The influence of the surface tension of the culture medium on bacterial growth

Abstract

Pellicle-forming bacteria such as the B. tuberculosis, B. subtilis and others of that group which habitually grow upon the surface of liquid medium, will grow throughout the body of the medium by depressing its surface tension from 59 dynes, the S. tension of ordinary broth, to 40-45 dynes. By analogy with the floating needle experiment it may be assumed that when the pellicle-forming bacteria are properly wetted they no longer grow upon the surface of the medium but throughout the body of the broth or even at the bottom of the flask.

The further observation has been made that the B. subtilis and B. antfzracis, when grown in media of low S. tension, finally become asporogenous. Cultures of B. anthracis grown under such conditions and sterilized by heat at 60° for 30 minutes protect guinea pigs. The enhanced wetting of the bacteria brought about by the addition of soap probably creates better nutritive conditions which cause the organisms to grow without forming spores. Castor oil soap when in aqueous solution is perfectly clear, does not hydrolyze as readily as most other soaps, and has therefore been used extensively in our experiments. It is more toxic to some bacteria than potassium or sodium stearate. This is probably due to the fact that castor oil soap is dialyzable and probably dialyzes into the cell and disturbs the salt balance by precipitating the calcium, magnesium and salts of the heavy metals.

Bacteria such as the pneumococcus and streptococcus will not grow on low tension media, while the organisms which inhabit the intestinal tract grow abundantly on media of low tension. This is not surprising since the contents of the intestines have a low S. tension due to the presence of bile, soaps and other S. tension depressants.