534' RESPIRATION AND FERMENTATION 



derivatives, as well as sulphuretted hydrogen, methylmercaptan, and other 

 malodorous gases. Carbon dioxide is usually produced, also hydrogen 

 commonly, and more rarely methane, carbon monoxide, and nitrogen 1 . 



Several of these products may be simultaneously produced by the same 

 organism, but in the fermentation of sugar by certain bacteria alcohol and 

 butyric acid are the two chief products, while about 5 to 6 per cent, of 

 the sugar is employed in the construction of various other substances. 

 A portion of the excrete products may arise during the preparatory 

 modifications which the food-material may undergo, or secondary reactions 

 may take place between them. Certain experimental results point indeed 

 in this direction, but the actual course of metabolism is even more difficult 

 to determine in the case of anaerobes than it is in that of aerobes (cf. 

 Sect. 77). Probably no one product of anaerobic respiration is produced 

 by a single organism only ; ethyl-alcohol is formed not only by yeasts but 

 also by different bacteria, and by the higher plants when insufficiently 

 supplied with oxygen. On the other hand not one of the products 

 is of universal occurrence, and even carbon dioxide appears to be absent 

 in certain cases, as in the lactic fermentation and in the denitrifying 

 fermentation of saltpetre 2 . Pammel describes certain fermentations in 

 which no gaseous products are evolved 3 , but in all cases continued activity 

 is possible only when the waste-products are excreted or removed. It is 

 possible on the other hand to conceive of anaerobic decompositions which 

 may yield a supply of energy and result in the formation of gaseous products 

 only, as is the case when formic acid decomposes into carbon dioxide and 

 hydrogen, or when nitroglycerine undergoes complete combustion. 



Historical. Pasteur 4 was the first to show that yeast and certain bacteria can 

 grow in the absence of oxygen. The objections raised against Pasteur's experi- 

 ments were not justified by the facts, for any traces of oxygen originally present 

 are rapidly consumed by the micro-organisms, which continue to increase for weeks 

 in a nutritive fluid containing no free oxygen. Moreover since the original germs 

 contained the merest trace of oxygen it is obvious that anaerobic existence is not 

 rendered possible by means of oxygen stored in these germs. Pasteur indeed 

 showed that in the absence of oxygen no growth is possible upon food-materials 

 which only permit aerobic existence. Growth continues only so long as a supply 

 of energy is assured, and Pasteur recognized that in the case of yeast the anaerobism 

 is strictly limited. It is still uncertain whether certain organisms are permanent or 

 temporary anaerobes, but that the former do actually exist has been shown by the 



1 Cf. Fliigge, 1. c., Bd. I, pp. 168, 219, where the chief literature is given. Cf. also Sect. 103. 



a Burri u. Stutzer, Centralbl. f. Bact., 1895, I, Bd. i, p. 427; Ad. Mayer, ibid., 1892, Bd. xn, 

 p. 99. Kayser (Ann. d. 1'Inst. Pasteur, 1894, T. vnr, p. 743) found that another lactic bacterium 

 evolves carbon dioxide. 



8 Pammel, Centralbl. f. Bact., 1896, II, Bd. II, p. 633. 



* Pasteur, Jahresb. d. Chem., 1861, p. 724; Etudes. 1. biere, 1876, p. 257. 



