MULTIPLICATION OF BACTERIA 775 



vitality. At higher temperatures, for example between 42°-5o° C, 

 growth ceases, and when kept at 55° or 60° C. for ten or 

 twenty minutes, the vegetative cells of most common species 

 of bacteria are killed. The spores, however, in some cases 

 resist the action of boiling water for three hours or more, and 

 have been found capable of development after exposure for an 

 hour to a dry heat of 130° C. 



(d) Oxygen. — A large number of bacteria are dependent upon 

 the presence of free oxygen in the atmosphere, and such species 

 are spoken of as aerobic. There are, however, many, such as 

 the butyric acid bacteria and the tetanus bacillus, whose vital 

 activity is checked altogether when they are exposed to this 

 gas in a free state; these are said to be anaerobic. Between 

 the obligate aerobic species which die unless well supplied 

 with air, and the obligate anaerobic types whose development 

 is arrested by traces of oxygen, a number of species exist which 

 are more or less indifferent^ to the presence or absence of 

 oxygen : those, such as the lactic-acid bacteria, the species 

 causing putrefaction, and the cholera bacillus, which are usually 

 aerobic but which can still maintain a certain degree of activity 

 when the amount of free oxygen is much reduced, are described 

 as facultative anaerobic bacteria, while those generally anaerobic 

 but capable of growth when surrounded by free oxygen are 

 spoken of as facultative aerobic species. 



(e) Food. — Food is necessary for all living organisms, and the 

 elements composing it are practically the same for bacteria as 

 for green plants (see p. 168). The phosphorus, sulphur, and 

 various essential metallic elements are readily obtained from 

 phosphates and sulphates of potassium, calcium, magnesium and 

 iron. 



A few species of bacteria are able to obtain the carbon they 

 need from carbon dioxide, but most of them require organic 

 compounds for the supply of this element. As sources for their 

 necessary nitrogen certain bacteria are able to utilise the free 



