Cystic fibrosis (CF) is the most common autosomal recessive genetic disorder in Caucasians. Lower respiratory tract of CF patients is colonized by specific bacteria, often leading to chronic infection and lung tissue damage. In this thesis we characterized 338 isolates of S. aureus from 92 Czech CF patients isolated in 2011-2013. Using spa typing and PFGE we detected high clonal heterogenity of this collection with the exception of MRSA strains (resistant to oxacillin; 5% prevalence) which were clonally related. The prevalence of S. aureus MLSB resistance in our collection was high (69 %), which is a serious problem due to common usage of these antimicrobials in clinical practice. A half of the MLSB resistant strains lacked any known determinant of this resistance (ermA, ermC, ermT, msrA). Sequencing of the ribosomal genes revealed a high number of S. aureus strains carrying target site mutations resulting in MLSB resistance (37 %). This is new important information about the staphylococcal strains associated with chronic infections in Czech Republic. Focusing on mutability of analysed strains, we also detected several strains with point mutations or deletions in their mutator genes mutS a mutL. Hypermutability could be responsible for the high rate of ribosomal mutations and for the presence of...