134 OUTLINES OF BACTERIOLOGY 



medium in which they are growing is raised to a comparatively 

 considerable degree. Bacillus lupuliperda is an example. We know 

 very little of these organisms, though they are present in large 

 numbers in the soil, and it is certain that they have a very wide 

 distribution. It is only when humid organic matter is massed together 

 that they obtain the conditions necessary for active growth and multi- 

 plication. It is not improbable that many of the well known soil 

 bacteria must be reckoned among the thermogenic organisms. On 

 the other hand there are bacteria which whilst not developing much 

 heat themselves can not only tolerate, but also actively multiply, at 

 temperatures which stop the growth of other organisms altogether. 

 To these the term " thermophilic " is applied. When the temperature 

 of moist organic matter approaches 70 C. all the organisms in the 

 decomposing mass, with the exception of the thermophilic kinds, 

 have been killed by the heat. Some organisms therefore, for example 

 some of those that are present in such matter when the temperature 

 approaches 70 C., are probably both thermophilic and thermogenic. 

 The best known thermophilic microbe is Bacillus thermophilus. 

 This organism thrives and reproduces with great activity at 70 C., 

 a temperature which produces painful burns on the skin, coagulates 

 egg-albumin and kills animal cells. Bacteria have been found in an 

 active condition in boiling springs, for example in one in Mexico 

 the temperature of which is 64 C. We may regard this power of 

 thriving at such high temperatures as having been induced by a 

 long process of acclimatisation, aided by natural selection. 



2. CHROMOGENIC BACTERIA. 



We may distinguish three kinds of chromogenic or colour-producing 

 bacteria. 



1. Those in which the colouring matter is connected in an important 

 way with the nutrition of the cell, such as the purple sulphur-bacteria, 

 and the bacteria which have a green colouring matter. These are 

 called Chromophorous bacteria. 



2. Those in which the colouring matter is a secretion of the proto- 

 plasm which is not extruded from the cell. These are called Para- 

 chromophorous bacteria. 



3. Finally, those in which the colouring matter is an excretion which 

 is thrust out of the cell, the bacteria themselves remaining colourless. 

 These are called Chromoparous bacteria. 



