SOURCES OF ENERGY IN ANAEROBES 533 



and also more economically with regard to the amount of material con- 

 sumed ] . 



As in the case of aerobes it is the specific character of an anaerobic 

 organism which determines whether a particular substance can be assimi- 

 lated, and to what uses it may be put, although at the same time the 

 products formed may vary to a certain extent according to the external 

 conditions, the stage of development, &c. It is, however, not surprising that 

 a facultative anaerobe may assimilate a variety of substances during aerobic 

 existence, but only a few when living as an anaerobe : thus yeasts are 

 able to ferment and live anaerobically upon particular forms of sugar, 

 whereas in the presence of oxygen they can grow upon a variety of 

 substances without exhibiting any fermentative activity 2 . Similarly many 

 bacteria require particular forms of sugar when living anaerobically, while 

 other anaerobic bacteria may be nourished with various substances : thus 

 certain butyric bacteria grow upon invert-sugar, mannite, or glycerine, 

 others upon lactic acid (Sect. 103), while for a few bacteria tartaric 3 , malic, 

 or citric acid may serve as the sole organic food. Peptone, either alone or 

 in combination with other substances, may suffice for many anaerobes, and 

 especially those which do not induce fermentation 4 . It is possible that 

 many organisms may be discovered which can exist both in the presence 

 and absence of oxygen without exhibiting any fermentative activity, as is 

 indeed the case when Bacillus prodigiosus is grown in the absence of sugar 5 . 



The products of anaerobic metabolism are many and various : thus 

 ethyl-alcohol is the chief product of the fermentation of sugar by beer-yeast, 

 whereas butyl-alcohol, lactic acid, or butyric acid are produced by certain 

 bacteria. When successfully nourished by a variety of different substances 

 the fermentative products remain in general the same. All substances 

 which are continually produced and excreted are to be regarded as direct 

 or indirect products of metabolic activity, and the whole host of fermentative 

 products comes under this category. The anaerobic fermentative products 

 include alcohols (ethyl-, butyl-, amyl-alcohol), acids (butyric, lactic, propionic, 

 oxalic, citric, valerianic, formic), while from proteid decomposition result 

 ammonia, fatty amido-acids, leucin, tyrosin, skatol and similar aromatic 



1 According to Rabinowitsch (Zeitschr. f. Hygiene, 1895, Bd. xx, p. 159), thermophile bacteria 

 grow better at low temperatures in the absence of oxygen. 



2 Cf. Laurent, Ann. d. 1'Inst. Pasteur, 1889, T. in, p. 114; Beyerinck, Centralbl. f. Bact, 

 1892, Bd. xi, p. 70; Bokorny, Prliiger's Archiv f. Physiol., 1897, Bd. LXVI, p. 114. 



3 Pasteur, Etude s. 1. bierc, 1876, p. 279. 



4 Liborius, I.e., p. 168 ; Beyerinck, Bot. Zeitung, 1891, p. 475. Cf. also Sect. 66, and Fliigge, 

 1. c., p. 219. The experiments have for the most part been made with ferment-organisms. 



5 Liborius, I.e., p. 168; Liideritz, Zeitschr. f. Hygiene, 1889, Bd. v, p. 154; Smith. Centralbl. 

 f. Bact., 1893, Bd. xiv, p. 865 ; 1895, Bd. xvm, p. i. Smith's supposition that only sugar can be 

 fermented is incorrect. 



