ROUTINE BACTERIOLOGICAL EXAMINATION. 109 



colonies present consist of the same bacterium ? The 

 final settlement of this question depends on microscopic 

 examination, but it is seldom necessary to examine all the 

 colonies in this way ; for particular bacteria when growing 

 in mass in a colony frequently present characteristic 

 appearances, which may be recognised even by the naked 

 eye or at least by a \ inch or i inch objective. The 

 shape of the colony, its size, the appearance of the margin, 

 the graining of the substance, its colour, etc. are all to be 

 noted. One precaution is necessary, viz. it must be noted 

 whether the colony is on the surface of the medium or in 

 its substance, as the same bacterium may exhibit differences 

 in its colonies according to their position. The arrange- 

 ment of the bacteria in a surface colony may be still more 

 minutely studied by means of impression preparations. A 

 cover-glass is carefully cleaned and sterilised by passing 

 quickly several times through a Bunsen flame. It is then 

 placed on the surface of the medium and gently pressed 

 down on the colony. The edge is then raised by a sterile 

 needle, it is seized with forceps, dried high over the flame, 

 and treated as an ordinary cover- glass preparation. In 

 this way very characteristic appearances may sometimes be 

 noted and preserved, as in the case of the anthrax bacillus. 

 The colonies on a plate having been classified, a microscopic 

 examination of each group must be made by means of 

 cover-glass preparations, and tubes of gelatine and agar are 

 inoculated from each representative colony. Each of the 

 colonies used must be marked for future reference, preferably 

 by drawing a circle round it on the under surface of the 

 plate or capsule with one of Faber's pencils for marking on 

 glass, a number or letter being added for easy reference. 



The general lines along which observation is to be made 

 in the case of a particular bacterium may be indicated as 

 follows : 



i. Microscopic appearances. Note (i) the form, (2) 

 the size, (3) the appearance of the protoplasmic contents, 

 especially as regards uniformity or irregularity of staining. 

 Has it a capsule? (4) the method of grouping, (5) the 



