Strategies for typing and properties of epidemic methicillin-resistant Staphylococcus aureus

Isolates of 17 strains of epidemic methicillin-resistant Staphylococcus aureus from outbreaks in ten hospitals in the UK were investigated with a variety of techniques both to explore their properties and to type them in order to confirm or refute known or suspected epidemiology. The techniques consisted of a biotyping system, peptidoglycan analysis, testing of antibiotic sensitivity to 21 agents, various phage-typing methods including heat shock, plasmid pattern analysis, and heat cure derivation of plasmid-less isogenic strains. All strains resembled those originally isolated in Australia, being in the possession of a large number of chromosomal resistance factors, pigmentation, ability to produce lipase and large molecular weight plasmids (c.15 Md to c.23 Md) which conferred resistance to gentamicin, propamidine, ethidium bromide, cetrimide and chlorhexidine. Some strains also had a c.3 Md plasmid conferring chloramphenicol resistance and others a c.1 Md cryptic plasmid. A large percentage of the population was resistant to 25 mg/l methicillin at 37 degrees C, an unusual feature. All the strategies, with the exception of peptidoglycan analysis, contributed to typing of the strains.