Clinical and bacteriologic findings in about 2000 cases of bacteremia illustrate the changes within the staphylococcal flora in Danish hospitals during the years 1957–66. The phage type complex 52, 52A, 80, 81, usually resistant to penicillin and streptomycin only, regressed with the increasing use of newer antibiotics. It was succeeded by a series of mutually related types of phage Group III, in which resistance to several antibiotics was acquired concomitantly with a stepwise blocking of susceptibility to phage, probably owing to "epidemic" phage activity. The latest members of the series were often methicillin resistant.

The combined effects of lysogenization, transduction and selection affect not only phage type and antibiotic resistance but also properties more directly connected with pathogenicity, such as lipase production, which facilitates the development of boils, and enhances mortality in cases of bacteremia.

Development of new staphylococcal types is accelerated in hospitals by abundant use of antibiotics and by the presence of massive bacterial reservoirs.