CHAPTER IX. 



THE EXAMINATION OF CULTURES. 



The study of cultures. The examination of cultures of 

 bacteria consists in a careful study of the appearance of the 

 growth in or upon the medium ; its effect upon the medium, 

 such as the liquefying of gelatin, the imparting of pigment, 

 coagulation of the casein in milk; and the microscopic ex- 

 amination of the bacteria themselves. The time at which -the 

 cultures are to be examined varies with the medium used, 

 the temperature at which the cultures were grown, and the 

 organisms themselves. The more common pathogenic bac- 

 teria will have completed their growth under favorable cir- 

 cumstances in from 24 to 36 hours. However, certain strep- 

 tococci will have developed their characteristic colonies in 

 from 30 to 16 hours; while the bacteria of glanders and tu- 

 berculosis will require a much longer time. When one ex- 

 amines pure cultures, the w~ork is a matter of simple descrip- 

 tion ; but in making bacteriological analyses where the num- 

 ber of species is unknown the process is more difficult. In 

 such examinations it is often desirable to determine the num- 

 ber of bacteria present, in case of quantitative work the 

 number of different species. In most quantitative work and 

 in procuring pure cultures of the species in question, plate 

 cultures are used. 



The examination of plate cultures. It is on the plate 

 culture that one is able to obtain the characteristic growth 

 of the colony. When the colonies are developed the plates 

 should be examined first by the unaided eye, making note of 

 the general character and number of the colonies. In an 

 impure culture the variety of colonies should be noted. In 

 such cases it is very important that the examination be made 

 early, before the colonies of bacteria that are more vigorous 

 in their growth or have a pronounced tendency to spread on 



