Antibiotic susceptibilities of genetically characterized Streptococcus milleri group strains

Previous studies of the antibiotic susceptibility of Streptococcus milleri group organisms have distinguished among species by using phenotypic techni ques. Using 44 isolates that were speciated by 165 rRNA gene sequencing, we studied the MICs and minimum bactericidal concentrations of penicillin, am picillin, ceftriaxone, and clindamycin for Streptococcus intermedius, Strep tococcus constellatus, and Streptococcus anginosus. None of the organisms w as resistant to beta-lactam antibiotics, although a few isolates were inter mediately resistant; one strain of S, anginosus was tolerant to ampicillin, and another was tolerant to ceftriaxone, Six isolates were resistant to cl indamycin, with representation from each of the three species. Relatively s mall differences in antibiotic susceptibilities among species of the S, mil leri group show that speciation is unlikely to be important in selecting an antibiotic to treat infection caused by one of these isolates.