254 LABORATORY EXERCISES IN BACTERIOLOGY. 



Exercise 73. Let each student make fractional smears from material 

 obtained from the fauces of a diphtheritic patient ; or if this be not at hand, 

 let the same be done with a mixture of known diphtheritic organisms and 

 some of the pus cocci diffused in a tube of bouillon. 



Exercise 74. Let each student isolate the organisms in a loopful of 

 normal saliva by diffusion in liquefied gelatine and agar. 



(B) PHYSIOLOGIC METHODS. 



In this group of procedures advantage is taken of some peculiarity in the life- 

 history of an organism sought to be obtained in isolation, either its peculiar resistance 

 to some influence capable of destroying associated bacteria before inoculation, or some 

 method of culture capable of restraining the development of organisms present with 

 it in the inoculated medium. These physiologic methods are selective, and for the 

 most part are employed where some definite organism is sought, and are not applicable 

 for the isolation of every species of any complex mixture. 



i. Selection by 'Living Animal Tissues. Advantage may be taken of the fact 

 that the living tissues of different animals by nature afford to some organisms a favor- 

 able opportunity for growth, while to others they are resistive in variable degree, 

 perhaps to the extent of actual destruction of the germs. It is to this fact largely 

 that we refer the distribution of disease in different varieties of animals. If a complex 

 mixture of bacteria in which there exists some form especially pathogenic for a certain 

 kind of animal be inoculated into an individual of this species, it is to be presumed 

 that the pathogenic germs will, from the favor of the conditions afforded, undergo 

 rapid development, probably showing a specific selection for some particular structure 

 of the animal body; while it is probable that the other organisms, finding less favorable 

 surroundings, will either not develop at all or but poorly (and in the latter case be 

 restricted to the immediate vicinity of the point of inoculation). It then is possible, by 

 planting upon proper laboratory media from the parts of the animal which have been 

 especially invaded by the pathogenic organisms, to obtain them free from their former 

 associates. If, for example, one will inoculate beneath the skin of a male guinea-pig 

 some of the nasal discharge from a horse affected by glanders, containing a mixture 

 of Bacterium mallei with various pathogenic and perhaps other organisms, the former 

 rapidly invade the animal, selecting the testicles as special points of growth, and 

 shortly kill it; while the pus germs are apt to remain restricted to the subcutaneous 

 tissues about the point of injection. By planting some of the material obtained from 

 the more or less necrotic testicles upon the laboratory media a pure culture of the 

 glanders organism may without special difficulty be obtained. 



Exercise 75. Early in the work of the class some sputum from a con- 

 sumptive case should have been dissolved in physiologic salt solution and 

 injected with a hypodermic syringe into the subcutaneous tissues of the 

 abdomen or chest of a number of guinea-pigs. In some of the animals it is 

 probable that the suppurative changes caused by the pyogenic germs in the 

 sputum may destroy life at a comparatively early period, but it may be 

 expected that in other instances the animals will in the course of some days 

 have overcome any general or local influences of these bacteria. In such, 



