A Case Report of Penicillin-Tolerant Streptococcus mitis Endocarditis Chang-Hua Chen, MD, MSc, PhD

There is no clear relationship between the serum inhibition test and clinical outcome for Streptococcus mitis (S. mitis) endocarditis. We report an 84-year-old male with endocarditis caused by penicillin-tolerant S. mitis. The results for the serum inhibitory test (SIT) and serum bactericidal test (SBT) showed a trough level of SIT = 1:256 and SBT = 1:4 and a peak level of SIT ≥ 1:1024 and SBT = 1:16. In addition, the SIT/SBT ratio was 64 at peak level and more than 64 at trough level, which is compatible with penicillin-tolerant S. mitis. Following a 42-day high-dose penicillin treatment (24 M IU/day, via a continuous drip), the patient made a good recovery. In vitro inhibitory and bactericidal test results were not a valid predictor of medical treatment failure. Physicians need to continue to evaluate the surgical indications when treating patients with S. mitis endocarditis.