Mastitis can be caused by a number of bacteria (Philpot & Pankey, 1975), among which Staphylococcus aureus is one of the most important, responsible for considerable economic loss to the dairy industry (Jasper et al. 1982). Carrier rates have been studied extensively in dairy herds suffering from mastitis (Davidson, 1961, 1963).

Subtyping bacteria is an important epidemiological tool; for example, antimicrobial susceptibility patterns (antibiograms) have been used for typing Staph. aureus in human medicine (Gillespie et al. 1990). Staph. aureus of bovine origin can be divided into several categories by biotyping (Devriese, 1984). Phage typing has proved useful in differentiating mastitis strains (Mackie et al. 1987) and plasmid profiling has been valuable in epidemiological studies of bovine Staph. aureus (Baumgartner et al. 1984).

During the last 5 years genomic fingerprinting of Staph. aureus became a powerful tool for epidemiological typing. Numerous techniques for comparison of staphylococcal isolates have been developed and are becoming important in investigations of strain origin, clonal relatedness and epidemiology.

The aim of this study was to determine the epidemiology of Staph. aureus isolates from mammary glands of cows from a single herd using modern molecular typing techniques, and to assess whether the same strain constantly colonizes the udder or cows are repeatedly infected by different strains. We investigated 26 Staph. aureus isolates obtained from quarter milk samples from 16 cows. These isolates were characterized biochemically, by their antibiotic resistance pattern, by two polymerase chain reaction (PCR) methods on the basis of coagulase (Coa) gene and protein A (Spa) gene (X region) polymorphism and by macrorestriction analysis of chromosomal DNA using pulsed-field gel electrophoresis (PFGE).