190 BACTERIOLOGICAL CHEMISTRY 



The products formed by the action of the eiido- 

 enzymes in the cell are usually essentially different from 

 the substance fermented, and are in the main useless, 

 often even harmful, to the organism. It is this accumula- 

 tion of end products which is often responsible for the 

 cessation of growth after a time ; for instance, in yeast 

 fermentation when about 8-5 per cent, of alcohol has been 

 produced, further growth of the yeast cells is inhibited. 



The majority of organisms can utilise several kinds of 

 food and therefore bring about various fermentations in 

 the course of their metabolism. They may even be able 

 to utilise the same food in different ways, according to 

 the conditions. For instance, certain typical sugar fer- 

 menting bacteria and the yeasts can be grown in the 

 absence of sugars ; lactic acid bacteria will grow on 

 peptone. Yeast normally ferments sugar to alcohol, but it 

 can also oxidise it completely to carbon dioxide and water 

 if a sufficient supply of oxygen is available. Most moulds 

 appear to be omnivorous, attacking almost any substrate 

 with which they may be supplied. At the other extreme 

 are the autotrophic bacteria, most of which can utilise 

 only one substrate as a source of energy (see Chapter VI) . 



The chemical activity of the micro-organisms is, 

 generally speaking, vastly greater than that of animals 

 or plants. For example, it has been calculated that 1 

 gram of Micrococcus urece can decompose 180 to 1,200 

 grams of urea per hour ; and that 1 gram of certain 

 lactose fermenting bacteria can hydrolyse 180 to 15,000 

 grams of lactose per hour. If man were capable of meta- 

 bolism on the same scale he would consume several 

 thousand tons of food per hour. These figures are pro- 

 bably subject to a certain margin of error, but they are 

 sufficiently accurate to indicate the enormous difference 

 between the metabolic activities of the bacteria and 

 animals. A major cause for this difference in activity 

 is the much greater area in bacteria available for the 

 absorption of nutrients. The area to weight ratio in 



