44 GENERAL MICROBIOLOGY 



5. Filter while hot through a plaited filter previously 

 washed with hot water. (Filtrate must be perfectly trans- 

 parent.) 



6. Tube, putting 8 c.c. in each tube. 



7. Sterilize for fifteen minutes on three successive 

 days. 



Microorganisms which will not produce ammonia or 

 nitrites from peptone may show this power if nitrogen is 

 added to this solution in the form of inorganic nitrogen as 

 potassium nitrate (0.2%). 



EXERCISE 11. NITRATE PEPTONE SOLUTION 



This solution is used to determine the power some 

 organisms have of reducing nitrates to nitrites, free ammonia 

 or nitrogen. 



Apparatus. 1000 c.c. distilled water; 1 gm. peptone, 

 Witte's; 0.2 gm. nitrite-free potassium nitrate; 'large agate- 

 ware pail; filling funnel; sterile test tubes; apparatus for 

 steam sterilization. 



Method. 1. Mix the following ingredients: 1000 c.c. 

 distilled water; 1 gm. Witte's, or other peptone; 0.2 gm. 

 nitrite-free potassium nitrate. Filter if necessary, 



2. Tube, placing 8 c.c. in each tube. 



3. Sterilize by heating for fifteen minutes on three suc- 

 cessive days or for five minutes in the autoclav at 120 C. 



CULTURES 



Definitions. A culture consists of the active growth 

 of microorganisms in or on a nutrient medium. 



A mixed culture is a culture composed of two or more 

 species of microorganisms growing together in or on a 

 nutrient medium. 



A pure culture is the growth of one species of micro- 

 organism only, in or on a nutrient medium, that was sterile 

 before inoculation. 



