Quinolone resistance due to reduced target enzyme expression.
We report for the first time low-level quinolone resistance mediated by decreased expression of topoisomerase IV in Staphylococcus aureus. A single-step mutant of wild-type S. aureus strain ISP794, P18 selected by using twice the MIC of premafloxacin, had four- and four- to eightfold greater MICs of premafloxacin and ciprofloxacin, respectively, than the wild type. Sequencing of parEC and gyrBA with their promoter regions revealed a point mutation (G-->A) 13 bp upstream of the start codon of parE. Genetic linkage studies showed that there was a high level of correlation between the mutation and the resistance phenotype, and allelic exchange confirmed the contribution of the mutation to resistance. Decreased expression of ParE and decreased steady-state levels of parEC transcripts in P18 and in resistant allelic exchange mutants were observed. The steady-state levels of gyrBA and topB transcripts were increased in P18 but not in two resistant allelic exchange mutants, and sequencing upstream of either gene did not reveal a difference between ISP794 and P18. The steady-state levels of topA transcripts were similar in the various strains. Growth competition experiments performed at 30, 37, and 41 degrees C with a susceptible allelic exchange strain and a resistant allelic exchange strain suggested that loss of fitness was associated with reduced levels of ParE at 41 degrees C. However, P18 had a growth advantage over ISP794 at all temperatures, suggesting that a compensatory mechanism was associated with the increased levels of gyrBA and topB transcripts. Thus, reduced levels of ParE appear to be compatible with cell survival, although there may be a fitness cost during rapid cell multiplication, which might be overcome by compensatory mechanisms without reversion of the resistance phenotype.
Ince
Dilek
D
Division of Infectious Diseases, Massachusetts General Hospital, Harvard Medical School, Boston, Massachusetts 02114, USA.
Hooper
David C
DC
D015152:Blotting, Northern; D015153:Blotting, Western; D027101:DNA Topoisomerase IV / Q000032:analysis / Q000235:genetics / Q000378:metabolism*; D024881:Drug Resistance, Bacterial; D005069:Evaluation Studies as Topic; D009154:Mutation; D017354:Point Mutation; D015363:Quinolones / Q000494:pharmacology*; D012333:RNA, Messenger / Q000032:analysis; D013211:Staphylococcus aureus / Q000187:drug effects* / Q000254:growth & development / Q000378:metabolism; D013696:Temperature
D015363:Quinolones; D012333:RNA, Messenger; D027101:DNA Topoisomerase IV