Relationships among Ciprofloxacin, Gatifloxacin, Levofloxacin, and Norfloxacin MICs for Fluoroquinolone-Resistant Escherichia coli Clinical Isolates

Abstract

Fluoroquinolones are some of the most prescribed antibiotics in the United States. Previously, we and others showed that the fluoroquinolones exhibit a class effect with regard to the CLSI-established breakpoints for resistance, such that decreased susceptibility (i.e., an increased MIC) to one fluoroquinolone means a simultaneously decreased susceptibility to all. For defined strains, however, clear differences exist in the pharmacodynamic properties of each fluoroquinolone and the extent to which resistance-associated genotypes affect the MICs of each fluoroquinolone. In a pilot study of 920 clinical Escherichia coli isolates, we uncovered tremendous variation in norfloxacin MICs. The MICs for all of the fluoroquinolone-resistant isolates exceeded the resistance breakpoint, reaching 1,000 g/ml. Approximately 25% of the isolates (n ‫؍‬ 214), representing the full range of resistant norfloxacin MICs, were selected for the simultaneous determinations of ciprofloxacin, gatifloxacin, levofloxacin, and norfloxacin MICs. We found that (i) great MIC variation existed for all four fluoroquinolones, (ii) the ciprofloxacin and levofloxacin MICs of >90% of the fluoroquinolone-resistant isolates were higher than the resistance breakpoints, (iii) ciprofloxacin and levofloxacin MICs were distributed into two distinct groups, (iv) the MICs of two drug pairs (ciprofloxacin and norfloxacin by Kendall's Tau-b test and gatifloxacin and levofloxacin by paired t test) were similar with statistical significance but were different from each other, and (v) ϳ2% of isolates had unprecedented fluoroquinolone MIC relationships. Thus, although the fluoroquinolones can be considered equivalent with regard to clinical susceptibility or resistance, fluoroquinolone MICs differ dramatically for fluoroquinolone-resistant clinical isolates, likely because of differences in drug structure.Fluoroquinolones, some of the most frequently prescribed antimicrobial agents worldwide, target the bacterial type II topoisomerases gyrase and topoisomerase IV. Type II topoisomerases are essential, ubiquitous enzymes involved in virtually every aspect of DNA metabolism. These enzymes cleave one DNA double helix, pass a second DNA molecule (or a different region of the first DNA molecule) through the break, and religate the broken DNA. Fluoroquinolones increase the longevity of the normally short-lived cleaved DNA-topoisomerase intermediates (reviewed in reference 7). DNA tracking machinery somehow is affected by these intermediates, resulting in multiple subsequent effects, such as chromosome fragmentation, the inhibition of DNA synthesis, and death (reviewed in reference 6).With regard to susceptibility or resistance defined by CLSI breakpoints (Table 1), the fluoroquinolones appear to exemplify a class effect, such that any decrease in susceptibility (i.e., increased MIC) to one drug means a simultaneous decrease for all (2). The fluoroquinolones, however, vary with regard to pharmacokinetic and pharmacodynamic parameters, including potency (reviewed in refer...

Evaluation of Quinolones for Use in Detection of Determinants of Acquired Quinolone Resistance, Including the New Transmissible Resistance Mechanisms qnrA , qnrB , qnrS , and aac (6′) Ib-cr , in Escherichia coli and Salmonella enterica and Determinations of Wild-Type Distributions

“…The differences observed seem to be related to the structural differences between the quinolones tested. Both quinolones and fluoroquinolones might be useful for the detection of the low-level resistance caused by these resistance determinants if the right breakpoints are set; so far, however, few data for other quinolone drugs are available (3,19). Nalidixic acid is known to be a very good drug for use for the detection of mutants; however, it would not be useful for the detection of isolates positive for qnr or aac(6Ј)Ib-cr mutations which might be found to have inhibition zones in the susceptible range.…”

Mechanisms Accounting for Fluoroquinolone Resistance in Escherichia coli Clinical Isolates

“…However, these mechanisms do not equally affect the MICs of all fluoroquinolones; in general, ciprofloxacin and norfloxacin MICs are affected by all of these mechanisms, and gatifloxacin and levofloxacin MICs are less affected or are not affected (14,23,26). In a companion study, we found tremendous variation among the MICs of ciprofloxacin, gatifloxacin, levofloxacin, and norfloxacin, such that the four fluoroquinolones were affected to the same extent only in a small subset of isolates (ϳ5%) (4). In addition to the tremendous diversity in MICs of the four tested fluoroquinolones, we recently uncovered clear correlations between fluoroquinolone resistance in Escherichia coli isolates and patient age, patient sex, hospital location, and culture site (3).…”

“…From our accompanying study of 214 fluoroquinoloneresistant and 27 fluoroquinolone-susceptible E. coli clinical isolates (4), we chose 153 isolates for Western blotting to determine AcrA levels. These isolates represented the full range of ciprofloxacin, gatifloxacin, levofloxacin, and norfloxacin MICs and included isolates ELZ4004, ELZ4032, ELZ4601, and ELZ4686, which had the unprecedented fluoroquinolone MIC relationships (4). For 78 of the isolates with quantified AcrA levels (including the four isolates with unusual resistance phenotypes), we sequenced the topoisomerase QRDRs and screened for the plasmid-located genes aac(6Ј)-Ib-cr and qnrA.…”

“…MICs for the clinical isolates were determined using the agar dilution method according to CLSI guidelines (15) except with a modified dilution scheme, as described previously (4). When the MIC was Ͻ1 g/ml by the agar dilution method, at least two Etest measurements were performed according to the manufacturer's instructions to quantify MICs.…”

Expression of Multidrug Efflux Pump Genes acrAB-tolC , mdfA , and norE in Escherichia coli Clinical Isolates as a Function of Fluoroquinolone and Multidrug Resistance

“…We previously found that known genotypic alterations could not explain the fluoroquinolone MICs in ϳ30% of the fluoroquinolone-resistant clinical isolates, suggesting that additional unknown mechanisms exist (2,6). In laboratory strains, the overexpression of mdfA or norE causes 2-to 4-fold increased ciprofloxacin and norfloxacin MICs but has no effect on levofloxacin MICs (17).…”