Consecutive isolation of homologous strains of methicillin-resistant and methicillin-susceptible Staphylococcus aureus from a hospitalized child

A multiply resistant, methicillin-resistant Staphylococcus aureus was repeatedly isolated from the anterior nares of a premature neonate hospitalized in an intensive-care unit and treated with multiple courses of antibiotics. Two months following cessation of antibiotic therapy, a strain of S. aureus with a similar antibiotic resistance profile, but susceptible to methicillin, was isolated from three consecutive nasal swabs. Total DNA of the methicillin-susceptible and -resistant isolates was digested with SmaI and resolved by pulsed-field gel electrophoresis. The SmaI restriction profile of the susceptible isolate was similar to that of the resistant one except for the replacement of a 207-kb fragment by a 147-kb fragment. In Southern hybridization, a mecA-specific probe hybridized to the 207-kb SmaI fragment of the methicillin-resistant strain but not to DNA of the susceptible strain. These results suggest that loss of the mecA gene can occur in vivo when antibiotic selective pressure is removed.