H2 BACTERIA AND THE CARBON DIOXIDE CYCLE 



similar zymogenic powers. There are a dozen different species of bacteria 

 that produce butyric acid and lactic acid. And, inasmuch as the descrip- 

 tions given are often far from exact, the student of these species finds himself 

 lost in a pathless forest of forms. For this reason it will be impossible here 

 to describe more than a few species. 



Some of the fermentation bacteria can acquire pathogenic properties. 

 Two of the butyric ferments found in soil are known to be the cause of 

 disease, Bacillus Chauvoei giving rise to ' quarter-evil ' in cattle, and another 

 form originating malignant oedema. The notorious B. coli communis is 

 a ferment organism in so far that it splits up grape sugar into lactic acid, 

 succinic acid, and ethyl and propyl alcohol, carbonic dioxide arising in the 

 process. By far the greater number of ferment organisms are, however, 

 harmless, and, considering the millions we take inside us every day in milk, 

 cheese, and other food, it is satisfactory to know this. 



The chemistry of fermentation is clearly understood only in those cases 

 where the process is oxidatory. The acetic fermentation belongs to this 

 category, the alcohol being combined with the oxygen of the air to form 

 aldehyde and water, the aldehyde being further oxidized to acetic acid, and 

 this again, if the process be allowed to go on, burnt up to carbonic acid and 

 water. The changes may be represented by equations thus : 



CH 3 CH 2 HO + O = CH 3 COH + H 2 

 CH 3 COH +0 =CH 3 COOH 

 CH 3 COOH +40= 2C0 2 +2H 2 



This fermentation is evidently allied to the respiratory process, and 

 the similar oxidatory processes of the nitrifying and sulphur bacteria. It 

 differs from most fermentative changes in the small number of resultant 

 substances, but is, of course, just as much a phase in the circulation of 

 carbon as any of the more complex processes, where large numbers of by- 

 products arise and obscure our view (see Chap. XIV). 



Of the fermentations of monacid alcohols only the acetic fermentation 

 of ethyl alcohol is of industrial importance (82). Alcoholic liquids, such as 

 beer or wine, if left exposed to the air in a warm place acquire a strongly acid 

 reaction, and become covered with a white skin or pellicle. This consists 

 of the ' vinegar bacteria,' which slowly oxidize the alcohol to acetic acid, 

 and finally to C0 2 and water. Sometimes the pellicle consists not of 

 bacteria but of a yeast {Mycoderma aceti), which transforms the alcohol at 

 once into C0 2 and water without the intermediate stage of acetic acid. 



The old form, Bacillus aceti, has been divided by Hansen into three 

 different species, B. aceti, B. Pasteurianus, and B. Kiitzingianus (Fig. 24). 

 All three are morphologically very similar, non-motile medium-sized rods 

 often growing out into chains, of which the pellicle mainly consists. They 

 are unable to grow in fluids containing more than T4 per cent, of alcohol. 



