Effect of Long Chain Fatty Acids on Bacterial Growth

We have shown elsewhere that certain water soluble lipids promote diffuse growth of tubercle bacilli in synthetic media, especially in the presence of serum albumin. 1-2 These same substances have now been found to enhance the growth of other microbial species, in particular of an unidentified micrococcus (strain C), recently isolated in our laboratory. Some understanding has also been gained of the conditions under which long chain fatty acids can stimulate bacterial growth. It is known that the soaps of fatty acids exert a bacteriostatic and bactericidal effect on certain micro-organisms, particularly on the Gram-positive and acid-fast species, and that unsaturated acids are more toxic than the corresponding saturated compounds. 3-9 For example, concentrations of oleic acid as low as 0.000001-0.00001% are sufficient to cause inhibition or retardation of growth of small inocula of human tubercle bacilli in synthetic liquid media. On the other hand, fatty acid esters (methyl oleate, triethan-olamine oleate, phosphatides) exhibit little or no primary toxicity. That the lack of toxicity is not due to poor solubility of the esters is indicated by the fact that the polyoxyethylene derivatives of oleic acid are essentially nontoxic, even though they are completely dispersible in water. 1 , 2 Thus, tubercle bacilli grow readily in synthetic media to which has been added 0.1-1.0% of Tween 80 (a polyoxyethylene derivative of sorbitan monooleate) purified to remove unesterified fatty acid. 10 Detoxification of the fatty acids can also be achieved, by adding to the medium native serum albumin. When an adequate amount of this protein is added to an opalescent soap emulsion (at neutral pH), there occurs an immediate clearing of the emulsion with concomitant disappearance of toxicity. It takes approximately 40 parts by weight of albumin to achieve complete detoxification of 1 part of oleic acid; however, growth of tubercle bacilli can be obtained in media containing 0.01% oleic acid and 0.5% serum albumin if a sufficiently large inoculum is used.