I 

 INHIBITION OF GROWTH 23 S 



Recent investigations have shbwn the rather unexpected fact that the 

 efficiency of a poison is not proportional to its concentration. If 

 a certain poisonous solution is diluted with an equal volume of water, 

 we might expect it to be half as poisonous as before, but depending 

 upon the chemical nature of the poison, it may be more poisonous 

 than expected, or considerably less. It follows from this that two dif- 

 ferent poisons of the same intensity, if diluted in the same proportion, 

 may not have the same intensity any more. 



Microorganisms will gradually become accustomed to certain poi- 

 sons, and become more resistant. This principle has been utilized 

 in the manufacture of distilled alcohol; yeasts have been cultivated 

 which can tolerate a high concentration of acid; the acid serves 

 to suppress bacteria producing undesirable fermentations. 



The age of the culture and the stage of development will naturally 

 change the resistance of a species materially. The old cultures 

 which are past the culmination of growth will be much more sensitive 

 to any poison unless a spore-producing organism is under test. In 

 this case, we find a greatly increased resistance, similar to the 

 increased resistance of spores against drying and heat. 



The classification of disinfectants is very difficult as long 

 as we cannot explain completely the process of poisoning. It is im- 

 possible to arrange them according to the intensity of action, because 

 the intensity of influence depends not only upon the disinfectant, 

 but also upon the species of organisms. Some yeasts can resist ten 

 'times as much alcohol as certain bacteria. Formaldehyde is not 

 nearly as strong an agent with molds as it is with bacteria. The dis- 

 infectant concentration of a poisonous substance is not absolute. The 

 simplest method of grouping is by chemical structure and qualities. 

 Of the following natural groups can be distinguished acids (inorganic 

 and organic), metallic salts, hydrocarbons (aliphatic and cyclic), 

 alcohols (aliphatic and cyclic), aldehydes, anaesthetics, essential oils, 

 oxidizing agents and reducing agents. 



The first three groups, acids, alkalies and salts, are distinguished 

 from the rest as electrolytes; the strength of acids and alkalies (chemic- 

 ally speaking) is measured by the degree of electrolytic dissociation. 

 The disinfectant value follows largely the same law. The strongest 

 acids in the chemical sense are also the strongest disinfectants. There 

 are exceptions, however, where, besides the poisonous effect due to 



