150 PHYSICAL INFLUENCES. 



are extremely resistant to strong sugar solutions, about 60-70 per cent 

 of cane sugar seems to be the limit of growth for Penicillium and Asper- 

 gillus species. Yeasts can also grow and ferment in very concentrated 

 solutions while bacteria in general do not tolerate solutions higher than 

 15 to 30 per cent, though many exceptions are known. 



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. prodigiosus 

 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. It was stated 

 before that the osmotic pressure depends upon the number of molecules 

 dissolved in a liquid; the larger the molecule the lower the osmotic pres- 

 sure. There is a group of substances known as colloidal bodies which 

 have a very large molecular weight, between i,oooand 10,000, it is estimated. 

 Their osmotic pressure even in very concentrated solutions would not be 

 high enough to interfere with microbial growth. Among these colloidal 

 bodies are found egg albumen, 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 decomposed. 

 The above statement refers only to solutions, while in natural media as 

 dried foods or soil, a combination of solid and dissolved substances is in- 



