138 GENERAL BACTERIOLOGICAL DIAGNOSIS 



the chief basis for research. In such a plate the first question 

 to be cleared up is : Do all the colonies present consist of the 

 same bacterium 1 The shape of the colony, its size, the appear- 

 ance of the margin, the graining of the substance, its colour, 

 etc., are all to be noted. One precaution is necessary, namely, 

 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 

 arrangement 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 may 

 be made by means of cover-glass preparations, and tubes of 

 gelatin 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 : 



1. Microscopic Appearances. For ordinary descriptive pur- 

 poses, young cultures, say of twenty-four hours' growth, on agar 

 should be used, though appearances in older cultures, such as 

 involution forms, etc., may also require attention. Note, (1) 

 the form ; (2) the size ; (3) the appearance of the protoplasmic 

 contents, especially as regards uniformity or irregularity of 

 staining ; (4) the method of grouping ; (5) the staining reactions. 

 Has it a capsule 1 Does the bacterium stain with simple watery 

 solutions'? Does it require the use of stains containing 

 mordants ? How does it behave towards Gram's method ? It 

 is important to investigate the first four points, both w r hen the 

 organism is in the fluids or tissues of the body and when growing 

 in artificial media, as slight variations occur. It must also be 

 borne in mind that slight variations are observed according to 

 the kind and consistence of the medium in which the organism 

 is growing. (6) Is it motile, and has it fiagella? If so, how 



