266 PHYSICAL INFLUENCES 



Colloidal Solutions. In order to determine the amount of water 

 which is absolutely necessary for microbial proliferation, only such 

 media can be used which do not cause osmotic pressure. If B. prodigio- 

 sus does not develop in a 10 per cent salt solution, this is not due to lack 

 of moisture, because the same bacillus will grow in a 30 per cent sugar 

 solution which contains 20 per cent less moisture. Another factor be- 

 sides the water content enters, which can be avoided only in solutions 

 without osmotic pressure. 



A few substances are known to give such solutions, namely, colloidal 

 bodies which have a very large molecular weight. Their osmotic pres- 

 sure even in very concentrated solutions would not be high enough 

 to interfere with microbial growth. Among these colloidal bodies 

 are found egg albumin, gelatin, peptones, all protein substances; 

 also starch, dextrin and gum arabic among the carbohydrates. None 

 of these substances has a retarding influence upon bacteria; some of 

 them can be mixed with water in all proportions; consequently, they 

 are the ideal medium to test the water requirements of microorganisms. 

 Experiments carried on with gelatin, powdered meat, crackers, 

 bread and potato, vary but little in results. A few bacteria cannot 

 grow in a medium with only 60 per cent water, but most organisms 

 develop slowly even with 50 per cent water and some may be able to 

 develop with only 40 per cent. Molds can grow very scantily in even 

 more concentrated media. Protozoa probably have to have a more 

 diluted medium for their development though no experiments bearing 

 upon their water requirements are known to the author. 



The fact that in a colloidal solution growth will cease if the moisture 

 is below 30 to 40 per cent does not necessarily indicate the conclusion 

 that any substance with less than 30 per cent water cannot be decom- 

 posed. The above statement refers only to solutions, while in natural 

 media as dried foods or soil, a combination of solid and dissolved 

 substances is involved. Butter is an excellent medium for many bac- 

 teria, yeasts, and molds, though it contains only 12 to 15 per cent of 

 moisture. If butter fat were soluble in water, the concentration of 85 

 parts of solid in 15 parts of liquid would certainly prevent any growth 

 whatever, but fat is insoluble, and the fat particles do not interfere 

 at all with the growth of microorganisms in the droplets of buttermilk 

 distributed all through the butter. The concentration in these small 

 droplets is the deciding factor. If the growth of microorganisms in 



