30 BACTERIOLOGICAL AND ENZYME CHEMISTRY 



apparatus. After twenty-four hours the Petri dish should be 

 examined, and signs of the development of colonies will then 

 be probably apparent, though it is generally necessary to 

 allow at least two days to elapse before making the sub- 

 culture. Specific subcultures are best made when the 

 number of colonies on the plate does not exceed 100; it is 

 generally, therefore, best to make two or three plates by 

 transferring a loop full of the inoculated and melted gelatine 

 from the first culture tube to a second and s imil arly to a 

 third, plates being poured in each case. Well-defijied colonies 

 having been obtained on the plate culture, separate colonies 

 can be removed by means of the platinum hook and transferred 

 to a tube of gelatine, there to develop. 



For the proper carrying out of these operations, manipula- 

 tive practice is necessary, in order to avoid accidental infection 

 by extraneous organisms from the air, etc., and also to acquire 

 rapidity and dexterity of handling. It is wise to consider 

 always that everything not actually sterilised is liable to be a 

 source of infection ; thus a platinimi wire after being laid down 

 on the bench must be re-sterUised, and cotton-wool plugs 

 re-singed after being held between the fingers. Such manipula- 

 tive details soon become a matter of habit. 



Examination of Bacteria under the Microscope.— As 



already mentioned, a high power is necessary for a satisfactory 

 examination of bacteria. Under a ^3 inch oil-immersion lens 

 it is possible to observe them either in the living condition, 

 in a drop culture or as a stained preparation. To examine 

 them in drop culture a small portion of growth either from a 

 plate or tube culture is removed by means of the platinum 

 loop, and quickly mixed with a drop of water on the under 

 side of an ordinary microscopic cover glass, which is then 

 placed on a specially made slide with a depression ground into 

 it (Fig. 5 d). On placing the cover slide with the drop on 

 the under side over the depression, the bacteria can be observed. 



