VARIETIES OF BACTERIA. 37 



gen a biological significance that had hitherto been 

 denied it. 



The so-called thiogenic bacteria convert sulphuretted 

 hydrogen into higher sulphur compounds. 



The progress of many forms of fermentation and 

 decomposition is accompanied by more or less elevation 

 of temperature. This is seen in the curing of manure, 

 tobacco, malt, etc. 



As stated, bacteria that produce disease are known 

 as pathogenic. They induce disease by their poisonous 

 action upon the tissues in which they are located. The 

 materials of which certain species of bacteria are con- 

 structed, and the products of growth of certain others, 

 are of the greatest importance in their relation to animal 

 and human pathology. Particular species, while not elim- 

 inating soluble poisons as a product of metabolism, are 

 nevertheless themselves built up of poisonous proteids, or 

 of proteids with which toxic materials are so intimately 

 associated that they can only be isolated by the most re- 

 fined and elaborate chemical manipulations. Others pro- 

 duce in the course of their growth soluble poisons that can 

 readily be separated by very simple methods from the bac- 

 teria that produced them. The proteid matters making 

 up the bodies of many species of bacteria, even those not 

 conspicuously pathogenic, have been shown by Buchner 

 to .induce disease when isolated and injected into the 

 tissues of animals ; in some cases causing only rise of 

 body-temperature, in others acute inflammatory proc- 

 esses with pus-formation. To such proteids Buchner 

 has given the name bacterial proteins. 



The poisonous soluble products of bacterial growth 

 are severally known as toxins, toxalbumins, and pto- 

 viains : toxins being, in general, uncrystallizable poisons 



