Distribution of methicillin-resistance and of protein A among Staphylococcus aureus strains isolated from different hosts.

The link between loss of protein A and methicillin-resistance, and the presence of both characters in relation to the hosts from which S. aureus strains have been isolated, was examined. The majority of methicillin-resistant strains lost protein A. Normal hosts were generally infected with methicillin-sensitive protein A positive strains, while in immunocompromised patients the finding of multiresistant-protein A deficient strains was more frequent. The importance of the host's immune system, together with the antibiotic pressure in selecting these mutants, is discussed.