MICROORGANISMS AND HUMAN WELFARE 17 



17. To determine conditions favorable and unfavorable 

 for the growth of bacteria. Laboratory demonstration. 



A. The effect of different degrees of temperature. Expose 



for ten minutes three Petri dishes of nutrient agar 

 to the air in a room or corridor when classes are 

 moving about. Cover the dishes and label them 

 No. 8, No. 9, and No. 10, respectively. Put dish 

 No. 8 in a temperature of 80 to 100 F., and dish 

 No. 9 in the refrigerator, or in some other equally 

 cold place. Dish No. 10 should be put in a steam 

 sterilizer and heated for thirty minutes on each 

 of three successive days; it should then be kept 

 in a warm, dark place. 



1. Describe the difference in the treatment of dishes 8, 



9, and 10. 



2. At the end of a week examine each of the three dishes. 



What difference do you find in the relative number 

 of colonies in them? 



3. What do you conclude, therefore, as to the influence 



of each of these three different degrees of tempera- 

 ture on the growth of bacteria? 



B. Pasteurization of milk. (Optional.) If possible secure a Pas- 



teurizer 1 (Fig. 8). Carefully clean with soap and hot 

 water, inside and out, four of the glass bottles, fill each 

 with milk that is fresh, and fasten on the stoppers. 



1 Home Pasteurizers, System Nathan Straus, each supplied 

 with bottles and stoppers, may be bought at the Nathan Straus 

 Pasteurized Milk Laboratory, 348 East 32d St., New York City, or 

 at any of the Laboratory depots situated throughout the city. The 

 manufacturer's price for the entire outfit is $1.50. The authors are 

 indebted to the Nathan Straus Laboratories for the cut of the Pas- 

 teurizer, and for the directions quoted above. The circular also 

 contains the following statements. "The advantage of Pasteuriza- 

 tion over other systems, such as sterilization or boiling, consists in the 

 lower degree of heat applied, which is sufficient to kill all noxious 

 germs, while the nourishing quality and good taste of the milk are 

 retained. . . . Before use, warm the milk in the bottles to 

 blood heat. Never pour it into another vessel. The milk must not 

 be used for children later than twenty-four hours after Pasteuriza- 

 tion. Never use remnants." 



