BACTERIA AS FERMENTS 91 



sulphide ( (NH 4 ) 2 S), the evolution of which produces the 

 characteristic odour of putrefaction. 



Besides holophytes and saprophytes there are included 

 among Bacteria many parasites, that is, species which feed 

 not on decomposing but on living organisms. Many of the 

 most deadly infectious diseases, such as tuberculosis, diph- 

 theria, and typhoid fever, are due to the presence in the 

 tissues or fluids of the body of particular species of microbes, 

 which feed upon the parts affected and give rise to the 

 morbid symptoms characteristic of the disease. 



Some Bacteria, like the majority of the organisms pre- 

 viously studied, require free oxygen for their existence, but 

 others, like Saccharomyces during active fermentation (see 

 p. 79), are quite independent of free oxygen and must there- 

 fore be able to take the oxygen without which their metabolic 

 processes could not go on, from some of the compounds 

 contained in the fluid in which they live. Bacteria are for 

 this reason divided into aerobic species which require free 

 oxygen, and anaerobic species which do not. 



As to temperature, common observation tells us that 

 Bacteria only flourish within certain limits. We know for 

 instance that organic substances can be preserved from 

 putrefaction by being kept either at the freezing-point, or at 

 or near the boiling-point. One important branch of modern 

 industry, the trade in frozen meat, depends upon the fact that 

 the putrefactive Bacteria, like other organisms, are rendered 

 inactive by freezing, and every housekeeper knows how easily 

 putrefaction can be staved off by roasting or boiling. Simi- 

 larly it is a matter of common observation that a moderately 

 high temperature is advantageous to these organisms, the 

 heat of summer or of the tropics being notoriously favourable 

 to putrefaction. In the case of Bacterium termo, it has been 



