title: Low-level resistance to rifampin in Streptococcus pneumoniae.
abstract: Rifampin is recommended for combination therapy of meningitis due to beta-lactam-resistant Streptococcus pneumoniae. High-level rifampin resistance (MIC, > or =4 mg/liter) has been mapped to point mutations in clusters I and III of rpoB of the pneumococcus. The molecular basis of low-level resistance (MICs, > or =0.5 and <4 mg/liter) was analyzed. Spontaneous mutants of clinical pneumococcal isolates were selected on Columbia sheep blood agar plates containing rifampin at 0.5, 4, 10, or 50 mg/liter. Low-level resistance could be assigned to mutations in cluster II (I(545)N, I(545)L). Sensitive (MIC, <0.048 mg/liter) wild-type strains acquired low-level resistance at a rate approximately 10 times higher than that at which they acquired high-level resistance (average mutation frequencies, 2.4 x 10(-7) for low-level resistance versus 2.9 x 10(-8) for high-level resistance [P < 0.0001]). In second-step experiments, the frequencies of mutations from low- to high-level resistance were over 10 times higher than the frequencies of mutations from susceptibility to high-level resistance (average mutation frequencies, 7.2 x 10(-7) versus 5.0 x 10(-8) [P < 0.001]). Mutants with low-level resistance were stable upon passage. Sequencing of a clinical isolate with low-level resistance (MIC, 0.5 mg/liter) revealed a Q(150)R mutation upstream of cluster I. The frequencies of mutations to high-level resistance for this strain were even higher than the rates observed for the in vitro mutants. Therefore, a resistance-mediating mutation located outside clusters I, II, and III has been described for the first time in the pneumococcus. In vitro low-level rifampin resistance in S. pneumoniae could be mapped to cluster II of rpoB. Mutants of pneumococcus with low-level resistance may be selected in vivo during therapy in tissue compartments with low antibiotic concentrations and play a role in the development of resistance.
authors.affiliation: Institute for Infectious Diseases, University of Bern, University Hospital Bern, Switzerland.
authors.forename: Patricia Stutzmann
authors.initials: PS
authors.lastname: Meier
authors.forename: Silvia
authors.initials: S
authors.lastname: Utz
authors.forename: Suzanne
authors.initials: S
authors.lastname: Aebi
authors.forename: Kathrin
authors.initials: K
authors.lastname: Mühlemann
mesh_terms: D000904:Antibiotics, Antitubercular / Q000494:pharmacology*; D004269:DNA, Bacterial / Q000235:genetics; D012321:DNA-Directed RNA Polymerases / Q000235:genetics; D024881:Drug Resistance, Bacterial; D008826:Microbial Sensitivity Tests; D009154:Mutation / Q000235:genetics; D011008:Pneumococcal Infections / Q000382:microbiology; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012293:Rifampin / Q000494:pharmacology*; D013296:Streptococcus pneumoniae / Q000187:drug effects*; D014169:Transformation, Bacterial
chemical_list: D000904:Antibiotics, Antitubercular; D004269:DNA, Bacterial; D012321:DNA-Directed RNA Polymerases; C105078:RNA polymerase beta subunit; D012293:Rifampin