REACTION OF THE NUTRIENT MEDIUM 205 



REFERENCES 



MARSHALL: Microbiology, pp. 319-320, 386-388. 



RUSSELL and HASTINGS: Experimental Dairy Bacteriology, pp. 89-91. 



LAFAR: Technical Mycology, Vol. II, Part I, pp. 142-144. 



EXERCISE 26. TO ILLUSTRATE THE EFFECT OF THE 

 REACTION OF THE NUTRIENT MEDIUM UPON 

 MICROORGANISMS 



Apparatus. One liter of ordinary broth (should be 

 enough for three students); normal NaOH; normal acid; 

 four sterile 1 c.c. pipettes; 10 c.c. pipettes; sterile test 

 tubes. 



Cultures. B. prodigiosus (broth culture); B. subtilis 

 (broth culture); Oospora lactis (broth culture); Torula 

 rosea (broth culture). 



Method. 1. By adding normal acid or alkali produce in 

 100 c.c. portions of ordinary broth the following reactions: 

 -40, -30, -20, -10, 0, +10, +20, +30, +40, +50 

 degrees Fuller's scale, and titrate after readjusting the 

 reaction, as a check. 



2. Tube, using 9.9 c.c. in each tube (mark the tubes 

 plainly), and sterilize (refiltration may be necessary before 

 tubing in some cases). 



3. Using a sterile 1 c.c. pipette, inoculate one set (ten 

 tubes) with 0.1 c.c. of the broth culture of each of the above 

 organisms (four sets) and incubate the tubes at room 

 temperature. 



4. Examine the tubes as often as possible for the first 

 twenty-four to thirty-six hours, and record the tube or 

 tubes in which macroscopic growth is first visible. What 

 do you conclude as to the effect of the reaction of the medium 

 in these instances? 



5. Examine the tubes every day for seven days. Tabu- 

 late your observation. Note the range of reaction in 

 which each organism is capable of growing. Does this 



