CULTIVATION OF BACTERIA 



31 



a suitable heat regulator. For the ordinary pathogenic 

 bacteria the incubator is set to maintain a constant tem- 

 perature of about 99 F. 



In studying the bacteria of water, a temperature of 

 60 F. is usually employed. The general type of incu- 

 bator is shown in Fig. 11, page 29. 



Fig. 13. Streak plate. (From Hiss and Zinsser, A Text-Book of 

 Bacteriology," D. Appleton & Co., publishers.) 



After remaining in the incubator for twelve to twenty 

 hours it will be noticed that the appearance of the beef- 

 broth has changed. Originally perfectly clear, it now 

 is more or less uniformly cloudy, or it may have a 

 cloudy sediment or be covered with a scum. If the cul- 



