CHAPTER IV. 



STUDY AND IDENTIFICATION OF BACTERIA GENERAL 

 CONSIDERATIONS. 



IN order to study bacteria it is necessary to isolate them in pure 

 culture. This may be accomplished by taking one or more loopfuls of 

 the material and mixing it in a tube of melted agar or gelatin. From 

 this first tube one or more loopfuls are transferred to a second tube of 

 melted agar or gelatin, and from this a third transfer is made, thereby 

 giving us tubes in which the distribution of the bacteria is one or more 

 hundred times less in the second than in the first tube, and equally 

 more dilute in the third than in the second. When we pour the con- 

 tents of the tubes into Petri dishes we would have the bacterial colonies 

 on the first plate so thick that it would be impossible to pick up a single 

 colony with a platinum needle without touching an adjacent one. On 

 the second plate the distribution might be such that we should have 

 discrete, well separated colonies, material from which could be taken up 

 on the point of the needle or loop without touching any other colony. 

 If the second plate did not meet these requirements, the third would. 



In clinical bacteriology we work almost entirely with organisms 

 preferring blood-heat temperature, hence it is necessary to use agar 

 or blood-serum as standard media for the obtaining of isolated colonies. 

 Gelatin is of little value for this purpose in medical work. In using 

 agar it will be remembered that it solidifies at a temperature slightly 

 below 40 C. and does not melt again until it is subjected to a tempera- 

 ture practically that of boiling. Again, if the temperature of the media 

 exceeds 44 C. it may affect injuriously the organisms we wish to study. 

 Consequently it requires careful attention and quick work to inoculate 

 the tubes, mix, transfer and pour into plates within the limits of a tem- 

 perature which injures the organisms, and one which brings about the 

 solidification of the agar. 



Again, we not only have colonies developing from organisms which 

 have been fixed at the surface as the agar solidified in the plate, but 

 more numerous ones developing from bacteria caught in the depths 



