The disappearance of multiresistant Staphylococcus aureus in Denmark: changes in strains of the 83A complex between 1969 and 1989.

During the 1960s multiresistant strains of Staphylococcus aureus were problematic in Denmark and other countries. An analysis of the phage types and antibiotic resistance patterns of S. aureus strains collected from approximately 20,000 Danish patients per year has given us a general picture of the evolution of S. aureus in Denmark during the last 30 years. Multiresistant S. aureus (i.e., strains resistant to penicillin, streptomycin, and tetracycline and often to methicillin and erythromycin as well) belonged mostly to the 83A complex, a relatively homogeneous subset of group III strains that can be further divided into six subtypes. The disappearance of multiresistant strains in Denmark began with a decline in the frequency of isolation of the most resistant subtypes, which was followed by a gradual decrease in the resistance of all 83A subtypes; thus strains of the 83A complex ultimately became no more resistant than other strains. As the proportion of strains of S. aureus accounted for by the 83A complex declined from 24% in 1969 to 6% by the late 1980s, this complex was replaced by strains of type 95, the 94,96 complex, and group II, all of which only rarely show resistance to multiple agents.