256 



BACTERIA AND DISEASE 



A pure cultuifc ut b^ctencx. ^NuLice 

 that the bacteria are all the same size 

 and shape. 



or the beef broth becomes cloudy, one form may be isolated from 

 the others by the following process: a platinum needle is first 



passed through a flame to sterilize 

 it. After the needle is cooled it is 

 dipped in a colony containing the 

 kind of bacteria we wish to study. 

 The needle is then quickly drawn 

 across the surface of a dish of 

 sterile culture medium, and the 

 dish is immediately covered to pre- 

 vent any other forms of bacteria 

 from entering. When we have suc- 

 ceeded in isolating a certain kind 

 of bacteria in a given dish, we have a pure culture. 



Size and Form. — In size, bacteria are the most minute plants 

 known. A bacterium of average size is about -g-^ of an inch in 

 length, and perhaps 2^^-00" c>f an inch in diameter. Some species 

 are much larger, others smaller. They are so small that several 

 million are often found in a large drop of impure water or sour milk. 

 Three well-defined forms of bacteria are recognized : a 

 spherical form called a coccus; a rod-shaped bacterium, 

 the bacillus; and a spiral form, the spirillum. Some 

 bacteria are capable of movement when living in a fluid. 

 Tiny lashlike threads of protoplasm called flagella 

 project from the body, and by a rapid movement cause 

 locomotion. Bacteria reproduce with almost incredible 

 rapidity. It is estimated that a single bacterium, by a 

 process of division called fission, might if unchecked 

 give rise to nearly 17,000,000 others in twelve hours. 

 Under unfavorable conditions bacteria stop dividing 

 and form rounded bodies called spores. The spore is 

 usually protected by a wall and can withstand very un- 

 favorable conditions of dryness or heat ; even boiling 

 for several minutes will not kill some forms. 



Where Bacteria are most Numerous. — As the result of our 

 studies, we may draw some inferences concerning the presence of 

 bacteria in our own environment. They are evidently present 

 in the air, and in greater quantity in air that is moving than in 





^ ^^ 



Different 

 forms of 

 bacteria. 



