i2 4 GENERAL BACTERIOLOGICAL DIAGNOSIS. 



plate the first question to be cleared up is : Do all the 

 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 colonies of the same bacterium may 

 exhibit differences 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 most accurate method, however, of examining the 

 arrangement of individual bacteria in a colony is by the 

 prolonged examination of hanging -drop cultures. For 

 applying such a method to an organism growing at 37 C. 

 a special incubator surrounding the microscope stage has 

 been devised. The colonies on a plate having been 

 classified, a microscopic examination of each group may 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. 



