42 METHODS OF STUDYING MICROORGANISMS 



Before the dyestuff is applied the smear must be fixed by 

 heat or alcohol or formaldehyde. This is for the purpose 

 of killing the albuminous material, keeping it exactly as it 

 was when removed from the body, and rendering it suscep- 

 tible of taking up and permanently retaining stains, a 

 property living tissues and fluid possess to a very limited 

 degree. Once fixed and stained, examination will reveal 

 the bacteria present, and the observer can form an opinion 

 of the probable nature of the infection. Reference is fre- 

 quently made in the text to Gram's stain, and it is desirable 

 that the reader be familiar with the term and its significance. 

 It is a combination of anilin oil, water, and gentian violet, 

 which stain can be fixed into some bacteria by after-treatment 

 with iodin solution, so that alcohol will not wash it out. The 

 test is of great importance in determining certain species. 



Animal Inoculation. Another method of studying bacteria 

 is by injecting them into susceptible animals. Thus can be 

 discovered their power of producing disease, its severity, 

 called virulence, and the nature of their action. When the 

 presence of bacteria in morbid matter cannot be demonstrated 

 by stain or by cultural methods, it may sometimes be shown 

 by injecting the suspected material into animals. If the 

 animal fall sick or die one can then obtain cultures of the 

 germs for study. The value of this method of discovering 

 bacteria is increased by the development of changes in the 

 animal's organs peculiar to certain germs. Thus the tubercle 

 bacillus, an organism not easy to find by direct examination, 

 produces definite alterations of organs and special kinds of 

 inflammation by which its presence is indicated and from 

 which it can be obtained. This is also true for other bacteria 

 streptococci, anthrax, and glanders bacilli. 



Protozoa. The study of protozoa varies according to the 

 source. The parasite of malaria may be found by direct 

 microscopic examination of the fresh blood. This is also 

 true of the organism of sleeping sickness. The protozoa 

 causing dysentery require the maintenance of a definite 



