44 DAIRY BACTERIOLOGY 



THE PUTREFACTIVE PROCESS 



The preceding sections have been devoted to certain carbo- 

 hydrate fermentations. We now pass on to the fermentations of 

 the proteins, which, according to our previous definition, come 

 under the heading of putrefaction. In this process two distinct 

 phases may be recognised : first, the protein hydrolysis or peptoni- 

 sation, due exclusively to the proteolytic enzymes, in which the 

 proteins are split up into soluble amino acids, and second, the 

 decomposition of these acids by other enzymes (amidases, oxidases 

 and reductases), with the formation of ammonia and various evil- 

 smelling substances. While the final products of carbohydrate 

 fermentation are always acid, those of protein fermentation are 

 always alkaline. As the action of proteolytic enzymes arid also 

 the growth of the organisms which secrete them are inhibited by 

 small quantities of free acid, it is easy to understand why the 

 presence of carbohydrates in appreciable amounts will prevent 

 putrefaction ; it is only when the carbohydrates and their acid 

 products have been destroyed or neutralised that the process of 

 putrefaction will obtain a proper start. As all bacteria which 

 secrete proteolytic enzymes may take part in a putrefactive 

 process, we have to deal with a large number of types. We will 

 first deal with the aerobic putrefactive bacteria, and then with 

 those which are obligate anaerobes, for the former initiate the 

 process and, consuming the available atmospheric oxygen, they 

 prepare the ground for the latter group, which then carry the 

 decomposition further ; it is especially during this second phase 

 that the evil-smelling products, *so characteristic of putrefaction, 

 are formed. 



A. The Aerobic Putrefactive Bacteria. 1. The 

 Fluorescent Bacteria. These are non-sporing motile rods with 

 polar flagellse, producing a fluorescent green colouring matter 

 (insoluble in chloroform) on neutral or alkaline media. The 

 commonest species are monotrich and Gram-negative. They do 

 not ferment lactose, and some are liquefying, others non -liquefy ing. 

 Several, especially the non-liquefying species, are denitrifying 

 organisms, i.e., they reduce nitrates to free nitrogen, and thus rob 

 plants of their nitrogenous nutrient matter. They are very 

 widely distributed in soil and water, and can as a rule grow at 

 temperatures only slightly above C. A species which is parti- 

 cularly active in liquefying gelatine, Bacterium fluorescens lique- 

 faciens, hydrolyses fats, and may therefore play an important part 

 in turning butter rancid. Bacterium pyocyaneum (Fig. 25) is a 

 nearly related form which also liquefies gelatine and hydrolyses 

 fats, but it grows so slowly at ordinary temperatures that it does 



