280 APPLIED BIOLOGY 



Sterilize half of the tubes again on second and third days. The 

 tubes sterilized once will probably become turbid within a few days 

 and microscopic examination will show that practically all the con- 

 tained bacteria are rod-like. Later they will form spores ; and this 

 is the secret of the bacteria appearing after once boiling. The spores 

 were able to withstand the heat ; but before the second or the third 

 heating they had germinated and the bacteria thus formed were 

 killed by the later heating. 



Pure Cultures. It often happens in the above experiment that 

 the tube heated once has practically a pure culture of a certain kind 

 of bacteria. This is due to the fact that all active bacteria are 

 killed, and when the spores germinate the conditions (food, acidity) 

 are best suited to the one kind of bacteria which flourishes. Even 

 if other kinds were suited to the existing conditions, the form which 

 is most abundant first may gradually crowd out the other kinds. 



Now, this is one way of getting approximately pure cultures of 

 certain bacteria; but for more accurate work and for separating 

 many species which live together, it is necessary to have a method 

 for isolation of individual bacteria. This is afforded by Koch's 

 culture-plate method, which uses a solid medium like gelatin instead 

 of a liquid medium like bouillon. In a liquid the bacteria are con- 

 stantly moving, and hence mixing ; but on a gelatin plate a bacterium 

 is fixed at a certain spot, and by division large colonies of its de- 

 scendants are developed. 



Gelatin Plates. (D) Run a thin layer of some melted gelatin 

 medium (gelatin melted in bouillon ; see directions in ' ' Teachers ' 

 Manual") into the bottom part of some Petri dishes, cover, and 

 sterilize for a half -hour. Repeat sterilization on second and third days. 

 Take care to keep the dishes level so that the melted gelatin does 

 not touch and glue the cover fast. After the last sterilizing, allow 

 the gelatin to cool so as to form a plate in the bottom of the Petri 

 dish. Wide-mouthed bottles, stoppered with cotton, can be used 

 in same way, simply adding enough gelatin to form a thin plate 

 on the side (Fig. 87). Small bottles and test-tubes may be used, but 

 they should be left inclined after the last sterilizing, so that the cooled 

 gelatin will be an inclined plate against a side of the tube or bottle. 



(D) Select some Petri dishes in which the gelatin plates have 

 remained sterile for several days. Remove the covers, and leave 

 the gelatin plate exposed to the air of the room for five, ten, or 

 twenty minutes. Then replace covers, and label. Look at the 

 dishes from day to day. If spots (white, yellow, or red) appear, 

 notice that they increase in size from day to day. Each spot is 



