66 GENERAL BACTERIOLOGY 



y Filter, tube and sterilize, 3 tubes. 



Sodium chloride 0.5 gm. ] 

 Peptone (Merck) 1. " I, 

 Potassium nitrate 0.2 ' 

 Water 1,000. gms.^ 



h. Litmus Milk. 



1) Freshly separated milk, or if this is not available, new milk 

 is placed in a separatory funnel in an ice chest over night to allow 

 the separation of the cream and the milk then drawn off. 



2) Litmus solution (e. above) is then added until medium is 

 faintly blue. 



3) Tube and sterilize in the steamer for 30-45 minutes on 3 or 

 4 consecutive days. During the summer months particularly very 

 resistant bacterial forms abound in the milk, so that it is necessary 

 to increase the number of applications or length of exposure. The 

 efficiency of the sterilizing process should be tested by placing the 

 milk in the incubator for several days to see if any change occurs, 

 2 tubes. 



In addition to the above have 15 tubes of bouillon (9 to contain 

 exactly 10 cc. for 41. and 44.) 10 tubes of gelatin, 15 tubes of agar, 

 6 water-blanks and 5 potato tubes. 



(If thought desirable the media required for Chapters IV. and V. 

 [Exercise 58], may be prepared at this time; this would then com- 

 plete all the media making required in Part I.) 



EXERCISE 38. EFFECT OF REACTION OF MEDIA ON GROWTH. 



General Directions. 



a. Melt 6 tubes of gelatin and add, under aseptic precautions, to 

 three of them, respectively, 0.1 cc, 0.3 cc, and 0.5 cc. of a normal 

 solution of hydrochloric acid, and to the other three the same 

 amounts of a normal sodium hydrate solution. 



6. Thoroughly mix, solidify gelatin in ice water and then inocu- 

 late (stab) each tube with the organism to be studied.. 



c. Make a control culture in a tube of neutral gela-tin. 



d. Incubate at 18° C. and note the effect of the chemicals on the 

 rate, amount and character of the growth. 



References. L. & N. 35 ; McF. 41. 



Special Directions. Use B. suhtilis and B. coli. Make sketches. 



EXERCISE 39. EFFECT OF CONCENTRATION OF MEDIA ON GROWTH. 



a. Pour about 2 cc. of ' ' condensed milk ' ' into each of two sterile 

 test-tubes, dilute one with five times the volume of sterile water. 



