/mL). Four had point mutations in 23S rRNA domain V and one had ermB. The remaining strain had no mutations. CAM MIC to the ermB-positive strain cultured on agar without CAM decreased to less than 1/16, from 64 μg/mL to 8 μg/mL.
Our results showed that an mefA- or ermB-positive strain exposed to low-concentration of CAM is made resistant by promoting the production of modification enzymes and efflux. It appears that clinical isolates had resistant genes but susceptible to macrolides in the phenotype will be resistant by inadequate chemotherapy with the antibiotics. Resistant genes such as mefA and ermB should thus be monitored to ensure appropriate chemotherapy.

Key word

Streptococcus pneumoniae, clarithromycin, mefA, ermB, point mutation

Received

August 12, 2008

Accepted

March 10, 2009

Jpn. J. Chemother. 57 (3): 203-207, 2009