Significance of methicillin tolerance in experimental staphylococcal endocarditis.

Methicillin-tolerant staphylococci are organisms that are inhibited by low concentrations of the drug but are resistant to its bactericidal effects. The clinical significance of this in vitro phenomenon is unknown. An experimental model of staphylococcal endocarditis in rabbits was used to determine whether methicillin-tolerant staphylococci are particularly difficult to eradicate from infected heart valves. In vitro sensitivity testing was used to identify a tolerant and a nontolerant strain of Staphylococcus aureus. Rabbits prepared by the insertion of a polyethylene catheter into the left ventricle and injected with a tolerant strain survived significantly longer than those injected with a nontolerant strain. No significant differences were demonstrated in the prevention of endocarditis with a single prophylactic dose of methicillin, or in the treatment of established endocardial infection with multiple doses of methicillin. In this study, methicillin had the same activity against experimental endocarditis caused by a tolerant and a nontolerant strain of S. aureus.