CHEOMOGENIC BACTERIA 135 



1. The Purple Bacteria. Under this name is comprised a group of 

 organisms differing very much from one another in shape, but all 

 agreeing in possessing a pigment of a purple colour, usually situated 

 just within the cell-wall. This pigment is diffused in the protoplasm 

 and not collected into definite bodies as in the majority of plants 

 showing pigments of this nature. This colouring matter is called 

 Baderio pvrpurin. The mode of life of these organisms is quite unique 

 in the bacterial world, for they seeJe the light, and do not avoid it, as do 

 the rest of the bacteria. The reason for this will be seen from the 

 following experiment : 



A drop of water containing these purple organisms is placed on a 

 slide, and a very small spectrum projected thereon. It is found that 

 the bacteria collect at certain points, viz. in the ultra red, to a smaller 

 extent in the orange, and to a still smaller extent in the green parts of 

 the spectrum. Now the spectrum of bacterio-purpurin shows absorp- 

 tion bands just at these three places, so that these bacteria, like green 

 plants, obtain energy by absorption of light, and it is only certain 

 parts of it, viz. the ultra-red, orange, and green components that are 

 absorbed. The bacteria are thus able to make use of the energy of 

 the sun's rays to build up food material. We shall return to these 

 interesting organisms when dealing with the sulphur-bacteria to which 

 they belong. 



Some chromophorous green bacteria have been described, but it is 

 not certain that these are not green algae which have been described as 

 bacteria. 



2. Bacteria producing Red Colouring Matter. The best and the longest 

 known of these is Bacillus prodigiosus, which in young cultures is 

 parachromophorous, whilst in older cultures it is chromoparous. When, 

 in olden times, red spots were seen on articles of food, it is small 

 wonder, considering the ignorance of those times, that the power of 

 witchcraft was invoked to account for their presence. This bacillus 

 grows on a variety of food stuffs. The colouring matter is at first 

 diffusely distributed throughout the cells, but later is thrust out and 

 collects outside the cell-membranes. This production of colour is 

 dependent on nutritive conditions, for no colour is developed if the 

 organism be cultivated on sterilised potatoes at a temperature of 

 38°-39°C. 



Other well known "red" microbes are, Bacillus erythrosporus. Bacillus 

 Kieliensis, Bacillus lactis erythrogenes, Bacillus corallinus, and Sarcina 

 rosea. When normal milk becomes red on standing, the cause of 

 the malady is usually a development of either Bac. prodigiosus or 



