46 METABOLISM 



medical bacteriologist, at least, has been niaiuly interested in other bacterial 

 activities ; but the time has quite clearly arrived when the chemical aspects of 

 bacteriology and immunity must be mastered by all serious students of these 

 branches of biology. 



The metabolism of bacteria, as of other living cells, is dependent on, and regu- 

 lated by, a complex system of enzymes and catalysts, whose activity is conditioned 

 by a variety of factors, such as temperature of incubation, the pH of the medium, 

 the presence or absence of molecular oxygen, and the presence or absence of a 

 particular food-stuff. In order that any given bacterium may grow and multiply, 

 these various conditioning factors must fall within a range limited by the reqiiire- 

 ments of the enzyme systems that are available. Many bacteria possess more 

 than one enzyme system for dealing with a given type of substrate, and a change 

 of environment is accompanied by a change in the predominating enzyme system. 



The prime needs of a bacterium are substances that it can assimilate and 

 synthesize into protoplasm ; and energy necessary for these syntheses, and for 

 movement (if it is motile), reproduction and the maintenance of structure. Both 

 are obtained by the decomposition of suitable substrates. No sharp dividing line 

 can be drawn between substrates that act mainly as sources of energy and those 

 which are needed for synthesis of bacterial proteins, carbohydrates, fats, enzymes, 

 etc. But it is convenient to distinguish the two types of activity, and discuss 

 the first under bacterial resj)iration and fermentation, and the second under 

 bacterial nutrition. Bacterial nutrition we shall leave till a later section, but 

 before examining the nature of respiration and fermentation in detail, we shall briefly 

 discuss the general implications of the energy-producing mechanisms in living cells. 



The Respiration of Bacteria. — ^The term respiration, with its connotations for 

 the student of mammalian physiology of the mechanical intake of oxygen for the 

 purpose of oxidizing food substances, is apt to confuse the student of bacteriology 

 unless he realizes that more than the direct utilization of atmospheric oxygen 

 is concerned. Respiration covers all those metabolic mechanisms that are em- 

 ployed in providing energy, as opposed to those that are concerned in synthesis. 

 It happens that many energy-yielding reactions, that is, exothermic reactions, 

 are oxidations in the simple sense of the word and take place in the cell when 

 molecular oxygen is supplied. But this concept of oxidation as the union of oxygen 

 with a given substance is as limited in its applications to biology as it is to inorganic 

 chemistry. In neither does the process of oxidation necessarily involve the transfer 

 of oxygen. 



The simple reaction 



FeClg + CI -> FeCla 

 is as much an oxidation as the more obvious 



2FeO + 0-> FejOg. 

 The common feature of these two oxidations, and indeed of aU oxidations, is change in the 

 electronic state of the substances concerned. If the oxidation of the ferrous to the ferric 

 chloride takes place in solution, in which the participants in the reactions are ionized, 

 the whole process may be written 



Fe++ + 2C1- + Cl^-Fe+ + + + 3C1-. 

 The ferrous salt is oxidized, and the chlorine atom reduced. The oxidation of the iron 

 may be written 



Fe+ + — >- Fe+ + + + one free electron 

 and the reduction of the chlorine by 



CI -f one free electron — > Ci~. 



