LECTURE IX. 



IN the preceding lecture the existence of many kinds of bacteria 

 has been indicated, which differ from one another in external 

 form and ciliation. These constitute the so-called morphological 

 genera. Among those which resemble one another in form there 

 are often great differences in the manner of nutrition and res- 

 piration, so that these morphological units must be subdivided 

 into physiological species. It will readily be understood that it 

 is the differences in metabolism which constitute the important 

 distinctions in these forms so far as their relation to mankind is 

 concerned. In this connection may be mentioned the vast 

 number of pathogenic organisms whose adaptations to special 

 supplies of nutritive materials, peculiar waste-products and powers 

 of reproduction are of course far more important than their mor- 

 phological form. 



Then again these same characteristics lend importance to other 

 bacteria which are feared for their destructive action or valued 

 for their useful intervention. 



In the destruction of food-stuffs the saprophytes already men- 

 tioned have a large share. The many varieties of putrefactive 

 bacteria are concerned in the spoiling of meat, and meat preserving 

 processes are essentially those of sterilisation and the cutting off 

 of the access of bacteria. These same putrefactive bacteria, or 

 some of them, may destr'oy milk, but the characteristic souring of 

 milk is caused pre-eminently by a special bacterium which pro- 

 duces lactic acid (CH 3 CHOH . COOH) from lactose (milk sugar 

 C 12 H2 2 O n ). The acidity thus developed is unfavourable for the 

 growth of the putrefactive bacteria. Another familiar example 

 of one of these processes depending on the special metabolism of 

 a bacterium is the souring of beer and wine. In this a bacterium 

 participates which oxidises ethyl alcohol (C 2 H 5 OH) and forms 

 acetic acid (CH 3 COOH). 



But certainly one of the most important processes (if not the 

 most important) in which bacterial metabolism plays an essential 

 role is that of the circulation of nitrogen in organic nature. 



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