32 APPLIED BACTERIOLOGY FOR NURSES 



ture was planted on sterile agar (a kind of gelatin) it 

 will now be found to be covered with a peculiar, more or 

 less slimy mass, or with a number of small, rounded spots, 

 This cloudiness in the fluid culture, or this slimy mass 

 on the solid cultures, is the new growth, and is made 

 up of. millions of tiny bacteria. Just as in planting seed 

 in the ground, a plant arises from each seed; so, in plant- 

 ing bacteria on the surface of a solid medium, wherever 

 a single bacterium was deposited a whole group of 

 similar bacteria develop. These groups become visible 

 to the naked eye, forming usually more or less rounded 



Fig. 14. — Petri dish for making plate cultures (McFarland). 



masses, varying in size from that of a pin's head to 

 disks J inch in diameter. Such masses of similar bac- 

 teria, the offspring of a single individual, are spoken 

 of as ^'colonies." The number of colonies developing 

 is thus an index of the number of living germs in the 

 material planted. If the bacteria originally planted 

 were very numerous, the colonies developing are so 

 closely crowded that they form one continuous film, 

 in which it is impossible to distinguish separate colonies. 

 It is apparent that we can discover the number of living 

 germs in a specimen of fluid by ]ilanting a known 

 quantity of the fluid on a solid medium, growing the 

 culture, and then noting the number of colonies which 

 develop. This is facilitated by using shallow flat glass 



