32 EXPERIMENTAL DAIRY BACTERIOLOGY 



tubes, cooling to 42-45 C., and marking them 1, 2, and 3. 

 With the sterile needle transfer a small amount of the growth 

 to be tested to the first tube of melted medium, replace the 

 plug, sterilize the needle, and mix the material by rotating the 

 tube between the hands. From this tube transfer three loop- 

 fuls to the second tube, mix, and again carry three -loopfuls to 

 the third tube. Pour each tube into a sterile Petri dish, spread, 

 and allow the medium to solidify. Incubate. On one of the 

 three dishes will usually be found such a number of colonies 

 that they will be able to attain the maximum size and devel- 

 opment. When preparing plates the probable number of liv- 

 ing organisms present must be considered and the amount of 

 original material taken, and the number of loopfuls trans- 

 ferred from one tube to another should be taken accord- 

 ingly. Three plates should always be made. An effort to 

 economize in material by making one or two plates is very 

 apt to end in failure. Sometimes one tube of medium may 

 be saved by using a water blank for the first transfer, as this 

 culture is generally so abundantly seeded that it is worthless 

 for study. 



Study of plate cultures. A macroscopical and microscop- 

 ical examination of the colonies appearing on the plates should 

 be made, as the general appearance is more or less constant 

 and characteristic for each kind of bacteria. The microscop- 

 ical examination should be made by inverting the Petri dish 

 on the stage of the microscope and studying the colonies with 

 a low power (16 mm. objective). The following points should 

 be noted in the examination : surface colonies, i.e. colonies 

 f which are wholly or partially on. the surface of the medium : 

 form of colony ; size of colony ; surface elevation ; topography 

 of surface ; microscopic internal structure of colony ; micro- 

 scopic structure of edge of colony ; color determined both by 



