198 BACTERIOLOGICAL CHEMISTRY 



Type III. — The hydrogen donator and acceptor are 

 different compounds, resulting in inter-molecular fermen- 

 tation. The Type I oxidation is a special case of Type III 

 in which the acceptor is oxygen. Examples of this type 

 are the anaerobic fermentations at the expense of the 

 oxygen of fumarates, nitrates, sulphates, or similar highly 

 oxidised substances. Ehizobium, for instance, can be 

 made to grow as much as 1 cm. below the surface of agar 

 containing a small amount of permanganate, whereas 

 normally it grows only on the surface of solid media. 

 Obviously for Type III reactions to occur the energy 

 liberated by the oxidation (dehydrogenation) of the 

 donator must be greater than that required to cause the 

 reduction (hydi'ogenation) of the acceptor (see also 

 Oxidation-Reduction Potentials, Chapter II). Besides 

 the factor of thermodynamic possibility the enzymic 

 activation of the substrates comes into play. This is 

 particularly well illustrated in the case of the streptococci 

 which cannot use oxygen, the best of all hydrogen 

 acceptors from the energy point of view. This is not due 

 to an actual sensitivity to oxygen since, for example, 

 Str. cremoris can grow in milk exposed to oxygen, but no 

 oxygen uptake can be measured, whilst Esch. coli under 

 the same conditions takes up oxygen freely. Again, 

 Str. lactis suspended in aerated buffer solution takes 

 up no oxygen, although the majority of aerobes and 

 facultative anaerobes take up from 5 to 25.jti1. of oxygen 

 per hour under the same conditions. Clostridium sporo- 

 genes under these conditions behaves like the strepto- 

 cocci, and cannot utilise oxygen since it, too, lacks the 

 appropriate enzyme system. Oxygen uptake is usually 

 stimulated by the presence of methylene blue and inhibited 

 by cyanide. 



The growth of organisms in oxygen usually involves 

 the production of hydrogen peroxide which is toxic to 

 most micro-organisms. Normally this hydrogen peroxide 

 is destroyed by the enzyme catalase with formation of 



