THE CHEMICAL CHANGES PRODUCED BY BACTERIA 51 



The Chemical Changes produced by Bacteria in various Substrates 



On the view outlined above, the oxidation of a complex substance by bacteria 

 may be pictured as the transfer of hydrogen from one substance to another, each 

 step being brought about by one of a graduated series of oxidation-reduction 

 systems with appropriate enzymes, carriers and co-enzymes, the whole resulting 

 in an even flow of energy for cell maintenance and syntheses. We shall now 

 examine in more detail the action of bacteria in the different types of foodstuffs. 



The Action of Bacteria on Carbohydrates and Allied Substances. — The majority 

 of the bacteria with which we are concerned in this book are able to attack the 

 hexose sugars. Occasionally the end result is complete oxidation to water and 

 carbon dioxide. This is, however, an unusual type of reaction, limited to a few 

 species and demanding a copious supply of oxygen. Various reactions leading 

 to less complete oxidation are more common ; and one of the most characteristic 

 types of bacterial fermentation is that in which molecular oxygen plays no direct 

 part, the reaction consisting essentially in the splitting of a complex molecule, 

 usually by hydrolysis, with a rearrangement of the oxygen atoms, so that one 

 portion of the hydrolysed molecule is oxidized while the other is reduced. An 

 example of this general type of reaction is afforded by the studies of Harden (1901, 

 1905), and of Harden and Walpole (1906) on the fermentation of dextrose by Bact. 

 coli. The main products of fermentation are lactic acid, acetic acid, ethyl alcohol, 

 carbon dioxide and hydrogen, and the reaction appears to be approximately repre- 

 sented by the formula, 



2CeHi206 + H2O = 2CH3CHOHCOOH + CHg-COOH + C2H5OH + 200^ + 2H2 



although small amounts of other substances, such as succinic acid, are produced. 



In some instances, as in the fermentation of dextrose by the typhoid bacillus, 

 no free gas is evolved. It was suggested by Harden that in this case that part 

 of the reaction which, with such an organism as Bact. coli, leads to the evolution 

 of equal parts of CO2 and Hj, stops short at the formation of formic acid, H-COOH 

 (see also Pakes and Jollyman 1901). This suggestion was strengthened by the 

 observation of Sera (1910), while Grey (1913-14) has shown that formic acid can 

 be identified as an intermediate product in the fermentation of dextrose by Bad. 

 coli. 



This type of reaction is, however, by no means the only one that occurs during 

 the cleavage of carbohydrates and allied substances by bacteria. 



The existence of wide variations is well illustrated by the results recorded by 

 Birkenshaw, Charles and Clutterbuck (1931). Using the carbon-balance-sheet 

 method employed by Raistrick and his colleagues (1931) in their extensive studies 

 of the metabolism of moulds, they determined the relative proportions of different 

 metabolic products formed by twenty different bacterial species growing in a 

 synthetic medium containing glucose. In the case of Bact. coli and of certain 

 nearly related organisms, up to 30-4 per cent, of the carbon of the glucose was 

 recovered after fermentation in the form of lactic acid, 5-0-14-4 per cent, was 

 recovered in the form of volatile acids, while the amount present as butylene 

 glycol (CHa-CHOH-CHOH-CHa) was negligible. On the other hand, a coliform 

 organism of a different type, Bact. asiaticum mobile, yielded 26'8-31-0 per cent, 

 of the carbon as butylene glycol, but none as lactic acid. Two anaerobes {CI. 

 saccharobutyricum and CI. pasteurianum) yielded a large proportion of carbon 



