Non-gyrA-mediated ciprofloxacin resistance in laboratory mutants of Streptococcus pneumoniae
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L J Piddock1, Y F Jin, M J Everett
1Antimicrobial Agents Research Group, Department of Infection, University of Birmingham, UK. l.j.v.piddock@bham.ac.uk
Abstract:
Two strains of Streptococcus pneumoniae, M4 (NCTC 7465, type strain) and M5 (clinical isolate), and their respective ciprofloxacin-resistant mutants, M4/C1, M5/C1 and M5/C3, were evaluated. All mutants were stable after one year's storage and all grew more slowly in Brain Heart Infusion broth than the parent. The MICs of ciprofloxacin, sparfloxacin and tosufloxacin were increased for the mutants of M4, whereas the mutants of M5 were less susceptible to ciprofloxacin only. The optimal bactericidal concentration (OBC) of each quinolone for all the strains was approximately ten-fold greater than the MIC. The OBCs for the mutants were increased for ciprofloxacin, but not for the other two quinolones. The DNA synthesis IC50 values of all quinolones correlated well with the MIC of each drug. All quinolones accumulated rapidly within all five strains; 10 mM magnesium chloride decreased the concentration of quinolone accumulated, but carbonyl cyanide m-chlorophenyl hydrazone had no effect. Mutant strains M4/C1, M5/C1 and M5/C3 accumulated less quinolone than their respective parent strains. DNA sequencing of those regions of gyrA and gyrB corresponding to the quinolone resistance-determining region in other bacteria did not reveal any differences between the parent and mutant strains.
Insights
This study investigated ciprofloxacin resistance in Streptococcus pneumoniae. Mutants showed slower growth and altered quinolone susceptibility, but no genetic changes in key resistance regions were found.
Area of Science:
- Microbiology
- Molecular Biology
- Pharmacology
Background:
- Quinolone antibiotics are crucial for treating Streptococcus pneumoniae infections.
- Emergence of antibiotic resistance in S. pneumoniae poses a significant public health threat.
- Understanding resistance mechanisms is vital for developing effective treatment strategies.
Purpose of the Study:
- To characterize ciprofloxacin-resistant mutants of Streptococcus pneumoniae.
- To investigate the susceptibility profiles and drug accumulation in resistant strains.
- To explore potential genetic underpinnings of quinolone resistance in these mutants.
Main Methods:
- Bacterial strains: Streptococcus pneumoniae M4 and M5, and their ciprofloxacin-resistant mutants.
- Antimicrobial susceptibility testing: determination of Minimum Inhibitory Concentrations (MICs) and Optimal Bactericidal Concentrations (OBCs).
- Drug accumulation assays and DNA sequencing of gyrA and gyrB genes.
Main Results:
- Mutants exhibited slower growth rates and altered susceptibility to quinolones, particularly ciprofloxacin.
- Optimal bactericidal concentrations (OBCs) were increased for ciprofloxacin in mutants.
- Reduced quinolone accumulation was observed in resistant strains, but no mutations in the quinolone resistance-determining regions of gyrA/gyrB were identified.
Conclusions:
- Ciprofloxacin resistance in these S. pneumoniae strains is associated with reduced drug accumulation and altered growth characteristics.
- The absence of mutations in key gyrA/gyrB regions suggests alternative resistance mechanisms may be involved.
- Further research is needed to elucidate the precise mechanisms of quinolone resistance in these strains.