396 BACTERIA 



It swells out and the bacillus looks like a drumstick. The diph- 

 theria bacillus is characterized by granular particles which turn 

 red with a certain stain and will not lose this stain. 



A method of identifying bacteria is through recognition of the types of colonies they form. 

 Some colonies are translucent, others are opaque ; some have regular margins, others have 

 irregular margins. One of the above plates has been exposed to bacteria, the other plate has 

 not been opened and is sterile. This is called the control. 



Media for cultivation of bacteria. Beef broth is the principal 

 medium used for the cultivation of bacteria. If a small quantity of 

 the seaweed, agar, is added to this, a solid medium results. Some- 

 times gelatin is used in place of agar, and prune juice, sugar 

 solution, or blood instead of beef broth. The nutrient medium 

 which will furnish the best nourishment for the particular bac- 

 terium is selected. 



Bacteria are usually grown in Petri dishes or on agar slants in 

 test tubes. The Petri dish was originally devised in 1887 by R. J. 

 Petri in a Berlin laboratory. It really consists of two dishes, 

 each one being a shallow glass saucer with a perpendicular rim. 

 One fits rather snugly as a dust-proof cover over the other. 



First, the agar medium is boiled and filtered two or three times. 

 The Petri dishes are thoroughly cleaned and sterilized. The hot 

 sterile agar is then poured into the dishes. Great care is taken, 

 in order to prevent any bacteria in the air from entering during 

 the pouring or plating of the agar. 



