SECTION IV. 



CHROMOGENIC, PHOTOGENIC, AND THERMOGENIC 

 BACTERIA. 



CHAPTER XII. 



CHROMOPAROUS BACTERIA, PRODUCING RED AND 

 YELLOW COLOURING MATTERS. 



86. Coloured and Colouring Bacteria. 



IN classifying the chromogenic (colour-producing) bacteria, the 

 situation, as well as the nature, of the colour has an importance 

 that cannot be disregarded. An examination for this first-named 

 characteristic in individual species quickly leads to the differentia- 

 tion of the chromogenic bacteria into coloured bacteria on the one 

 hand and colouring bacteria on the other ; the cells in the latter 

 being themselves quite colourless, but excreting a coloured trans- 

 formation product: these species have been designated chromo- 

 parous by Beyerinck. 



In the coloured bacteria, on the contrary, the colouring matter 

 remains within the cells. This group may be divided into two 

 sub-groups, the one comprising those coloured bacteria the colour- 

 ing matter of which performs an important physiological function, 

 as in the case of the purple bacteria treated of in the following 

 chapter ; such bacteria are termed chromophorous. On the other 

 hand, the second sub-group includes those coloured bacteria in 

 which the colouring matter has no such function, and must be 

 regarded as a purely passive metabolic product, which is, neverthe- 

 less, not excreted (as in the chromoparous species), but remains 

 within the cell without manifesting any apparent activity. These 

 bacteria are termed parachromophorous. 



The chemical properties of the bacterial colouring matters and 

 their importance for distinguishing one species from another were 

 discussed by PAUL SCHNEIDER (I.) after exhaustive experiments 

 with thirty different species. His results in this connection may 

 be thus summarised : (i.) The bacterial colouring matters can to 

 some extent be differentiated by their behaviour towards solvents. 

 (2.) A given species, grown under identical conditions, always pro- 

 duces the same colouring matter. (3.) Two species, differing as 



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