138 BACTERIOLOGICAL DIAGNOSIS. 



those which are described in the section on the blood. 

 There will be slight differences, but not enough to lead 

 to error if the examination of the colonies is supple- 

 mented by an inspection of stained films. 



The pneumococcus, gonococcus, the fungus of actino- 

 myces, and the tubercle bacillus will not grow on these 

 plates ; and the bacillus of glanders will grow feebly, if 

 at all. 



In a day or two longer the plates will, in some cases, 

 be found to have undergone a decided change. If 

 liquefying organisms are present the colonies will soon 

 become depressed below the general surface of the 

 medium and will be surrounded by haloes which 

 consist of liquefied gelatin. This will happen with the 

 staphylococci and the bacillus pyocyaneus ; not with 

 the streptococci, the typhoid bacillus, nor with the 

 bacillus coli. 



The bacillus of blue pus can readily be distinguished 

 from the staphylococci by its morphological appearance 

 (it is a slender rod) and by the fact that the gelatin 

 round the colony is coloured blue or bluish-green, the 

 growth itself being nearly white. 



INTERPRETATION OF RESULTS. 



The information which is obtained by a study of the 

 bacteria in pus is of more scientific interest than of 

 practical importance. It is the situation of the collec- 

 tion of pus rather than the bacteria causing it which 

 influences treatment and prognosis. A list of the more 

 important results which are produced by the chief pyo- 

 genic bacteria may be of interest. 



Staphylococci are the chief producers of localised 

 suppuration in the skin such, for instance, as that 



