CLASSIFICATION 



77 



on the knowledge that Bacteria are the agents which cause inflam- 

 mation and blood poisoning in surgical operations. 



Pigment Bacteria. Many Bacteria develop characteristic pigments. 

 A very small number show traces of chlorophyll, but in most cases 

 the pigments are of very different kind, and probably not associated 

 with carbon assimilation unless possibly the purple-red pigment of 

 some of the Sulphur Bacteria. In other Bacteria the color is not 

 contained within the protoplast, but is an excretion which stains 

 more or less intensely the gelatinous matrix in which the colorless 

 cells are imbedded ; such, for instance, is the red pigment of Bacillus 

 prodigiosiis. 



Iron Bacteria. A small number of Bacteria, e.g. Cladothrix dicho- 

 toma, possess the power of depositing iron-oxide in the sheath in 

 which the cells are im- 

 bedded. It still seems 

 somewhat uncertain, how- 

 ever, how far the presence 

 of iron is an essential for 

 the growth of these Iron 

 Bacteria. 



Sulphur Bacteria. The 

 Sulphur Bacteria (Fig. 53) 

 comprise a considerable 



D 



number of forms which 

 are distinguished by their 

 ability to oxidize sulphu- 

 retted hydrogen, the sul- 

 phur being set free in 

 the form of conspicuous 

 granules of pure sulphur 

 within the protoplast. It 

 is supposed that these organisms obtain energy by the oxidation of 

 hydrosulphuric acid instead of by ordinary respiration, in which 

 respect they differ from all other known organisms. Many of the 

 Sulphur Bacteria possess a purplish pigment (Bacterio-purpurin), 

 which may possibly be related to chlorophyll in its properties, but 

 this is still by no means clear. 



FIG. 53. A, Chromatium Weissii, a purple Sul- 

 phur Bacterium, the round granules are sulphur. 



B, motile cell stained to show the flagellum. 



C, Thiocystis violacea, a purple Coccus-form. 



D, Beggiatoa alba. (All X about 1000.) 



Aerobic and Anaerobic Bacteria 



While many Bacteria, like other active organisms, require free 

 oxygen in order to live, there are very many of them which grow 

 normally only in the absence of free oxygen. These are the so-called 

 Anaerobic Bacteria, which include a large number of the organisms 

 causing decay in organic substances. 



