TECHNIQUE 85 



This will insure a more uniform spreading of the medium over the 

 bottom of the dish. 



To pour the liquefied agar or gelatin from the tubes, remove the 

 cotton plug and flame the mouth of the tube so as to kill any 

 bacteria or spores that may be present; raise one side of the cover 

 just high enough to permit bringing the tube to the middle of the 

 dish and pour contents into the dish over the material planted into 

 the middle of the dish. Let cover of the dish sink into place and 

 by very slight tilting of the Petri dish induce the culture medium 

 to spread evenly over the bottom of the dish before the medium 

 has had time to coagulate. As the medium spreads it also causes 

 the spreading of the planted material. 



Many workers use 5 cc. of the medium for plating, instead of 

 10 cc. as above recommended. The smaller amount is satisfactory 

 when I cc. quantities are to be planted or inoculated. However, 

 in order to make sure that the entire area of the bottom of the 

 dish is well covered, 10 cc. quantities should be used. The larger 

 amount also minimizes the influences which the changes in 

 evaporation in the media may have upon the quantitative results. 



4. Method of Making the Plate Cultures. — Absolutely clean 

 sterilized (dry heat of 150° C. for i hr.) Petri dishes of the 

 standard size (10 cm. diam.) are used, o.i cc. quantities of 

 the substance to be cultured, or dilutions thereof, are planted or 

 delivered into the middle of the dish, an absolutely clean and 

 sterile i cc. pipette accurately divided into tenths. The cover of 

 the dish is to be lifted just high enough to permit placing the pipette 

 in position, and is to be replaced just as soon as possible. 



In the usual water analysis work, i cc. quantities are generally 

 planted, instead of o.i cc. quantities as above recommended. 

 For purely quantitative results, the smaller amounts should be 

 planted because the larger amounts may include enough of the 

 inoculating liquid to interfere with the uniformity of results. 



Formerly it was customary to mix the material to be planted 

 with the medium in the tube before plating. This method has 



