W. p. LARSON 



iSi 



At the present time interfacial tensions cannot be measured or even calculated 

 with any degree of accuracy. The studies of Harkins' and Halvorson and Green^ indi- 

 cate there are all gradations of zones at the particle-water interface. Such zones may 

 vary from one extreme, where they must be well defined, to the other, where they 

 flow together, with all possible intermediate gradations. A pellicle-forming micro- 

 organism rich in "fat," like the tubercle bacillus, probably is a good example of the 

 first case, while the pneumococcus in a solution of bile salts represents the latter 

 (not "wet" in one case and dissolved in the other). 



SURPACE TENSION AND BACTERIAL BEHAVIOR 



In discussing the effect of the surface tension of the culture medium on bacterial 

 growth it may be well to begin with the pellicle-forming organism, since its effect is 

 more apparent with this group. 



The earlier bacteriologists regarded pellicle formation as an expression of obligate 

 aerobiosis. It is true that pellicle-forming bacteria are aerobic, but they are not all 

 obligate aerobes. Bacteria which are facultative anaerobes may be developed to 

 grow in pellicle. Upon reflection it is apparent that the quality of aerobiosis is not 

 the mechanism which causes bacteria to grow upon the surface of a licjuid medium 

 since pellicles are invariably of greater density than the fluid upon which they are 

 growing. Benton^ has shown that the pellicle of B. subtilis may be sedimented at 

 every stage of its development by centrifugation. It is obvious, therefore, that 

 bacteria growing on the surface of a liquid are supported in this position by some 

 force. Since aerobiosis does not constitute a force, we must look elsewhere for the 

 explanation. It is undoubtedly the tension in the surface of the medium which sup- 

 ports the bacteria in this position. An analogy may be found in the floating steel 

 needle. If a steel needle is coated with a film of oil and carefully placed upon the 

 surface of water it also will remain supported upon the surface. Here there can be no 

 question of aerobiosis. In the language of the physicist, the needle is not "wet" by 

 the water; its weight is not sufficient to break the water surface. It is thus supported 

 by the surface tension of the liquid. Reducing the surface tension by the addition of 

 a little soap or other surface-tension depressant causes the needle to sink promptly. 

 There is little doubt but that pellicle formation is a property of the surface tension 

 of the medium, although the character of the bacterial surface is also an important 

 factor, as we shaU see presently. 



If the surface tension of broth is depressed to some point below 40 dynes by the 

 addition of sodium ricinoleate and inoculated with the hay bacillus,'' it will be ob- 

 served that the growth is either diffuse throughout the medium, or, as often occurs, 

 it grows at the bottom of the flask. In this connection the question arises as to the 

 relation of the surface tension of a liquid to its ability to dissolve oxygen. If oxygen 

 were more soluble in water of reduced surface tension, this might account for the 

 different type of growth. 



' Harkins, W. D., in Alexander: op. cit., p. 192. 1926. 



^Halvorson, H. O., and Green, R. G.: Colloid. Symp. Monograph, 2, 185. 



3 Benton, A. G.: Proc. Soc. Exp. Biol, and Med., 20, 513. 1923. 



^ Larson, W. P., Cantwell, W. F., and Hartzell, T. B.: Jour. Infect. Dis., 25, 41. 1919. 



