Therapy of methicillin-resistant Staphylococcus epidermidis experimental endocarditis

Antibiotic therapy of methicillin-resistant Staphylococcus epidermidis endocarditis was investigated with the rabbit endocarditis model. Time-kill studies in vitro demonstrated that gentamicin and rifampin had the most rapid early bactericidal rates. With rifampin alone, rifampin-resistant subpopulations emerged. Combinations of antibiotics with gentamicin or rifampin in vitro did not significantly alter the killing rate but prevented emergence of subpopulations resistant to the latter. In the rabbit endocarditis model, gentamicin and vancomycin were the most effective single antibiotic regimens in terms of ability to reduce the bacterial densities on cardiac valve vegetations. Five treatment regimens were equally effective, including vancomycin, gentamicin, vancomycin plus rifampin or gentamicin, and rifampin plus gentamicin. The three-drug combination of vancomycin, rifampin, and gentamicin did not significantly improve the results. Cephalothin therapy was significantly less effective than any of the regimens noted above. It was no more effective than no treatment at 2 days and was only slightly more effective at 4 days. This result with cephalothin treatment was not predicted by routine types of in vitro antibiotic susceptibility testing. Treatment of rabbits with methicillin or cephalothin was associated with an increase in the subpopulation of bacteria resistant to the respective drugs. A number of regimens show potential for therapy of these infections, including vancomycin plus rifampin or gentamicin, rifampin plus gentamicin, and vancomycin alone.