Bacterial resistance studies using in vitro dynamic models: the predictive power of the mutant prevention and minimum inhibitory antibiotic concentrations

Abstract

In light of the concept of the mutant selection window, i.e., the range between the MIC and the mutant prevention concentration (MPC), MPC-related pharmacokinetic indices should be more predictive of bacterial resistance than the respective MIC-related indices. However, experimental evidence of this hypothesis remains limited and contradictory. To examine the predictive power of the ratios of the area under the curve (AUC24) to the MPC and the MIC, the selection of ciprofloxacin-resistant mutants of four Escherichia coli strains with different MPC/MIC ratios was studied. Each organism was exposed to twice-daily ciprofloxacin for 3 days at AUC24/MIC ratios that provide peak antibiotic concentrations close to the MIC, between the MIC and the MPC, and above the MPC. Resistant E. coli was intensively enriched at AUC24/MPCs from 1 to 10 h (AUC24/MIC from 60 to 360 h) but not at the lower or higher AUC24/MPC and AUC24/MIC ratios. AUC24/MPC and AUC24/MIC relationships of the areas under the time courses of ciprofloxacin-resistant E. coli (AUBCM) were bell-shaped. A Gaussian-like function fits the AUBCM-AUC24/MPC and AUBCM-AUC24/MIC data combined for all organisms (r(2) = 0.69 and 0.86, respectively). The predicted anti-mutant AUC24/MPC ratio was 58 ± 35 h, and the respective AUC24/MIC ratio was 1,080 ± 416 h. Although AUC24/MPC was less predictive of strain-independent E. coli resistance than AUC24/MIC, the established anti-mutant AUC24/MPC ratio was closer to values reported for Staphylococcus aureus (60 to 69 h) than the respective AUC24/MIC ratio (1,080 versus 200 to 240 h). This implies that AUC24/MPC might be a better interspecies predictor of bacterial resistance than AUC24/MIC.

Fig 1

Determination of MPC. Estimated values are indicated by italicized text. Symbols: ○, E. coli ATCC 25922; □, E. coli 4300; ▽, E. coli 4454; △, E. coli GM2995.

Fig 2

Simulated pharmacokinetics and time courses of susceptible (0× the MIC) and resistant (4× and 16× the MIC) subpopulations of E. coli exposed to ciprofloxacin. Antibiotic dosing is indicated by arrows. The simulated AUC/MIC ratios (in hours) are indicated by numbers at each pharmacokinetic profile. The descriptions of the symbols are the same as those presented in the legend to Fig. 1.

Fig 3

AUC₂₄/MIC- and AUC₂₄/MPC-dependent effects of ciprofloxacin on resistant to 4× the MIC subpopulations of E. coli. Combined data on four bacterial strains (bottom row) are fitted by equation 2. AUC₂₄/MIC: Y₀ = 0, x₀ = 1.896, a = 414.3, b = 0.6338, c = 4.304. AUC₂₄/MPC: Y₀ = 0, x₀ = 0.5809, a = 436.1, b = 0.5567, c = 2.000. The descriptions of the symbols are the same as those presented in the legend to Fig. 1.

Fig 4

AUC₂₄/MIC-dependent effects of ciprofloxacin on susceptible subpopulation of E. coli fitted by equation 1. Y₀ = 0, x₀ = 2,697, a = 375, b = 0.1025. The descriptions of the symbols are the same as those presented in the legend to Fig. 1.

Fig 5

Ciprofloxacin effects of on resistant to 4× the MIC subpopulations of E. coli as related to different predictors fitted by equation 3. Y₀ = 0, a = 101000 (AUC₂₄/MIC), 700 (AUC₂₄/MPC), or 525 (T_>MPC), and b = 3.39, 4.34, or 0.0369, respectively. The descriptions of the symbols are the same as those presented in the legend to Fig. 1.