PLANT PARASITES. 



155 



ment of great value in the use of solid media for the cultivation of bacteria. They 

 usually grow within or upon the surface of the solid nutrient media in sharply circum- 

 scribed masses, called colonies, and different species may grow side by side in the same 



FIG. 77. A PETRI GELATIN PLATE CULTURE OF BACTERIA FROM MILK. 



In one plate there are few colonies showing difference in size and character. In the other the colonies 

 are much more abundant. 



receptacle for considerable periods without in the slightest degree interfering with one 

 another or tending to mix. The mode of growth and general appearances of the pro- 

 liferating bacterial masses on the solid medium often present very characteristic differ- 

 ences between different forms, and thus not only furnish valuable means of identifying 

 species, but render possible an early detection of contamination from chance admixture 

 of species. A given species of bacteria may be cultivated through a series of genera- 

 tions by transferring, with proper precautions, a minute portion from a growing colony 

 to fresh sterilized culture media. After cultivation through several generations the 



species may be presumed, and by micro- 

 scopical examinations proved, to be en- 

 tirely pure, and the effects, if any, pro- 

 duced by its inoculation into healthy 

 animals, to be due to it alone. 



THE CULTURE SUBSTANCES. There 

 are many culture media, some of which 

 are best suited for one, some for another 

 species of bacteria. Those most com- 

 monly used are meat broth (bouillon), 

 broth rendered solid by gelatin or agar- 

 agar (called " nutrient gelatin " or " nu- 

 trient agar"), boiled potatoes, milk, 

 coagulated blood serum, pleuritic " chest 

 serum," or other transudates into the 

 serous cavities. Glucose is often added 

 to the media for special purposes, and 



This cut shows a small portion of the culture, Fig. 

 77, magnified. Some of the colonies are very small, 

 others of a different species are large and spreading. 



FIG. 78. -COLONIES IN GELATIN PLATE CULTURE. 



the incorporation of litmus serves to 

 mark the formation of acids or alkalies 

 as the result of bacterial metabolism. 

 Various special forms of artificial media 



are also employed. (For the details of the methods of preparing culture media con- 

 sult special works on bacteriology, such as Abbott's " Principles of Bacteriology " or 

 Park's " Bacteriology in Medicine and Surgery.") These, various forms of media are 



