INVESTIGATION OF MICROBIAL DISEASES 119 



a particular locality or tissue, or are present only at one 

 stage of the infection. In addition to the microscopical 

 examination, cultures must be made on various media, 

 those media being chosen which will probably be suitable 

 for the growth of the organism present in the particular 

 condition ; for example, in the examination of animal 

 diseases, media rich in protein, such as blood-serum, 

 nutrient agar and gelatin, will be the most serviceable. 

 In the examination of plant diseases, vegetable infusions 

 prepared from the plant itself or from other sources, and 

 enriched by the addition of vegetable proteins, and carbo- 

 hydrates, should be chosen. In fermentations, beer-wort, 

 grape or fruit juice, and saccharine solutions should be 

 made use of ; while for the nitrifying organisms, solutions 

 containing nitrates and nitrites, salts of ammonia, urea, 

 and asparagin will have to be employed. In addition, 

 it will in most cases be advisable, and in all safer, in order 

 to isolate the various species, to make plate cultivations, 

 either in Petri dishes (p. 78), or by streaking several sloped 

 tubes of agar, etc. (p. 81). Having obtained pure cultiva- 

 tions it will be necessary to determine the species of 

 organism, 1 if it has been previously isolated and described, 

 or to give a careful description of it, if it be a new one, 

 for the use of subsequent investigators. In the identifica- 

 tion or description of an organism all the following features 

 must be carefully noted : 



1. The morphology of the organism under various conditions, its 

 size, form, and motility, the presence of flagella, and their number, 

 arrangement, and character. 



2. The presence or absence of spore formation, its nature, the 



1 The descriptions of a large number of species of bacteria have been 

 collected and tabulated in convenient form by Chester in A Manual 

 of Determinative Bacteriology (Macmillan and Co., 1901). The terms he 

 suggests for describing bacterial growths, etc., might well be adopted 

 by bacteriologists. A committee of the Society of American Bacterio 

 legists has drawn up an elaborate chart for the description of species 

 of organisms. 



