136 AGEICULTUEAI, AND INDUSTEIAL BACTERIOLOGY 



the microorganism. Some forms can utilize inorganic 

 materials exclusively, others require the most complex 

 of organic compounds for their best growth. 



Effect of Concentration of Nutrients. — For each nutri- 

 ent there must be an optimum concentration at which 

 the microorganisms will grow most rapidly. An increase 

 in concentration beyond this point will more or less 

 rapidly cut down the rate of growth, that is increase the 

 length of the generation time. 



Stimulating Effects of Dilute Poisons. — Substances 

 which are ordinarily regarded as distinctly poisonous to 

 microorganisms, may when sufficiently dilute exert a stim- 

 ulating effect. Certain of the aniline dyes, for example, 

 when present in sufficient concentration prevent all 

 growth of microorganisms, but when greatly diluted they 

 may increase the rate of growth. 



Effect of Autogenic Substances. — ^Practically all micro- 

 organisms more or less rapidly produce substances 

 inimical to their own development. In some instances the 

 nvunber of bacteria which develop in a culture medium 

 is limited by the lack of some particular food constituent. 

 More frequently in the laboratory it is limited by the 

 accumulation of these toxic substances. Just what these 

 are is not known in most cases. Apparently in some 

 instances they are destroyed by heat and may be removed 

 by filtration through porcelain filters. As these substances 

 accumulate, the rate of development decreases, the gene- 

 ration time increases, until finally the bacteria are dying 

 off more rapidly than they multiply. 



Effect of Chemicals upon Eate of Death 



In any culture or mass of bacteria in which there is no 

 multiplication the cells are dying. The deatli rate may 

 be very small, but the bacteria are, nevertheless, decreas- 

 ing in number. 



