Apparent failure of endocarditis prophylaxis caused by penicillin-resistant Streptococcus mitis

Antibiotic resistance among viridans streptococci has increased with Streptococcus mitis being more resistant than other viridans species. In a case presented in this report, it is possible that antibiotic resistance contributed to an apparent failure of endocarditis prophylaxis. The patient had undergone periodontal surgery on 2 separate occasions and in both instances was administered 2 g of amoxicillin orally 1 hour before each procedure. He subsequently developed a subacute illness and had multiple blood cultures drawn that grew S. mitis with a minimum inhibitory concentration of 1.0 microg/mL for penicillin. Transesophageal echocardiogram provided further evidence of infective endocarditis with vegetations seen on the anterior leaflet of the mitral valve. Combination therapy with high-dose intravenous aqueous crystalline penicillin G and gentamicin sulfate for 4 weeks was curative. Clindamycin, rather than amoxicillin, has since been used as dental prophylaxis for subsequent procedures.