210 



BACTERTOLOGICAL CHEMISTRY 



addition oi" SLdpliuric acid to such a culture in peptone 

 water a red colour develops (the so-called " cholera red 

 reaction "), due to the nitroso -indole reaction between 

 nitrite and indole. 



In intra-molecular fermentations of Type II (see 

 p. 197) complete oxidation does not occur as a rule. In 

 these cases the energy is supplied by a shift of the oxygen 

 in the molecule, usually towards the ends of the chain. 

 This may be illustrated by a comparison of glucose and 

 its fermentation products, alcohol and carbon dioxide, 

 as the result of yeast fermentation, or laotic acid following 

 bacterial fermentation : — 



CO, 



CH. 



C'H, 



CHOH 

 CHOH 



O CHOH 



Yeast 



I 

 CH2OH 



+ 

 CO2 



(alcohol + carbon dioxide) 



Bacteria 



COOH 

 CHOH 



CH3 



+ 

 CH, 



CHOH 



CH CHOH 



CH2OH COOH 



(glucose) (lactic acid) 



Compounds in which this accumulation of oxygen at the 

 ends of the carbon chain has already occurred are not 

 caj)able of serving as energy sources by intra-molecular 

 fermentation. If they are fermented at all it is by the 

 mechanism of Type III, in which some outside hydrogen 

 acceptor is necessary. Thus the simple alcohols, fatty 

 acids and dibasic acids, like oxalic, malonic or succinic 

 acids, require either oxygen or some hydrogen acceptor 

 like nitrate in order that they may be utilised by the 

 organism. It is the uniform distribution of oxygen along 

 the chain of carbohydrate molecules which renders them 

 so valuable as nutrient materials. Hydroxy-acids, like 

 tartaric or lactic acids and amino -acids, which yield 

 liydroxy-acids on hydrolytic deamination (see Chapter 

 Xiri), can also be fermented anaerobically by Type II 

 reactions. 



