,l8 LABORATORY GUIDE IN BACTERIOLOGY 



4. Increase to 10 pounds and hold for 5 minutes. 



5. Increase to 15 pounds and hold for 5 minutes. 



6. Open the steam outlet and keep at 15 pounds 

 pressure for 20 minutes. 



It is advisable to sterilize the tubes again on the 

 following day by slowly bringing the pressure to 15 

 pounds with the steam outlet slightly open and keep- 

 ing at this pressure for 20 minutes. 



SECTION s 



PREPARATION OF CULTURE MEDIA 



EXERCISE I. PREPARATION OE DTTNHAM'S PEPTON 

 SOLUTION AND OF PEPTON BROTH (bOUILLON) 



1. Weigh the saucepan, measure into it 1,000 c.c. of 

 tap water, and heat over a flame or a water bath. 



2. Dissolve in this, when hot, but not boiling, 10 

 grams Witte's pepton. 



3. When dissolved, replace the evaporated amount 

 of water and divide into two equal parts of 500 c.c. 

 each. 



4. One-half is then filtered until perfectly clear, 

 tubed, and sterilized in the autoclave for 5 minutes at 

 120° C. -This is Dunham's Pepton Solution. 



5. Dissolve 1.5 grams extract of beef in the remain- 

 ing 500 c.c. 



6. Adjust the reaction with phenolphthalein paper 

 or by titration against n. NaOH. 



Note. — The reaction of culture media is a matter of vital 

 importance. Bacteria, especially pathogenic bacteria, grow 

 preferably in a medium which is neutral or slightly acid to 

 phenolphthalein. The neutral point of litmus is about 2 per 



