CHAPTER II 



MORPHOLOGY continued 



Pigments, Intracellular Products ; Movement and Organs of Locomo- 

 tion; Cell-division; Spore-formation and Germination. 



MOST bacteria are individually colourless, and appear, even when 

 massed together in a pure culture, either white or of a yellowish tinge. 

 There are, however, a considerable number of species, the chromogenic or 

 pigment-bacteria, remarkable for the brilliant colouration of their cultures. 

 Some of the Sarcinae, for instance, have a bright yellow tint, Staphylococcus 

 pyogemis aureus is golden yellow or orange, B. brunneus yellowish brown, 

 Micrococcus agilis, Bacillus prodigiosus, and Spirillum rubrum each a different 

 shade of red, B. cyanogenus (the bacillus of blue milk) blue, B. violaceus 

 a deep violet ; and many kinds of water-bacteria, as well as the bacillus of 

 blue pus, give origin to brightly fluorescent pigments. The production of 

 all these colouring matters is very variable and largely dependent upon the 

 conditions of growth, upon the composition and reaction of the culture 

 media, and upon the influence of oxygen, light, and heat. 



Most pigment-bacteria appear under the microscope to be colourless, 

 and the question at once suggests itself whether the colouring matter 

 is really contained within the cell. In B. prodigiostis^ the cause of the 

 ' bleeding host,' the pigment can be seen in the form of granules scattered 

 about between the colourless bacteria, so that in this case there can be no 

 doubt as to the seat of the colour of the cultures. The various fluorescent 

 substances also are secretions of the bacteria with which they are associated, 

 and they diffuse into the culture medium, which often fluoresces brightly 

 throughout its whole thickness. The pigment of B. cyanogenus also is 

 dissolved in the culture medium. In most pigment-bacteria similar con- 

 ditions prevail, that is to say, they are ' chromoparous ' (6). Some species, 

 on the other hand, are actually ' chromophorous', i. e. the protoplasm itself 

 is coloured. This is the case with the sulphur bacteria Chromatium and 

 Thiocystis, and with some sap-green species (B. virens). As regards these 

 last, however, it is undecided whether they are bacteria or minute algae (7). 



