82 MICRO-ORGANISMS AND FERMENTATION. 



life. Nevertheless, it is possible to cultivate them e.g., in 

 blood serum at the temperature of the body. 



The metatrophic bacteria, which form the vast majority, 

 and are of special interest to the fermentation physiologist, 

 are not equally responsive to the different carbonaceous and 

 nitrogenous food-stuffs. Peptone and amides are good sources 

 of nitrogen. Many bacteria can also utilise ammonium salts 

 and nitrates under given conditions, thus Bact. aceti can 

 assimilate ammonia in presence of acetic acid. Similarly, 

 according to Henneberg, certain species of acetic acid bacteria 

 can utilise potassium nitrate and ammonium tartrate as sources 

 of nitrogen, if the culture material contains sufficient dextrose. 



The carbohydrates constitute the most important source 

 of carbon. Of the different varieties of sugar, grape sugar 

 forms an excellent food for bacteria. 



According to the conditions of nourishment, bacteria may 

 bring about varying decompositions of the substrata. 



The different nutritive fluids and gelatines that are used in 

 the culture of bacteria are described in Chap, i., sec. 6. 



Pasteur made the important discovery that there are 

 certain bacteria and other micro-organisms which do not 

 require free oxygen, but are capable of effecting active decom- 

 position of the fermenting material, even when oxygen is 

 excluded. He, therefore, distinguished two classes of micro- 

 organisms, aerobic and anaerobic. Whilst the aerobic bacteria 

 breathe in a similar manner to all other organisms, and thereby 

 convert organic substances (non-nitrogenous) into carbon 

 dioxide and water, and bring about similar decompositions 

 with the nitrogenous compounds, the anaerobic bacteria com- 

 prise on the contrary those whose life activity is sustained 

 without free oxygen. To this class belong some of the butyric 

 bacteria, as well as the bacteria that ferment ceUulose. 



Since Pasteur's discovery (1861) numerous bacteria have 

 been investigated in this respect, and it has been proved that 

 there is every possible transitional stage between the obligatory 

 aerobes (amongst which the hay bacillus, Bac. subtilis, must 

 be classified) and the obligatory anaerobes. A number of 

 facultative anaerobic species are now known which grow well 

 with access of air, but also develop, to a degree varying with 



