Isolation of Bacteria 215 



In this tube we may have no more than ten thousand organ- 

 isms, and if the same method of dilution be used again, the 

 third tube may have only a few hundreds, and a fourth only 

 a few dozen colonies. 



After the tubes are thus inoculated, one of the sterile glass 

 plates is caught by its edges, removed from the iron box, and 

 placed beneath the bell-glass upon the cold plate covering 

 the ice-water of the leveling apparatus. The plug of cotton 

 closing the mouth of tube No. i is removed, and to prevent 

 contamination during the outflow of the gelatin the mouth of 

 the tube is held in the flame of a Bunsen burner for a moment 

 or two. The gelatin is then cautiously poured out upon the 

 plate, the mouth of the tube, as well as the plate, being 

 covered by the bell-glass to prevent contamination by germs 

 in the air. The apparatus being level, the gelatin spreads 

 out in an even, thin layer, and, the plate being cooled by the 

 ice beneath, it immediately solidifies, and in a few moments 

 can be removed to the moist chamber prepared to receive it. 

 As soon as plate No. i . is pre- 

 pared, the contents of tube No. 

 2 are poured upon plate No. 

 2, allowed to spread out and 

 solidify, and then superimposed ' Fig 42 ._ Glass 

 on plate No. i in the moist 

 chamber, being separated from the plate already in the 

 chamber by small glass benches (Fig. 42) made for the purpose 

 and previously sterilized. After the contents of all the tubes 

 are thus distributed, the moist chamber and its contents are 

 stood away to permit the bacteria to grow. Where each 

 organism falls a colony develops, and the success of the 

 whole method depends upon the isolation of a colony and its 

 transfer to a tube of new sterile culture media, where it can 

 grow unmixed and undisturbed. 



From the description it must be evident that only those 

 culture media that can be melted and solidified at will can be 

 used for plate cultures viz., gelatin, agar-agar, and glycerin 

 agar-agar. Blood-serum and LofHer's mixture are entirely 

 inappropriate. 



The chief drawbacks to this excellent method are the 

 cumbersome apparatus required and the comparative im- 

 possibility of making plate cultures, as is often desirable, in 

 the clinic, at the bedside, or elsewhere than in the labora- 

 tory. The method therefore soon underwent modifications, 



