64 BACTERIOLOGICAL CHEMISTRY 



organisms contain 0-5 to 2-0 per cent, of phosphatides, 

 whilst the tubercle iDacillus contains about 6-5 per cent. 

 Yeasts also contain a high proportion of phosphatides. 



Pigments. — Many species of bacteria, yeasts, and fungi 

 are pigmented ; the fungi nearly all contain pigment in 

 some form, but the majority of bacteria, on the other 

 hand, show only minimal pigmentation. Beijerinck 

 has classified the pigment producing micro-organisms 

 according to the site of the occurrence of the pigment : 

 Chromophoric organisms contain the pigment in the 

 protoplasm, as in the case of bacterio-purpurin, the 

 pigment of certain sulphur bacteria. Parachromophoric 

 organisms carry the pigment in some other part of 

 the cell than the protoplasm, usually in the cell wall 

 or in the capsule. As examples may be quoted some 

 blue bacteria, the moulds and species of Torula (coloured 

 yeasts). Chromoparous organisms excrete the pigment as 

 such into the medium, as does Serratia marcescens 

 {B. prodigiosus) ; or a colourless leuco -compound may 

 be excreted and this may become oxidised in the medium 

 to the coloured compound, which is the case with the 

 green pigment of Pseudomonas aeruginosa {B. pyocyaneus). 

 The mould HelmintJiosporiu7n gramineum produces a 

 pigment, helminthosporin, which sheathes the mycelium 

 with crystals. 



In comparatively few cases has the constitution of 

 these pigments been worked out, but in general they 

 are either carotenoid pigments or melanins. 



The pigments will be considered further in Chapter 

 XXI. 



Coloured colonies of certain bacteria are produced for 

 diagnostic purposes by the use of special media. This 

 may be illustrated by the production of the pink colonies 

 of lactose fermenting organisms on MacConkey's medium, 

 and by the black colonies of the diphtheria bacillus on 

 McLeod's tellurite medium, in which cases a product of 

 the metabolism of the organism reacts with a substance 



