18 GENERAL MICROBIOLOGY 



four papers of proper size for wrapping Petri dishes and is 

 inexpensive. 



Three or more Petri dishes may be wrapped together 

 if all are to be used at the same time. Mark each plainly 

 with the desk number. 



Pipettes. Place a piece of cotton in the bottom of a test 

 tube, plug the top only of the pipette with cotton (not too 

 tightly}, leaving but little of the cotton projecting out. 

 Wrap a small portion of cotton around the lower third of 

 the pipette, insert the pipette into the test tube until the 

 tip rests on the cotton, making the cotton wrapping serve 

 as a plug for the tube. 



Wrap pipettes so prepared in paper (one layer) and tie 

 and mark them plainly with the desk number. 



A covered metal case is often used for holding pipettes 

 to be sterilized. The upper end of the pipettes are plugged 

 with cotton, the pipette inserted in the case, the open end 

 of the case plugged with cotton, and the cover replaced. 

 (This latter method is not recommended for the new student, 

 as the necessity of careful technic in removing a sterile 

 pipette from the case without contaminating those remaining 

 is difficult to impress upon him) . 



Fermentation tubes are plugged with cotton as directed 

 for test tubes; the cotton plug should not project into the 

 bulb. 



Deep culture dishes are wrapped singly in paper as 

 directed for Petri dishes. 



Slides and cover-glasses are generally sterilized by 

 flaming, but only as needed. 



NUTRIENT MEDIA 



" Chemically, like all other living cells, microorganisms 

 consist of organic and inorganic nitrogen and mineral salts; 

 it is therefore necessary in order to grow a microorganism, 

 that these three classes of substances be made available, 

 together with oxygen, which is an essential to the life of all 



