IletJiods of Determination of Pathogenic Blicrobes. 129 



inoculated tubes receives no more than one 2:erm : 

 the culture is then pure. 



Method of isolation by plate cultures. — Koch has re- 

 commended a method of plate culture now practiced 

 in all the laboratories. A trace of the substance 

 containing the bacterial mixture is inoculated to a 

 tube of gelatin which is then fluidified by heating at 

 30°. The gelatin is then agitated so as to uniformly 

 distribute the germs throughout its substance, and 

 poured upon a sterilized glass plate which is placed 

 horizontally on a cold surface, and covered with a 

 bell jar. The gelatin spreads out in a thin solid layer 

 in which the germs find themselves isolated. The 

 plate is then placed on a small bench in the moist 

 chamber, in a room kept at a temperature of about 

 20°. Each germ gives an isolated colony, the appear- 

 ance of which can be studied and from which seed 

 can be obtained for starting new cultures. In this 

 process the germs which grow on the surface of the 

 layer of gelatin and which come, for the most part, 

 from the exterior should not be taken into account. 



Instead of using a plate the gelatin may be spread 

 out in the tube itself or in any vessel presenting a 

 very large interior surface.* This mode of procedure 

 has the advantage over the original technique that it 

 exempts the culture from all risk of external infection 

 and allows of anaerobic cultures. 



The substitution of agar-gelatin for gelatin consti- 



* [The original "plate process" of Koch is now, for the most 

 part, supplanted by the more convenient and safer method of 

 Petri, in which the fluidified gelatin, after inoculation in the test 

 tube, is poured Into a shallow glass dish provided with a cover 

 (crystallizers) . — D.] 



