CULTURE MEDIA 51 



contain a comparatively thin layer of culture media, so that media becomes dry 

 in them after several days. Consequently, Petri dishes only have media placed 

 in them at the time one intends to plant bacteria. 



The media intended for use in dishes is stored in flasks or tubes. When a 

 plate is to be planted, a flask or tube of sterile media is liquefied in a water bath 

 and poured into a sterile dish; when solid the substance to be planted is dropped 

 on the surface or streaked over it with a sterile platinum wire or glass rod. 

 Another method is to liquefy a tube of culture media, cool it to 4oC., drop the 

 substance to be cultured into it, mix by turning the tube upside down several 

 times, and then pour the contents of the tube into a Petri dish. As a precau- 

 tion against contamination the cover of a Petri dish is lifted as little as possible 

 and media is not allowed to run over the edges. Dishes are not moved nor 

 placed in an incubator until the contents have solidified. 



CULTURE TECHNIQUE 



Liquids containing bacteria that are to be cultured are transferred to tubes 

 of culture media with sterile glass pipettes or with loops of platinum wire; solids 

 and macroscopic masses of bacteria are commonly handled with loops of plati- 

 num wire except when stab cultures are to be made, then a straight piece of 

 wire or a needle is used. 



When a tube of culture media is to be planted it is desirable to have the air 

 as still and as free of dust as possible, windows and doors are closed and one 

 avoids breathing directly upon or over exposed surfaces of media. The cotton 

 plugs are removed from flasks or tubes containing culture media by grasping the 

 plug between the little finger and palm of the hand, or between the other fingers, 

 care being taken to avoid touching the inside of the tube. While the plug is 

 held between the fingers, that part which enters the tube must not come into 

 contact with anything. Just before replacing the plug it is well to pass it 

 through a flame to burn off any organisms that may have settled on it. The less 

 frequently tubes are opened and the shorter the time the plug is out of the tube, 

 the less danger of contamination. After removing the plug, the open end of the 

 tube is flamed before introducing or removing bacteria; it is again flamed before 

 replacing the plug. 



If a pipette is used to inoculate media its contents are dropped into or upon 

 the media without bringing the pipette into contact with the media or the upper 

 portion of the tube. When inoculating liquid media with platinum loop, the 

 loop is held so that it does not touch the side of the tube until submerged in the 

 media, then it is given a quick turn and removed with the same care as when 

 inserted. 



Material containing bacteria to be cultivated on the surface of solid media 

 (either slants in tubes or plates) must be gently dropped, smeared or streaked 

 across the surface to avoid breaking the smooth surface. * 



To determine the character of growth beneath the surface, to observe gas 

 formation and other phenomena, what are known as shake cultures are sometimes 

 made in solid media, usually in gelatin. A tube of media is liquefied and cooled 



