Further evolution of a strain of Staphylococcus aureus in vivo: evidence for significant inactivation of flucloxacillin by penicillinase.

A strain of Staphylococcus aureus (no. FAR4) has been isolated at intervals, for 32 months, from the sputum of a patient with cystic fibrosis of the lung. Changes in the properties of isolates of this strain over the first 18 months have been reported previously (Lacey et al., 1973 and 1974). During the last 14 months (May 1973 to July 1974), further evolution has occurred to produce a total of 31 distinct phenotypes. Recent changes are as follows. 1. The ability of isolates to produce penicillinase in vitro was closely correlated with flucloxacillin therapy. Inactivation of flucloxacillin by penicillinase was demonstrated by diffusion testing (but not MIC determination) in vitro and may have occurred to a significant extent in vivo. 2. Lincomycin-resistant mutants slowly disappeared from the sputum after the termination of clindamycin therapy. 3. All of the recent isolates were resistant to erythromycin, possibly because of the linkage of the genes coding for erythromycin resistance with those coding for the production of delta-haemolysin; delta-haemolysin may be an important "virulence factor".