Investigation of preculture solid medium and incubation period in mass spectrometric identification of mycobacteria using MALDI Biotyper^®

Abstract

Mass spectrometry is an effective and widely used method for identifying mycobacteria, but there is no clear recommendation for the type of preculture medium or preculture period. Therefore, in this study, we investigated the culture medium and culture period for the slow-growing nontuberculous mycobacteria (SGM)/Mycobacterium tuberculosis (MTB) group and the rapid-growing mycobacterium (RGM) group. We used the Kyokuto 2% Ogawa medium (Ogawa medium) and Vital Media 7H11-C agar medium (7H11C plate), and cultured the samples for about 1 to 2 weeks. As a result of the investigation, both SGM and RGM showed the highest species-level identification rate and score value (SCV) for mass spectrometry identification performed after 1 week of culture using the Ogawa medium. The SCV for identifying RGM was low after two weeks of culture, and the SCV was significantly lower on the 7H11C plate. The reason for the low identification rate at the species level on the 7H11C plate cultured for 1 week was considered to be the insufficient amount of growing bacteria. The reason why the SCV of RGM was low after two weeks of culture is considered to be that the culture period was too long and a good spectrum could not be obtained. In conclusion, when a solid medium is used for preculture, the Ogawa medium gives better results than the 7H11C plates, and the preculture periods for mass spectrometry identification are about 1–2 weeks for SGM and 1 week for RGM. It is inferred that this leads to good identification results.