i82 SURFACE TENSION OF THE MENSTRUUM 



Green' studied this problem, using standard methods of gas analysis. She found 

 that the dissolved oxygen did not vary with the surface tension. The conclusion, 

 therefore, that pellicle formation is a property of the surface tension of the medium 

 rather than the property of aerobiosis seems to be justified. 



Since wetting is a function of the surface tension, it may be stated that bacteria 

 will grow on the surface of a liquid medium if the latter is unable to wet the surface 

 of the organism. Since not all bacteria grow upon the surface of broth of a given sur- 

 face tension, it follows that some of the factors determining the type of growth are in- 

 herent in the bacterial cells. 



In discussing this phase of the question it may be assumed that the surfaces of 

 bacteria attract water in varying degrees. In case the water molecules have a greater 

 attraction for each other than for the bacteria, the latter are caught and held in the 

 surface of the fluid, and, finding conditions favorable to growth, develop in pellicle. 

 On the other hand, where there is greater attraction between water and bacteria — 

 as is the case between water and a clean, fat-free, glass surface, the bacteria are drawn 

 into the medium, and, finding conditions favorable to growth, grow diffusely. Thus 

 it is apparent, if this conception be true, that the type of growth in a liquid medium 

 will depend upon two factors, viz.: the liquid-air tension of the medium and the 

 nature of the bacterial surfaces. 



Since wetting is a function of the surface tension, it follows that an organism may 

 behave differently in two media of different surface tension, as is the case with the 

 hay bacillus in media with surface tensions of 60 and 35 dynes. 



If pellicle-forming bacteria are extracted with acetone and ether and the extract 

 compared with that of non-pellicle formers, it will be found that the former are rela- 

 tively rich in acetone-ether soluble substances. The tubercle bacillus is a striking 

 example of this group of bacteria, possessing as high as 40 per cent of fatlike sub- 

 stances, compared with about 7 per cent in non-pellicle-growing organisms. 



Bacteria, which ordinarily grow diffusely throughout liquid media, will develop in 

 pellicle' if grown on a medium containing a carbohydrate or glycerol, which they will 

 not ferment. The staphylococcus, for example, when grown for a few generations 

 upon 3 per cent glycerol broth, produces a pellicle resembling very much the growth 

 of the tubercle bacillus; the medium beneath the pellicle remains perfectly clear, and, 

 macroscopically, it resembles the growth of the tubercle bacillus. The ether-acetone 

 extract of such a culture represents 39.9 per cent of the weight of the dry matter of 

 the organisms, as compared with approximately 7 per cent of the same strain of the 

 staphylococcus grown upon broth without glycerol. 



It has been shown by numerous investigators that surface-tension depressants 

 have a marked effect upon bacterial growth. Indeed, it is the operation of the laws 

 of surface energy which makes it possible for bacteria to obtain food. It has been 

 pointed out earlier in this chapter that many organic compounds are surface-tension 

 depressants; that is to say, that they concentrate in the surfaces and interfaces of the 

 liquid. The nutritive material in culture media must, therefore, be concentrated at 

 the bacteria-water interfaces, which make them immediately available to the bacterial 



' Green, B. S.: Papers from Mayo Found, and Med. School, Univ. of Minn., p. 578. 1921-22. 



^ Larson, W. P., and Larson, L. W : Joiir. Infect. Dis., 31, 407. 1922. 



