Antibiotic Susceptibilities of Genetically Characterized Streptococcus milleri Group Strains

Abstract

Previous studies of the antibiotic susceptibility of Streptococcus milleri group organisms have distinguished among species by using phenotypic techniques. Using 44 isolates that were speciated by 16S rRNA gene sequencing, we studied the MICs and minimum bactericidal concentrations of penicillin, ampicillin, ceftriaxone, and clindamycin for Streptococcus intermedius, Streptococcus constellatus, and Streptococcus anginosus. None of the organisms was resistant to beta-lactam antibiotics, although a few isolates were intermediately resistant; one strain of S. anginosus was tolerant to ampicillin, and another was tolerant to ceftriaxone. Six isolates were resistant to clindamycin, with representation from each of the three species. Relatively small differences in antibiotic susceptibilities among species of the S. milleri group show that speciation is unlikely to be important in selecting an antibiotic to treat infection caused by one of these isolates.Three species compose the Streptococcus milleri group: Streptococcus intermedius, Streptococcus constellatus, and Streptococcus anginosus (5,12,13). Investigators who have used phenotypically differentiated strains within the S. milleri group have suggested that these three species have similar antibiotic susceptibilities (1,4,7,8). Phenotypic identification to the species level, however, has been shown to be difficult and at times unreliable (3,6,9). In order to compare the antibiotic susceptibilities of species within the S. milleri group, we determined the MICs and minimum bactericidal concentrations (MBCs) of four clinically relevant antibiotics for 44 genotypically characterized strains of the S. milleri group. MATERIALS AND METHODSBacteria. Forty-four clinically significant isolates of the S. milleri group that had been isolated from different patients between 1985 and 2000 were studied. All had been implicated as the causative organisms in infection (2). Their sources of isolation are summarized in Table 1. These strains were assigned to the S. milleri group based on the results of API 20 Strep system (bioMérieux Vitek, Hazelton, Mo.) tests and were further speciated by PCR amplification and sequence analysis of a segment of the 16S rRNA gene (3). They were stored at Ϫ70°C after having been passaged no more than two or three times. In addition to two well-characterized strains of Streptococcus pneumoniae that have been studied repeatedly in our laboratory, the following American Type Culture Collection (ATCC) strains were included as reference strains: ATCC 27335 (S. intermedius), ATCC 9895 and ATCC 33397 (S. anginosus), and ATCC 29213 (Staphylococcus aureus).MIC and MBC testing. Todd-Hewitt broth (Difco, Detroit, Mich.) containing 0.5% yeast extract (Difco) (THY), a broth medium previously shown by our laboratory to be optimal for testing the MICs of S. pneumoniae (10), was used in this study. Preliminary studies showed that this medium supported the growth of S. milleri group isolates more reliably than Mueller-Hinton or tryptic soy broth. Penicillin G (Sigma...

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Periodontal Bacteria in Rabbit Mandibular and Maxillary Abscesses

“…Of all potential pathogens tested, 100% were susceptible to clindamycin and chloramphenicol; 96% were susceptible to penicillin, ceftriaxone, and cefazolin (MIC Յ 2 g/ml); 86% were susceptible to azithromycin (MIC Յ 0.5 g/ml) and tetracycline; 54% were susceptible to metronidazole and ciprofloxacin (MIC Յ 1 g/ ml), and only 7% were susceptible to trimethoprim-sulfamethoxazole (MIC Յ 2/38 g/ml). All strains of the Streptococcus milleri group were susceptible to clindamycin, penicillin, and ceftriaxone, although occasional resistance in humans has been reported (9,27). Although metronidazole was largely effective against anaerobic gram-negative bacilli and gram-positive cocci in this study, it was ineffective against anaerobic grampositive bacilli and streptococci.…”

“…For empirical therapy when an adequate culture cannot be performed, veterinary clinicians should consider the potential susceptibilities of the types of oral microbes isolated in our study, as well as of those reported in human (3)(4)(5)(6)(7)27) and other mammalian (13,14) studies. New methods of nonoral delivery of clindamycin, ceftiofur, and penicillins are currently under investigation.…”

Perfil fenotípico e susceptibilidade antimicrobiana de Streptococcus equi isolados de equinos da região Sul do Brasil

“…Portanto, os resultados obtidos através do método de disco difusão foram confirmados pela CIM de benzilpenicilina, assegurando a correlação dos resultados. Em outro estudo, Tracy et al (2001) avaliaram a susceptibilidade de 44 isolados de S. milleri frente à penicilina; CIM <0,12μg/mL confirmaram a sensibilidade desta espécie a este antimicrobiano. No entanto, Heffelfinger et al (2000) interpretam como sensível quando a CIM for <1,00μg/mL de penicilina para Streptococcus pneumoniae.…”