BACTERIA AND BIOLOGICAL FACTOES 19 



SECTION V 



RELATION OF BACTERIA TO BIOLOGICAL FACTORS 



Exercise 23. Rate of Growth of Bacteria 



This question will be studied by noting the rate of growth of 

 B. coli in a flask of beef broth inoculated by the instructor. A 

 schedule will be made, assigning students to draw and plate 

 samples at hourly intervals from the moment of inoculation. 

 Students will take samples from the flask, according to direc- 

 tions, and put them into sterile Petri dishes, adding immedi- 

 ately a tube of melted agar. Mix the contents of the Petri 

 dish by tilting it back and forth, allow to cool in a horizontal 

 position, and place in the incubator. Each student will com- 

 pute the number of organisms per cubic centimeter in his 

 samples and report the number to the instructor for the com- 

 posite result. Take notes on the entire experiment and draw 

 a graph to illustrate the rate of increase. Where does the graph 

 rise most rapidly? Why? 



Exercise 24. Chemotaxis 



Free-swimming organisms, such as motile bacteria, show a re- 

 sponse to the presence of chemicals commonly serving as nutri- 

 ents. This response can be conveniently studied by placing the 

 chemicals to be tested in small capillary tubes and observing the 

 behavior of organisms near the open ends of the tubes. 



Proceed as follows: 1. Heat a small glass tube in a flame 

 until soft, withdraw from the flame, and quickly draw it out to 

 a slender filament. Break up the thinnest part of the filament 

 into pieces 8-10 mm. long and fuse up one end of each in the 

 flame. Place these tubes in a small dish of beef bouillon and 

 exhaust the air under the receiver of an air pump. 



2. A culture of motile bacteria may be obtained by boiling a 

 few kernels of corn or peas for a minute, to kill them, and 

 allowing them to putrefy in an open dish. Mount a drop of this 

 culture on a slide ; taking one of the capillary tubes in a pair of 



