Development of selective isolation media for detecting the genera Actinomyces and Schaalia from oral specimens containing indigenous bacteria.

To isolate specific bacteria from samples constituting the microbiota, it is essential to employ selective media that suppress the growth of resident bacteria other than specific target bacteria. Selective media for clinically important Actinomyces (including Schaalia, which was previously taxonomically classified as part of the genus Actinomyces) have been limited because they have been designed for a limited range of species within the genus and require ingredients which are difficult to prepare and handle. This study aimed to develop a selective medium [referred to as Actinomyces and Schaalia Selective Medium (ASSM)] for the isolation of a broad range of Actinomyces and Schaalia species from samples mixed with resident bacteria. The composition of ASSM includes yeast extract, agar, brain heart infusion (BHI), levofloxacin (LVFX), fosfomycin (FOM), colistin (CL) and metronidazole (MNZ). Evaluation of the medium using 24 swab samples serially collected from the roots of the teeth of a healthy individual for whom metagenome sequencing data of a saliva sample are publicly available revealed that ASSM adjusted to concentrations of LVFX 0.5 mg l-1, FOM 5 mg l-1, CL 1 mg l-1 and MNZ 2 mg l-1 and cultured anaerobically at 35 °C for 7 days enabled the isolation of Actinomyces species from 37.5 % of the samples. The inclusion of CL and MNZ in ASSM can also be useful for samples harbouring other bacterial species. The selective isolation medium is expected to contribute to studies investigating the relationship between these bacteria and their pathogenesis or disease.