MAKING PLATE AND ESMARCH ROLL CULTURES 31 



gelatin is liquefied at a temperature of 45 C ; (2) the plates 

 when made are to be kept in the locker the same as the gelatin 

 stab cultures; (3) in hot weather it is sometimes necessary 

 to put a piece of ice in the reservoir under the glass plate on 

 the leveling tripod to congeal the gelatin. 



The directions given above for making the dilutions are 

 applicable only when the original culture is moderately clouded. 

 If there are comparatively few bacteria in the liquid, a larger 

 quantity of the culture will be necessary. If there are many 

 more, as in turbid bouillon or in slant- agar-culture cultures, 

 it will be necessary to take a much smaller quantity for the 

 first dilution. It is often desirable to make the first dilution 

 in a tube of sterile water or bouillon instead of gelatin or agar, 

 and to make 2 rather than 3 plates. It is sometimes desirable 

 to make 4 or more cultures. 



45. Making Esmarch roll cultures. For this purpose gela- 

 tin is ordinarily used. Agar does not adhere readily to the 

 sides of the tubes, but is sometimes used. Take the desired 

 number of large tubes of gelatin, liquefy, inoculate, label, and 

 number the dilutions as in making gelatin plate cultures. 

 Place a block of ice about 6 inches long in an agate-iron or 

 glass tray. Melt a slight, nearly horizontal groove in the ice 

 with a test tube containing hot media or water. The inocu- 

 lated tubes are tipped and rolled so that the liquid gelatin 

 moistens the inside of the tube to within about a centimeter 

 of the plug. Then roll the tube rapidly in the groove on the 

 ice until the medium becomes solid. The gelatin should not 

 come in contact with the plug. In rolling the tube the plugged 

 end should always project beyond the ice. (See illustration in 

 text-books.) 



