118 GENERAL BACTERIOLOGICAL DIAGNOSIS 



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 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 examina- 

 tion 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 24 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? Does the bacterium stain with simple watery 

 solutions'? Does it require the use of stains containing 

 mordants 1 How does it behave towards Gram's method ? It is 

 important to investigate the first four points both when 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 flagella ? If so, how are 

 they arranged 1 (7) Does it form spores, and if so, under what 

 conditions as to temperature, etc. 1 



2. Growth Characteristics. Here the most important points 

 on which information is to be asked are, What are the 

 characters of growth and what are the relations of growth (1) to 



