MAKING PLATE AND ESMARCH ROLL CULTURES 29 



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 levelling tripod to congeal the gelatin. 



The directions given above for making the dilutions are appli- 

 cable only when the original culture is moderately clouded. If 

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

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

 more, as in turbid bouillon or slant agar culture cultures, it will 

 be necessary to take a much smaller quantity for the first dilu- 

 tion. 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. 



43. Making Esmarch Roll Cultures. — For this purpose gela- 

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

 sides of the tubes. It 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 inoculated tubes 

 are tipped and rolled so that the liquid gelatin moistens the 

 inside of the tube to within about a centimetre 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 con- 

 tact with the plug. In rolling the tube the plugged end should 

 always project beyond the ice. See illustration in text-books. 

 (Fig. 29, p. 133, Abbott.) 



