Decrease in Bacterial Load Versus Resistance Selection of Pneumococcal Subpopulations by β-Lactam Physiological Concentrations over Time: An In Vitro Pharmacodynamic Simulation

Abstract

To investigate β-lactam effects on Streptococcus pneumoniae–mixed cultures, a computerized pharmacodynamic model simulating over 24-hr concentrations obtained after several β-lactam regimens was used. Strain 1 (no penicillin binding protein [PBP] mutations) and strain 2 (mutation in pbp1a) were penicillin/amoxicillin susceptible. Strain 3 (mutations in pbp1a, pbp2x, and pbp2b) and strain 4 (mutations in pbp1a, pbp2x, and pbp2b [10 changes]) were penicillin/amoxicillin resistant. Initial inoculum was approximately 6 × 106 CFU (colony forming units)/ml (with a 1:1:1:1 proportion of each strain). Population analysis profile was performed pre- and postsimulations. Strain 1 exhibited the best fitness (growth over 24 hr) in individual cultures, and strain 2 did so in mixed cultures in antibiotic-free simulations. In antibiotic simulations with the mixed inocula, penicillin/amoxicillin-susceptible strains were eradicated with all study drugs (time that concentrations exceed the minimal inhibitory concentration [T > MIC ≥ 43%]). Penicillin-resistant strains showed different evolution depending on the antibiotic: (a) cefditoren produced >2 log10 reduction of initial inocula at 12–24 hr (T>MIC ≥45%), with a remaining population growing in plates with ≥4 mg/L amoxicillin; (b) cefuroxime, cefixime, and cefaclor did not decrease initial inocula at 12–24 hr (T > MIC = 0%), with minor subpopulations growing in plates with 4 mg/L amoxicillin; (c) amoxicillin produced 2.6 log10 decrease of initial inocula at 12 hr (T>MIC = 47.5%), but 1.1 log10 increase of initial inocula at 24 hr, with a significant population growing in plates with 4 mg/L amoxicillin. Antibiotic activity against mixed inocula (susceptible and resistant strains) depends on intrinsic activity (as well as its subsequent pharmacodynamic activity: T > MIC against resistant strains), and on possible selection of intra-strain-resistant subpopulations.