Colony Morphology of Tubercle Bacilli. II. Influence of pH of Culture Medium on Colony Morphology and Virulence

Abstract

Extensive studies have been made with reference to the range of pH in which tubercle bacilli will grow and the changes in pH which take place during growth on fluid medium. These studies have been reviewed by Wells and Long. 1 Recently Birkhaug 2 has studied the changes in pH induced in fluid medium during growth of R and S forms of avian and mammalian tubercle bacilli. However, the influence of pH on colony morphology of Mycobacteria has been less extensively investigated. Steenken, Oatway, and Petroff 3 found that R colonies of degraded virulence were characteristic of the growth at pH 6.1 on either Petroff's or Calmette's medium; whereas Petroff's medium at pH 7.2 supported the growth of S variants of greater virulence. Recently it was observed in this laboratory 4 that by adjusting the pH of Corper's medium to various values, three principal types of colonies could be obtained: rough, smooth, and intermediate. The present report is an extension of the latter work.

Twenty-five strains of tubercle bacilli were used in the experiments. Included in these were: R and S variants of an avian strain, a freshly isolated avian strain, 11 bovine, and 11 human strains. Ten of the human strains were isolated within the last 16 months. Corper's glycerinated egg-yolk medium with Congo Red was used throughout. 450 cc. lots of the latter were adjusted to various pH values between 6.0 and 7.2 by intervals of 0.2 pH, the pH determinations being done by the glass electrode method through the courtesy of Dr. D. A. MacInnes. Phosphate buffer (M/1.5 KH₂PO₄ or M/1.5 Na₂HPO₄) was added in the quantity just sufficient to bring to the desired pH value.