The grIA Mutation in Norfloxacin-Resistant First-Step Mutants and Clinical Isolates of Staphylococcus aureus

Curing bacteria of antibiotic resistance: reverse antibiotics, a novel class of antibiotics in nature

By screening cultures of soil bacteria, we re-discovered an old antibiotic (nybomycin) as an antibiotic with a novel feature. Nybomycin is active against quinolone-resistant Staphylococcus aureus strains with mutated gyrA genes but not against those with intact gyrA genes against which quinolone antibiotics are effective. Nybomycin-resistant mutant strains were generated from a quinolone-resistant, nybomycin-susceptible, vancomycin-intermediate S. aureus (VISA) strain Mu 50. The mutants, occurring at an extremely low rate (<1 × 10(-11)/generation), were found to have their gyrA genes back-mutated and to have lost quinolone resistance. Here we describe nybomycin as the first member of a novel class of antibiotics designated 'reverse antibiotics'.

Relationship between mutations in the DNA gyrase and topoisomerase IV genes and nadifloxacin resistance in clinically isolated quinolone-resistant Staphylococcus aureus

We analyzed the relationship between resistance to nadifloxacin, a fluoroquinolone antimicrobial agent, and mutations in the quinolone resistance-determining regions (QRDRs) of the A subunit of DNA gyrase and topoisomerase IV, in 24 clinical isolates of Staphylococcus aureus. Seven known mutations were found in the QRDRs. The minimum inhibitory concentration (MIC) of nadifloxacin increased in a manner that was dependent on mutations in the A subunit of DNA gyrase, and did not appear to be related to mutations in the A subunit of topoisomerase IV. The type 9 mutant, which included four mutations, was highly resistant to ofloxacin, norfloxacin and sparfloxacin, but only moderately resistant to nadifloxacin (MIC, 12.5 microg/ml). One of the norfloxacin-resistant strains that expressed high levels of norA was not resistant to nadifloxacin. To the best of our knowledge, this is the first report describing a fluoroquinolone antimicrobial agent whose primary target is suggested to be DNA gyrase in S. aureus.

Antibacterial activity of gatifloxacin (AM-1155, CG5501, BMS-206584), a newly developed fluoroquinolone, against sequentially acquired quinolone-resistant mutants and the norA transformant of Staphylococcus aureus

Alternate mutations in the grlA and gyrA genes were observed through the first- to fourth-step mutants which were obtained from four Staphylococcus aureus strains by sequential selection with several fluoroquinolones. The increases in the MICs of gatifloxacin accompanying those mutational steps suggest that primary targets of gatifloxacin in the wild type and the first-, second-, and third-step mutants are wild-type topoisomerase IV (topo IV), wild-type DNA gyrase, singly mutated topo IV, and singly mutated DNA gyrase, respectively. Gatifloxacin had activity equal to that of tosufloxacin and activity more potent than those of norfloxacin, ofloxacin, ciprofloxacin, and sparfloxacin against the second-step mutants (grlA gyrA; gatifloxacin MIC range, 1.56 to 3.13 microg/ml) and had the most potent activity against the third-step mutants (grlA gyrA grlA; gatifloxacin MIC range, 1.56 to 6.25 microg/ml), suggesting that gatifloxacin possesses the most potent inhibitory activity against singly mutated topo IV and singly mutated DNA gyrase among the quinolones tested. Moreover, gatifloxacin selected resistant mutants from wild-type and the second-step mutants at a low frequency. Gatifloxacin possessed potent activity (MIC, 0.39 microg/ml) against the NorA-overproducing strain S. aureus NY12, the norA transformant, which was slightly lower than that against the parent strain SA113. The increases in the MICs of the quinolones tested against NY12 were negatively correlated with the hydrophobicity of the quinolones (correlation coefficient, -0.93; P < 0.01). Therefore, this slight decrease in the activity of gatifloxacin is attributable to its high hydrophobicity. Those properties of gatifloxacin likely explain its good activity against quinolone-resistant clinical isolates of S. aureus harboring the grlA, gyrA, and/or norA mutations.

Vancomycin resistance in staphylococci

Recent emergence of vancomycin resistance in methicillin-resistant Staphylococcus aureus (VRSA) has posed a new threat to hospital infection control and antibiotic chemotherapy. Relatively low-level resistance of VRSA compared to that of vancomycin-resistant enterococci (VRE), and prevalence of S. aureus clinical strains heterogeneously resistant to vancomycin (hetero-VRSA), challenge the value of routine antibiotic susceptibility tests as a tool for the prediction of clinical efficacy of vancomycin therapy. This review summarizes the history of emergence of glycopeptide resistance in staphylococci and considers the mechanism of resistance in these organisms.