Decrease in bacterial load versus resistance selection of pneumococcal subpopulations by beta-lactam physiological concentrations over time: an in vitro pharmacodynamic simulation.
To investigate beta-lactam effects on Streptococcus pneumoniae-mixed cultures, a computerized pharmacodynamic model simulating over 24-hr concentrations obtained after several beta-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 x 10(6) CFU (colony forming units)/ml (with a 1:1:1:1 proportion of each strain). Population analysis profile was performed pre- and post-simulations. 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 >or= 43%]). Penicillin-resistant strains showed different evolution depending on the antibiotic: (a) cefditoren produced >2 log(10) reduction of initial inocula at 12-24 hr (T>MIC >or=45%), with a remaining population growing in plates with >or=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 log(10) decrease of initial inocula at 12 hr (T>MIC=47.5%), but 1.1 log(10) 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.
Cafini
Fabio
F
Microbiology Department, School of Medicine, Universidad Complutense, Madrid, Spain.
Aguilar
Lorenzo
L
Sevillano
David
D
Giménez
Maria-Jose
MJ
Alou
Luis
L
Fenoll
Asuncion
A
Echevarría
Olatz
O
Torrico
Martha
M
González
Natalia
N
Coronel
Pilar
P
Prieto
Jose
J
D019881:Aminoacyltransferases / Q000235:genetics; D000900:Anti-Bacterial Agents / Q000493:pharmacokinetics / Q000494:pharmacology*; D019540:Area Under Curve; D001426:Bacterial Proteins / Q000235:genetics; D004305:Dose-Response Relationship, Drug; D006207:Half-Life; D066298:In Vitro Techniques; D046915:Penicillin-Binding Proteins / Q000235:genetics; D010458:Peptidyl Transferases / Q000235:genetics; D020641:Polymorphism, Single Nucleotide; D013296:Streptococcus pneumoniae / Q000187:drug effects* / Q000235:genetics; D013997:Time Factors; D018440:beta-Lactam Resistance
D000900:Anti-Bacterial Agents; D001426:Bacterial Proteins; D046915:Penicillin-Binding Proteins; C072223:PBP 2x protein, Streptococcus; D019881:Aminoacyltransferases; C478701:penicillin-binding protein 2b, Streptococcus; C480727:PBP1a protein, Streptococcus pneumoniae; D010458:Peptidyl Transferases