DIVISION IV 

 CHEMICAL INFLUENCES 



CHAPTER I 

 STIMULATION OF GROWTH* 



The influence of chemical substances upon microorganisms may 

 be helpful or harmful, or not noticeable. As helpful must be con- 

 sidered above all the food compounds. Unless given in such large 

 doses as to cause a physical or osmotic effect they will stimulate 

 the development. Other substances, not food, can also act as 

 stimulants. It is a recognized fact of long stand- 

 ing that many poisons in very small doses will 

 stimulate. This applies to the most highly 

 developed animals and plants as well as to micro- 

 organisms. Raulin noticed in 1869 that Asper- 

 gillus niger grew very much better in a nutrient 

 solution if a small amount of zinc salt was added. 

 He considered the zinc, therefore, as a necessary 

 constituent of the mold cells. Alcoholic fermenta- 

 tion can be stimulated by metallic salts. It is be- 

 ' /: ' ' - ' lieved by some physiologists that, as a law of nature, 



FIG. 120. Chem- every substance that is injurious in a certain con- 



o taxis. (After ce ntra tion is a stimulant in a lower concentration. 

 Fischer.) ... . .. . , . , 



A similar action of certain chemical compounds 



upon enzymes has been noticed, retarding in high concentrations, 

 stimulating in weaker solution. 



CHEMOTROPISM AND CHEMOTAXIS. Microorganisms manifest their 

 preference for certain foods not by a stimulated growth alone. They 

 also make efforts to obtain better food by growing or moving toward it, 

 which isjiot a manifestation of a rudimentary intellect. Such reactions 

 of microorganisms may be accounted for largely by chemical or osmotic 



* Prepared by Otto Rahn. 



286 



