title: Pharmacodynamic modeling of the evolution of levofloxacin resistance in Staphylococcus aureus.
abstract: Previously, we demonstrated the importance of low-level-resistant variants to the evolution of resistance in Staphylococcus aureus exposed to ciprofloxacin in an in vitro system and developed a pharmacodynamic model which predicted the emergence of resistance. Here, we examine and model the evolution of resistance to levofloxacin in S. aureus exposed to simulated levofloxacin pharmacokinetic profiles. Enrichment of subpopulations with mutations in grlA and low-level resistance varied with levofloxacin exposure. A regimen producing average steady-state concentrations (Cavg ss) just above the MIC selected grlA mutants with up to 16-fold increases in the MIC and often additional mutations in grlA/grlB and gyrA. A regimen providing Cavg ss between the MIC and the mutant prevention concentration (MPC) suppressed bacterial numbers to the limit of detection and prevented the appearance of bacteria with additional mutations or high-level resistance. Regimens producing Cavg ss above the MPC appeared to eradicate low-level-resistant variants in the cultures and prevent the emergence of resistance. There was no relationship between the time concentrations remained between the MIC and the MPC and the degree of resistance or the presence or type of mutations that appeared in grlA/B or gyrA. Our pharmacodynamic model described the growth and levofloxacin killing of the parent strains and the most resistant grlA mutants in the starting cultures and correctly predicted conditions that enrich subpopulations with low-level resistance. These findings suggest that the pharmacodynamic model has general applicability for describing fluoroquinolone resistance in S. aureus and further demonstrate the importance of low-level-resistant variants to the evolution of resistance.
authors.affiliation: Division of Infectious Diseases, Department of Internal Medicine, Room MN672, University of Kentucky Medical Center, 800 Rose Street, Lexington, KY 40536-0298, USA.
authors.forename: Jeffrey J
authors.initials: JJ
authors.lastname: Campion
authors.forename: Philip
authors.initials: P
authors.lastname: Chung
authors.forename: Patrick J
authors.initials: PJ
authors.lastname: McNamara
authors.forename: William B
authors.initials: WB
authors.lastname: Titlow
authors.forename: Martin E
authors.initials: ME
authors.lastname: Evans
mesh_terms: D000890:Anti-Infective Agents / Q000493:pharmacokinetics / Q000494:pharmacology*; D027081:DNA Gyrase / Q000235:genetics; D024881:Drug Resistance, Bacterial / Q000235:genetics*; D019143:Evolution, Molecular; D006801:Humans; D064704:Levofloxacin; D016106:Methicillin Resistance; D008826:Microbial Sensitivity Tests; D008954:Models, Biological; D009154:Mutation; D015242:Ofloxacin / Q000493:pharmacokinetics / Q000494:pharmacology*; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000235:genetics / Q000254:growth & development
chemical_list: D000890:Anti-Infective Agents; D064704:Levofloxacin; D015242:Ofloxacin; D027081:DNA Gyrase