EXAMINATION BY CULTURAL METHODS 1 99 



is present, haemoglobin will dissolve out of the corpuscles and 

 colour the broth. 



If colonies appear on the agar tube, they are' to be carefully 

 examined with a lens, and their characters noticed. The 

 organisms which will be most likely to develop are strepto- 

 cocci, staphylococci, anthrax bacilli, pneumococci, typhoid 

 bacilli, the bacillus of plague, or the B. coll; the gonococcus 

 may also develop, for it will obtain the haemoglobin necessary 

 for its development from the blood itself. 



Streptococci form small white colonies which show no tend^ 

 ency to run together to form a film. The centre of each colony 

 is often more opaque than its periphery. 



Staphylococci form a more or less uniform film, the colonies 

 extending laterally and fusing together. The growth is 

 opaque, and is of a dead white, lemon, or orange colour, ac- 

 cording to the nature of the staphylococcus present (albus, 

 citreus, or aureus). 



Anthrax bacilli form small white colonies, having* the "bar- 

 rister's wig " appearance already described. 



The colonies of the pneumo coccus are small flat white 

 points, which do not tend to fuse together. They are difficult 

 to see when they are young, and, in case of doubt, the tube 

 should be returned to the incubator. 



The colonies of the typhoid bacilli and the B. coll are 

 whitish and opalescent. They usually have an angular or 

 polyg'onal appearance when small, and tend to run together 

 when older if they are thickly sjst. Their discrimination must 

 be left to an expert. 



The bacillus of plague forms white colonies which are 

 circular or have a crenated outline ; they tend to run together, 

 and form a uniform film over the surface of the medium. 



The gonococcus, if it develops, forms very minute trans- 

 parent colonies which have been compared to droplets of dew. 

 They do not become confluent. This organism will not grow 

 if transplanted on to the surface of ordinary media, unless a 

 film of blood be previously spread over it. 



After cultures have been obtained they are to be examined 

 microscopically by the method described on p. 22, and the 

 morphological appearances compared with those of the patho- 

 genic organisms which we have enumerated. It is especially 

 important to test whether the organism which has been 

 isolated stains by Gram's method or not. 



