The Growth of Mycobacterium tuberculosis in Semi-solid Agar Media

Further experience of semi-solid agar media has fully confirmed the earlier opinion (Knox, 1955a) of their great value for culture of Mycobacterium tuberculosis. The media mainly used were those of Kirchner, Fisher and Dubos, made semi-solid with 0.125% agar and containing serum or bovine albumin. Large inocula of M. tuberculosis H 37 Rv strain in Kirchner semi-solid agar gave easily visible growth in 2–3 days when the simple method of viewing described was used; small inocula gave countable colonies in 10–14 days, or in 7–10 days when a hand-lens was used. Maximum or nearly maximum viable counts were reached in about 2 weeks, and the final number of viable organisms recovered was as high as or higher than in any other media used. The medium also gave rapid growth of other strains of M. tuberculosis including drug-resistant variants of H 37 Rv, strains of M. tuberculosis var. bovis, and of B.C.G. and strains isolated from patients’ sputa. Semi-solid media are easy to prepare and safe to handle. Contamination rates are low, evaporation does not occur when rubber stoppers are used, and large numbers of cultures can be inoculated and incubated in a small space. Repeated readings can be made without opening the culture tubes. Viable counts and drug sensitivity tests are easy to read and record. For drug sensitivity tests semi-solid media, besides giving, when serial drug dilutions are used, an initial end-point which is as easy to read as with liquid media, also give information not given by liquid media as to the proportion of resistant organisms present in a given culture. Semi-solid agar media are of great value for any work involving speed in culture, drug sensitivity tests or viable counts of M. tuberculosis.