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

Microb Drug Resist. Spring 1998;4(1):45-9. doi: 10.1089/mdr.1998.4.45.


Penicillin-resistant oral streptococci constitute the genetic reservoir for beta-lactam resistance in S. pneumoniae. Here we report the isolation of clinical strains of S. mitis with unusually high MIC values for beta-lactam antibiotics; resistance to benzylpenicillin was 64 microg/ml and to cefotaxime 128 microg/ml. Among the beta-lactam compounds tested, only the carbapenems imipenem and meropenem showed MICs below 32 microg/ml. Both S. mitis strains were resistant to tetracycline and were highly resistant to aminoglycosides. Pulse field mapping of chromosomal DNA revealed identical patterns in both strains, indicating clonal identity of the two isolates. Using chromosomal S. mitis DNA, the laboratory strain S. pneumoniae R6 could be transformed in four successive steps to cefotaxime and benzylpenicillin resistance of 64 microg/ml. The results exemplify the importance of commensal streptococci for the development of cefotaxime resistance in S. pneumoniae.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Anti-Bacterial Agents / pharmacology
  • Bacterial Proteins*
  • Carrier Proteins / analysis
  • Cefotaxime / pharmacology
  • Cephalosporin Resistance / genetics
  • Child, Preschool
  • Electrophoresis, Gel, Pulsed-Field
  • Hexosyltransferases*
  • Humans
  • Infant, Newborn
  • Microbial Sensitivity Tests
  • Muramoylpentapeptide Carboxypeptidase / analysis
  • Penicillin G / pharmacology
  • Penicillin Resistance / genetics
  • Penicillin-Binding Proteins
  • Peptidyl Transferases*
  • Streptococcus / drug effects*
  • Streptococcus / genetics*
  • Streptococcus / isolation & purification
  • Transformation, Genetic
  • beta-Lactam Resistance*


  • Anti-Bacterial Agents
  • Bacterial Proteins
  • Carrier Proteins
  • Penicillin-Binding Proteins
  • Peptidyl Transferases
  • Hexosyltransferases
  • Muramoylpentapeptide Carboxypeptidase
  • Cefotaxime
  • Penicillin G