ENZYMES, TOXINS, PTOMAINS 49 



a gradual disintegration, and the injurious effects observed are said 

 by these observers to be not purely mechanical. 



4. Chemotaxis. Bacteria respond to various chemical stimuli. 

 Substances which can be used by them for nutritional purposes, as 

 various constituents of laboratory media, appear to attract bacteria. 

 Harmful substances, as acids or alkalis, may act in the reverse manner. 

 Oxygen is a powerful chemotactic agent for many aerobic bacteria, 

 while many anaerobes are repelled by it. The mutual chemotactic 

 relations of bacteria and leukocytes, and the well-defined tendency 

 of certain invasive bacteria to localize in definite tissues or organs 

 of the animal body are interesting fields for speculation. Nothing 

 conclusive is known about these relations. 



K. ENZYMES, TOXINS, PTOMAINS. 



Enzymes. The phenomena of chemical interchange between 

 bacteria and their environment indicate that enzyme activity plays 

 an important part in bacterial metabolism. 



Enzymes may be defined as substances of unknown composition 

 produced by living cells which incite specific chemical reactions with- 

 out permanently combining with the products of reaction. A small 

 amount of enzyme acting under favorable conditions will cause a 

 relatively extensive transformation of substance without itself being 

 used up or inactivated. There is, however, a limit to the amount of 

 transformation which a given amount of enzyme can accomplish, 

 for the accumulation of reaction products tends to restrict enzyme 

 action; the removal of reaction products appears to extend enzyme 

 action somewhat. All bacterial cells appear to produce or to possess 

 enzymes, probably several, which may be divided somewhat arbi- 

 trarily into two classes, the extracellular or exo-enzymes, and the 

 intracellular or endo-enzymes. 



Exo-enzymes. Exo-enzymes are those which are excreted from the 

 organism and appear as soluble, filterable and, frequently, diffusible 

 enzymes, which may be obtained in an active state from filtrates of 

 cultures of bacteria. Their diffusion from the bacterial cell and their 

 filterability suggests that they may be relatively simple in molecular 

 aggregation. Their function is essentially a "preparatory" one, for 

 they transform potential nutritional substances, as proteins, carbo- 

 hydrates or fats, to simpler compounds which are assimilable by the 

 bacteria. It is very probable that the exo-enzymes work uneconom- 



4 



