title: In vitro selection of resistance in Streptococcus pneumoniae at in vivo fluoroquinolone concentrations.
abstract: OBJECTIVE
  To compare the ability to select for resistance in Streptococcus pneumoniae of levofloxacin, moxifloxacin, ciprofloxacin and prulifloxacin.
  
  
  METHODS
  Twenty strains of S. pneumoniae susceptible to fluoroquinolones were used. The frequencies of spontaneous single-step mutations at plasma and epithelial lining fluid (ELF) peak and trough antibiotic concentrations were calculated. Multi-step selection of resistance was evaluated by performing 10 serial subcultures on agar plates containing a linear gradient from peak to trough antimicrobial concentrations, followed by 10 subcultures on antibiotic-free agar. Resistant strains selected after multi-step selection were characterized for DNA mutations by sequencing gyrA, gyrB, parC and parE genes.
  
  
  RESULTS
  Levofloxacin and moxifloxacin showed the lowest frequencies of mutations (median <10(-11)) at plasma peak and at ELF concentrations, while medians ranging from 10(-8) to 10(-6) were observed for ciprofloxacin and prulifloxacin. In a multi-step selection assay, ciprofloxacin and prulifloxacin selected for the highest number of resistant strains (19 and 31, respectively). No selection of resistance was observed for levofloxacin at ELF concentrations and for moxifloxacin at plasma and ELF concentrations. Mutations in parC, parE and gyrA genes were found in ciprofloxacin- and prulifloxacin-resistant strains, while only parC mutations were found for levofloxacin.
  
  
  CONCLUSIONS
  Levofloxacin and moxifloxacin are characterized by a lower propensity to select in vitro for resistance in S. pneumoniae than ciprofloxacin and prulifloxacin, when tested at plasma and lung concentrations.
authors.affiliation: Laboratory of Microbiology, Department of Preclinical Sciences LITA Vialba, University of Milan, Milan, Italy.
authors.forename: E
authors.initials: E
authors.lastname: De Vecchi
authors.forename: L
authors.initials: L
authors.lastname: Nicola
authors.forename: F
authors.initials: F
authors.lastname: Ossola
authors.forename: L
authors.initials: L
authors.lastname: Drago
mesh_terms: D000900:Anti-Bacterial Agents / Q000494:pharmacology*; D001426:Bacterial Proteins / Q000235:genetics; D027081:DNA Gyrase / Q000235:genetics; D027101:DNA Topoisomerase IV / Q000235:genetics; D024881:Drug Resistance, Bacterial; D024841:Fluoroquinolones / Q000494:pharmacology*; D006801:Humans; D008826:Microbial Sensitivity Tests; D020125:Mutation, Missense; D013296:Streptococcus pneumoniae / Q000187:drug effects* / Q000235:genetics
chemical_list: D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D024841:Fluoroquinolones; D027101:DNA Topoisomerase IV; D027081:DNA Gyrase