Tryptic Soy agar supplemented with bacitracin represents a remarkable advancement as a selective culture medium for Streptococcus anginosus

Yu Fukuda, DDS,a Susumu Imai, PhD,b Yoshiki Hamada DDS, PhD,a and Nobuhiro Hanada, DDS, PhDb

aDepartment of Oral and Maxillofacial Surgery, and bDepartment of Translational Research, Tsurumi University School of Dental Medicine, Yokohama, Japan

Purpose: To develop a new selective culture medium specifically for Streptococcus anginosus.
Materials and Methods: Three antibiotics, sulphamethazine (s), aztreonam (a), and bacitracin (b), were tested by adding to Tryptic Soy (TS) agar at different concentrations. Initially, sulphamethazine (1.0 mg/mL) and aztreonam (0.2 mg/mL) were added to TS agar to obtain TS-sa agar. In total, 24 strains of oral bacteria including three clinical S. anginosus isolates were used during the entire study. A minimal inhibitory concentration (MIC) test was performed by using 96-well plate to determine the optimal bacitracin concentration for the selective medium for S. anginosus under anaerobic condition at 37oC for 48 hours. Finally, bacitracin was added at a concentration of 2.0 U/mL to TS-sa agar to prepare TS-sab agar medium and selectivity was tested using all 24 strains of bacteria.
Results: Most strains of S. anginosus could grow on the TS-sab agar medium but other bacteria except for S. intermedius and S. sobrinus could not be detected on this medium. S. intermedius and S. sobrinus occasionally showed some growth on TS-sab agar medium with low detection rate. Although some clinical isolates of S. anginosus were not enriched like other strains of S. anginosus, a remarkable enhancement was achieved in this study in the development of a selective culture medium for S. anginosus.
Conclusion: TS-sab may serve as a remarkably improved semi-selective culture medium for S. anginosus. Continued studies are required to improve the selectivity specific for S. anginosus.
(Asian Pac J Dent 2012; 12: 37-44.)
Key Words: aztreonam, bacitracin, selective medium, Streptococcus anginosus, sulphamethazine