THE CHEMICAL CHANGES PRODUCED BY BACTERIA 57 



indole from tryptophan, which is widely used as a qualitative biochemical test in the 

 identification of bacterial species, provides a good example of the breakdown of an amino- 

 acid. Thick washed suspensions of Bad. coli in phosphate buffer will convert this amino- 

 acid (/S-indole a-amino propionic acid) to indole in the presence of oxygen. The oxygen 

 taken up corresponds to the complete oxidation of the side chain to carbon dioxide and 

 water (Woods 1935). In the absence of air, the compound is only deaminated, with the 

 formation of a-indole jjropionic acid. The mechanism of the oxidative attack is obscure 

 and many complex series of steps have been proposed for this aerobic dissimilation, but 

 the recent work of Fildes (1938), and Baker and Happold (1940), suggest that one enzyme, 

 a trjrptophanase, is responsible for the action, breaking the link between the indole ring 

 and the a-carbon atom of the side chain. The oxidation of the side chain to ajnmonia, 

 CO2 and water follows after the liberation of the indole. A similar mechanism, but one 

 involving a disruption and resynthesis of the indole nucleus, is proposed by Krebs, Hafez 

 and Eggleston (1942). 



The observation that a free supply of a fermentable carbohydrate will " spare " 

 amino-acids, peptides or proteins in a medium was established by several workers 

 (see Hirschler 1886, Smith 1897, Peckham 1897, Glenn 1911, Kendall, Day and 

 Walker 1914, Kendall and Walker 1915, Jones 1916). The hypothesis of protein- 

 sparing formulated on the analogy with mammalian physiology was deduced 

 from the fact that ammonia production in the medium was largely inhibited by 

 the carbohydrate. But, as Stephenson (1939) points out, the analogy is imperfect, 

 for, in the mammalian case, the sparing is judged by a diminished excretion of 

 urea, an end-product of metabolism, whereas ammonia may be a source of energy 

 for a bacterium. Indeed, Raistrick (1919) and Eaistrick and Clark (1921) have 

 shown that in a medium containing known amino-acids, their decomposition is 

 increased, not lessened, by the addition of the fermentable substance glycerol. 

 The ammonia produced in these circumstances is undetectable, since it is utilized 

 by the bacteria for synthesis. It may be noted also that Berman and Rettger 

 (1918) were unable to demonstrate any protein-sparing as the result of the addition 

 of carbohydrates, except in those instances in which the rapid fall in pH resulting 

 from carbohydrate breakdown caused an inhibition of further bacterial growth ; 

 and that Heap and Cadness (1924) have shown that the presence of glucose greatly 

 increases the rate of HjS production from peptone by an organism that forms 

 this gas during protein-cleavage. In regard to the cleavage of more complex 

 protein molecules de Bord (1923) has shown that the addition of a fermentable 

 carbohydrate to a protein-containing medium causes an increase in the concen- 

 tration of amino-nitrogen induced by bacterial growth. These findings are in 

 accord with later observations by Kendall (1922), which indicate that the effect 

 of added carbohydrate is to lessen the utilization of proteins as a source of energy, 

 not as material for synthesis. 



A number of anaerobic bacteria belonging to the Clostridium group are incapable 

 of gross utilization of carbohydrates (see Chapter 36), and are dependent upon 

 amino-acids as energy sources. Although bacteria have been described which 

 find adequate energy sources in the employment of chemical mechanisms that, 

 from the formal thermodynamic point of view are comparatively unrewarding, 

 the vigorous growth of the Clostridia in media containing protein or protein digests 

 has stimulated the search for other anaerobic energy-yielding mechanisms. Stick- 

 land (1934, 1935) demonstrated a number of amino-acid dehydrogenase systems 

 in CI. sporogenes. Certain amino-acids (alanine, valine and leucine) were hydrogen 

 donators, and others (glycine, proline and hydroxyproline) hydrogen acceptors. 



