RESUMO
Ceftaroline is a unique cephalosporin with activity against methicillin resistant Staphylococcus aureus (MRSA). It was approved for clinical use in the USA, Europe and Russian Federation since 2010 for the treatment of the skin and soft tissue infection and community-acquired pneumoniae. In the present study there was used molecular typing of 24 isolates of MRSA with reduced susceptibility to ceftaroline. For 8 isolates belonging to different genetic lines (ST8, ST239 and ST228) and requiring MICs there were determined antibiotic concentrations preventing formation of resistant mutants (mutant prevention concentration) and the ranges of the mutant selection window (MSW). The last majority of the isolates with reduced susceptibility to ceftaroline (MIC of 2 mcg/ml) belonged to the clonal line ST228. The whole genome sequencing of two isolates of ST228 showed that they belonged to the epidemic South Germany genetic line and were characterized by the presence of mutations in PBP2a (N146K) and PBP2 (C197Y) responsible for reduced susceptibility. The highest rates of MPC (32 mcg/ml) and MSW (2-16 mcg/ml) were observed in the clinical isolates belonging to the genetic line ST8. The isolates of ST239 and ST228 had the selection window within 2-4 mcg/ml. No dependence of the MIC and MPC/MSW levels was detected.
RESUMO
On diffusion of an antibiotic from the disk to the nutrient medium, gel concentration of an antibiotic changes. This process is individual for each antibiotic. The action of different concentrations of the antibiotic at different intervals results in the suppression of microbial growth around the disk (formation of a zone). The relationship between the antibiotic concentration in the disk and the diameter of a microbial growth suppression area is proportional in the limited range of concentrations. The relationship between the concentration of an antibiotic and the diameter of a growth suppression area may greatly vary for various taxonomic groups of microorganisms and for various strains. The established fact may be responsible for the disagreement of results of a microbial antibiotic sensitivity test, by using different methods and makes the standardization of the disk-diffusion test difficult.
