42 A MANUAL OF BACTERIOLOGY 



coloured pigments. They are termed " chromogenic 

 bacteria," examples of which are the Sarcina lutea and 

 Micrococcus cereus, var. flavus, which form citron-yellow 

 pigments ; the Bacillus prodigiosus and Spirillum rubrum, 

 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 liquefaciens. 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 to produce occasional 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 

 surrounding 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 



