METHODS FOR THE MICROSCOPIC STUDY OF BACTERIA 215 



media the growth of the -same organism is usually luxuriant, moist, 

 and spreading. 



3. By the composition and reaction of the medium the addition 

 of specifically nutritive substances, as of fresh sterile tissue to media 

 for the cultivation of anaerobes; of utilizable carbohydrates to media 

 for the cultivation of carbohydrophilic bacteria; of fresh, defibrinated 

 blood to media for the cultivation of hemoglobinophilic organisms; 

 these may improve conditions otherwise unfavorable for bacterial 

 development. 



The reaction of the medium, furthermore, is important; many 

 bacteria are extremely sensitive to slightly acid media; the aciduric 

 bacteria thrive in media too acid for the existence of other organisms. 

 Even the ordinary laboratory media, made according to a definite 

 formula, vary sufficiently in chemical and physical properties to 

 influence materially the appearance of bacterial colonies. The degree 

 of influence is more pronounced in the feebly growing forms, but it 

 may affect the appearance of colonies of the more hardy types as well. 



4. The rate of growth of bacteria also affects the appearance of 

 colonies. 



It is useless, as a scientific procedure, to attempt to recognize dif- 

 ferences of greater refinement than the accuracy of the method permits 

 of, and for this reason the descriptions of bacterial colonies should 

 not be carried to extremes. In general, bacterial growths on solid 

 media are described as solids in space the average size, form, color, 

 lustre and texture. This applies equally well to colonies, slant and 

 stab cultures. The really valuable information gleaned from a study 

 of bacterial growths is the recognition of types of growth. For example, 

 spore-forming bacteria (aerobic) produce rather heavy, opaque, floc- 

 culent colonies; members of the Alcaligenes dysentery, typhoid, 

 paratyphoid group grow characteristically as rather small, round, 

 transparent colonies. 



(b) The Enumeration of Bacteria. A very practical application of 

 the plating method for the isolation of bacteria is the enumeration of 

 bacteria in water, milk and other similar substances. The principle 

 involved depends upon the development of colonies of bacteria from 

 single cells. If a definite volume of water, 1 c.c. for example, is dis- 

 tributed in melted agar, thoroughly mixed in the tube by rotation 

 between the hands, and poured carefully into a sterile Petri dish, the 

 number of colonies which develop within a definite period of incuba- 

 tion may be regarded as a measure of the number of living bacterial 



