STUDY OF MICROORGANISMS 45 



tion. The inosl valuable of the methods used for this pur- 

 pose is tluit devised by Gram. If bacterial preparations are 

 stained with rentian violet dissolved in a saturated solution 

 of aniline oil in water, and then treated with iodine dis- 

 solved in a solution of potassium iodide, the combination 

 of the protoplasm of the cell and the dye is, in the ease of 

 certain organisms, of such a nature as to be soluble in alco- 

 hol : in the ease of other bacteria it is insoluble. The latter 

 retain in the presence of alcohol the color imparted to them 

 by the dye, and are termed Gram-positive organisms. The 

 others are called (1 nun-negative, since the color is removed 

 by washing the preparation in alcohol. This method is of 

 value in demonstrating Gram-positive bacteria in tissues 

 that are themselves Gram-negative. 



Systematic study of bacteria. The classification of 

 higher plants and animals and their identification is based 

 wholly upon their morphology, their form, and their struc- 

 ture. In the case of such simple organisms as the bacteria, 

 the variation in form is so slight as to make this character 

 of limited value in the study and identification of a par- 

 ticular pure culture. Many different kinds of bacteria are 

 so alike in the form of the cell itself that it becomes neces- 

 sary to include other characteristics in differentiating one 

 type from another. 



Cultural characteristics. The appearance of a mass of 

 cells that has developed in or on a culture medium is often 

 so characteristic that it can be used in separating one spe- 

 cies from another. For example, if a pure culture pro- 

 duces a uniform turbidity in a liquid, it is evident that the 

 cells will be found as single cells, or in very small aggre- 

 gates. If the growth is massed at the top or the bottom of 

 the culture medium, it implies that the cell-aggregates will 

 be large, and that the cells may be found in long chains, 

 or in zoogloea-like masses. The medium may remain per- 



