264 GENERAL MICROBIOLOGY 



In what compounds is phosphorus found in soil? Are 

 these available as plant food? What are the functions of 

 bacteria in this connection? 



What relation has phosphorus to decay and nitrogen 

 fixation? 



10. Give results and any conclusions in detail. Point 

 out any possible practical applications. 



REFERENCES 



MARSHALL: Microbiology, pp. 287-288. 



LOHNIS: Laboratory Methods in Agricultural Bacteriology, pp. 115- 



116. 

 SACKETT, PATTEN and BROWN: The Solvent Action of Soil Bacteria 



Upon Insoluble Phosphates, etc., Spec. Bui. 43, Mich. Exp. Sta., 



1908. 



DAIRY MICROBIOLOGY 



EXERCISE 1. A COMPARATIVE STUDY~OF THE NUM- 

 BER AND TYPES OF MICROORGANISMS AND 

 OTHER CELLS IN MILK 



A. PLATING METHOD 



Apparatus. Sterile Petri dishes; sterile 1 c.c. and 10 

 c.c. pipettes; tubes of sterile litmus lactose agar; 90 c.c. 

 and 99 c.c. dilution flasks; tubes of sterile litmus milk. 



Culture. Fresh milk from any source desired. 



Method. 1. Shake the milk vigorously one hundred 

 times to obtain a homogeneous sample, and plate in dilu- 

 tions, 1-100, 1-10;000 and 1-1,000,000 in litmus lactose 

 agar. 



2. Incubate the plates inverted at room temperature 

 for five days. 



3. Count at the end of this time, estimate the average 

 number of bacteria per cubic centimeter and approximate 

 the numbers of the different types of colonies. 



4. Are acid colonies present? Chromogenic colonies? 

 B. subtilis or B. mycoides? 



