PIGMENT FORMATION 37 



red pigments ; the Bacillus violaceus forms a rich violet 

 one ; and the Bacillus pyocyaneus, a blue. A large number 

 of chromogenic organisms require oxygen for the production 

 of the pigment, and potato is often the most favourable 

 culture medium. In some cases the medium may become 

 coloured, and the property of fluorescence be conferred 

 upon it, as is the case with the Bacillus fluorescens lique- 

 faciens. Usually the pigment is extra-cellular, occasionally, 

 as in B. violaceus, it is intra-cellular. 



A group of organisms producing purplish pigments has 

 been described under the name of " purple bacteria." 

 It is doubtful if these organisms are true bacteria, and the 

 pigment may exercise a respiratory function analogous 

 to chlorophyll. 



Phosphorescence, or light-production, is developed by 

 some bacteria, notably by many marine forms, and is 

 well seen in decomposing fish. Some spirilla are also 

 known occasionally to produce phosphorescence. 



A necrotic action on the tissues is produced by many 

 pathogenic organisms. For example, the tubercle and 

 glanders bacilli cause necrosis and caseation of the sur- 

 rounding tissues. 



Gas production. This is common to many organisms. 

 The gas may consist of carbonic acid, hydrogen, or marsh 

 gas, and in some cases of foul-smelling sulphur compounds, 

 sulphuretted hydrogen, mercaptans, etc. 



Sulphuretted hydrogen may be detected by the blackening of 

 lead acetate paper. Methyl mercaptan may be detected by 

 aspirating a current of air through the culture, through a calcium 

 chloride drying-tube, and then through a test-tube or small flask 

 containing isatin dissolved in concentrated sulphuric acid. The 

 red colour of the isatin solution is changed to olive- or grass-green 

 by the mercaptan. 



Toxic bacterial products. Almost without exception the 

 pathogenic action of bacteria is brought about by means 



