The Effect of Organic Acids on Mammalian Tubercle Bacilli

The growth of tubercle bacilli in vitro was inhibited by the addition of the sodium salts of very low concentrations of certain organic acids to a variety of liquid and agar culture media containing whole serum or serum albumin. Capric acid was the most active of the compounds tested, but inhibition of growth occurred also with the shorter aliphatic acids. Lactic acid was also growth-inhibitory, whereas the keto and dicarboxylic acids tested were inactive in this respect. The inhibitory activity of the aliphatic acids and of lactic acid increased as the pH of the medium was lowered by addition of HCl. It was greater in media enriched with serum or with oleic acid-albumin complex, but was otherwise fairly independent of the composition of the medium. The inhibitory effect appears to be bacteriostatic rather than bactericidal and to depend upon a disturbance of the normal metabolic processes of the bacilli. Some of the long chain fatty acids caused a marked enhancement of growth when used in low concentrations and in admixture with enough serum albumin to overcome their toxicity. The significance of these findings is discussed with reference to the survival and multiplication of tubercle bacilli in vivo within inflammatory and caseous areas, which are known to be often acidic and to contain high concentrations of organic acids.