George E. Holm

Salt effects in bacterial growth: IV. The physical nature of bacterial growth in various concentrations of neutral salts.

The physical nature of growth produced by various organisms has generally been attributed to the characteristics of the organisms themselves. Those organisms producing a pellicle upon the media in which they are growing were thought to do so because they were more strongly aerobic than those which did not form pellicles. Of recent years, however, this view has been changed and it is known that organisms which grow in the surface of the media or settle to the bottom as they grow, do so, not alone because of the characteristic of the organism, but largely because of the physical properties of the media. It has been shown by Larson, Cantwell and Hartzell 1 that the nature of the growth of B. subtilis, Mycobact. tuberculosis and other organisms can be entirely changed by the addition of castor oil soap. In the case of B. subtilis 45 dynes prevents the formation of pellicles and growth takes place down in the body of the medium. Neutral salts when added to the media in small quantities affect the physical nature of the growth of various organisms. Bact. coli when grown in medium of 1 per cent peptone broth whose H ion concentration has been adjusted between pH 5 and pH 8, shows great and varying degrees of turbidity, but the majority of the organisms settle out spontaneously. It has been noticed in our work, however, that when a small quantity of a neutral salt is added the majority of the bacteria are held in suspension and a very diffused growth is obtained. Observations have been made upon media of varying H ion concentration in order to determine whether or not the action of the neutral salts is primary or secondary and to determine the relation of turbidity to the rate of growth in various concentrations of neutral salts.