102 MICROBIOLOGY 



(See Kitasato's method for isolating B. tetani.) In the study 

 of pathogenic bacteria the methods usually employed are 

 plate cultures and animal inoculation. 



Plate cultures. Plate cultures consist of liquefied gela- 

 tin or agar which has been inoculated with the material in 

 question and poured on glass plates or in Petri dishes. Each 

 organism present in this medium is enabled to develop or 

 multiply by itself, forming a colony. Formerly, glass plates 

 were employed for this purpose and covered with sterile 

 bell-jars. This was followed by the Esmarch rolls. 1 Later 

 Petri substituted the covered round glass dish which bears 

 his name. They are made in several sizes. 



Making plate cultures. Take 3 large tubes of agar or 

 gelatin and liquefy them. Then cool by standing the tubes 

 with a thermometer in a cup of water at a temperature of 

 about 45 C. When the temperature of the agar reaches 

 that of the water and the temperature of the whole has low- 

 ered to 40 C. it is ready for use. For convenience in label- 

 ing, number the tubes 1, 2, and 3. 



Place 3 sterilized Petri dishes on the leveling tripod and 

 adjust it by means of a spirit level. If shelves are properly 

 adjusted the leveling tripod is not necessary. With the wire 

 loop proceed by the same method as followed in making tube 

 cultures. Take one loopful of the bouillon culture, place it 

 in tube 1 and mix by carefully shaking. Flame the wire 

 and transfer 2 loopfuls from tube 1 to tube 2 and mix as 

 before. Again name the loop and transfer 3 loopfuls from 

 tube 2 to tube 3 and mix as with tubes 1 and 2. After the 

 tubes are inoculated, pour the agar into the Petri dishes. In 

 doing this remove the plug, carefully flame the mouth of the 

 tube, and after quickly cooling, raise with the left hand the 

 edge of the cover on one side of the Petri dish sufficiently to 

 allow of inserting the mouth of the tube. After the agar is 

 poured out of the tube replace the cover immediately. Label 

 and number the Petri dishes to correspond with the dilutions 



Esmarch. Zeit. f. Hygiene, Bd. I (1886) p. 293. 



