Growth of mycobacteria in relation to the pH of the medium

The influence of pH on the growth of mycobacteria on Loewenstein and Dubos OAA media was studied with 56 strains belonging to 10 species of rapidly growing and 16 species of slowly growing mycobacteria. pH gradients were obtained by the use of McIlvaine's citrate and Sörensen's citrate buffer. The sensitivity of mycobacteria at different pH values of the medium was independent of the culture medium and buffer used. Minimal, maximal and optimal pH values were specific for each species, all strains of the same species producing identical results. Some slowly growing mycobacteria such as Mycobacterium lepraemurium developed within a very narrow optimal pH range: between 5.8 and 6.1. Other species developed over a wide optimal pH range, e.g. M. nonchromogenicum: from 5 to greater than 7.4. For slow growers, in general, with the exception of M. lepraemurium, optimal pH was between 5.8 and 6.5. In general, rapidly growing mycobacteria developed over a wider pH range of more than 2 units. All strains, with the exception of M. chelonei, developed optimally between pH 7 and 7.4. For M. chelonei, as for slower growing species, the optimal pH was between 5.4 and 6.5. Taxonomically related species such as M. tuberculosis and M. bovis, M. kansasii and M. gastri, M. microti and M. bovis BCG, and M. vaccae and M. parafortuitum, were inhibited or stimulated at identical pH values. This specificity constitutes a supplementary taxonomic character. At certain pH values, some species otherwise difficult to separate may be differentiated: e.g. M. avium, not growing at pH 4.6, is distinguished from M. scrofulaceum which does grow at pH 4.6. For routine diagnostic purposes, Loewenstein Jensen medium at pH 7 is not optimal; Ogawa medium, with its pH of 6, is more appropriate.