184 BACTERIOLOGY 



an organism, such 'as the production of indol, reduction of 

 nitrates to nitrites, the formation of ammonia, production 

 of gas in media containing different carbohydrates, or the 

 reducing power of the organism on aniline dyes, etc. 



Influence of External Agencies. Note the vitality of the 

 organism under the influence of various physical and 

 chemical agents. Determine the temperature at which it 

 thrives best, as well as the lowest and highest temperatures 

 at which growth is possible. Determine the thermal death- 

 point of the organism by subjecting it to various degrees 

 of temperature from 55 to 75 C. for ten minutes. Deter- 

 mine its resistance to drying; to the influence of light; to 

 the influence of germicidal substances. Determine the 

 influence of different gases upon the growth of the organisms, 

 such as hydrogen, nitrogen, or carbon dioxide. Determine 

 the chemical reaction of the culture-media best adapted for 

 its growth. 1 



BIOCHEMIC CHARACTERS. 



If the organism exhibits chromogenic properties, ascertain 

 whether the pigment is intra- or extracellular. Ascertain 

 under what conditions of temperature, reaction, and con- 

 stitution of media, or under what atmospheric conditions 

 this function is best exhibited. Note the influence of dif- 

 ferent reagents upon the pigment, such as chloroform, ether, 

 alcohol, water, acids, or alkalies. Note whether the organism 

 exhibits photogenic properties, and if so, ascertain what 

 conditions are most suitable for the manifestation of this 

 phenomenon. 



1 For more detailed description of the variations in the character of 

 the macroscopic and microscopic appearance of the cultures in the different 

 media, the student is referred to Chester's Determinative Bacteriology and 

 Eyre's Bacteriologic Technique. 



