Streptococcus mitis with unusually high level resistance to beta-lactam antibiotics

Penicillin-resistant oral streptococci constitute the genetic reservoir for beta-lactam resistance in S. pneumoniae. Here we report the isolation of c linical strains of S. midis with unusually high MIC values for beta-lactam antibiotics; resistance to benzylpenicillin was 64 mu g/ml and to cefotaxim e 128 mu g/ml. Among the beta-lactam compounds tested, only the carbapenems imipenem and meropenem showed MICs below 32 mu g/ml, Both S. midis strains were resistant to tetracycline and were highly resistant to aminoglycoside s. Pulse field mapping of chromosomal DNA revealed identical patterns in bo th strains, indicating clonal identity of the two isolates. Using chromosom al S, midis DNA, the laboratory strain S. pneumoniae R6 could be transforme d in four successive steps to cefotaxime and benzylpenicillin resistance of 64 mu g/ml. The results exemplify the importance of commensal streptococci for the development of cefotaxime resistance in S. pneumoniae.