Adaptation of methicillin-resistant Staphylococcus aureus during antibiotic therapy.

Colonization of a patient by methicillin-resistant Staphylococcus aureus (MRSA) of a single phage-type for over four years is described. During this period we observed the appearance and disappearance of resistance to erythromycin, clindamycin, gentamicin, kanamycin, tobramycin, neomycin and mupirocin. We also saw stepwise increases in methicillin resistance and reversible changes in physical appearance and the colonizer pathogen role. Correlation of clinical observations, details of antibiotic therapy and laboratory studies demonstrated that adaptation of MRSA during antibiotic therapy favoured MRSA establishment and predominance.