MICROORGANISMS AND FERMENTATIONS 15 



bacterial invasion unless they have previously been weakened by 

 the action of toxins. To counteract this, the higher organisms 

 secrete substances, known as anti-toxins, which play an important 

 part in recovery from disease and the subsequent more or less 

 permanent period of immunity from the disease in question. 



VARIABILITY 



Microorganisms are classified together when they resemble one 

 another in structure and habits of living and growth. Many 

 microorganisms, however, are very liable to vary in appearance 

 and in their biological characteristics, so that we may often have 

 to deal with dissimilar forms which nevertheless belong to the 

 same species. As we have really no sure guide enabling us to 

 distinguish between essential and non-essential characteristics, it 

 may often be difficult to determine whether we are dealing with 

 different species or only variants of the same species. Undoubtedly, 

 the biological properties, such as those which relate to nutrition, 

 growth and energy, are more important in this respect than out- 

 ward form, and, again, the means by which energy is produced are 

 of more importance than the particular raw materials from which 

 the energy is derived. Though it may be granted that an organism 

 which derives its energy from an alcoholic fermentation instead of 

 a complete oxidation to carbon dioxide and water would hardly be 

 classified among the higher organisms, yet it is often observed that 

 closely related organisms are able to utilise different nutrient 

 materials. If a lactic acid organism undergoes variation, it does not 

 follow that it will start producing alcohol or butyric acid instead 

 of lactic acid, but it may easily lose its power to ferment a poly- 

 saccharide, such as milk sugar, which requires a special enzyme, 

 lactase, to convert it into fermentable material. In other words* 

 the intracellular enzymes are much more characteristic of a given 

 microorganism than the extracellular enzymes. It may be assumed 

 that the chief products of a fermentation process will always be 

 the same under similar conditions, but the by-products may vary 

 considerably according to the condition of the organism. The 

 ability to produce certain substances which affect the taste, smell 

 or colour of the medium is especially variable. The nature of the 

 cell wall may also vary, so that an organism may appear sometimes 

 with and sometimes without a capsule, or the cell wall may dis- 

 integrate into a mucilaginous mass. Bacteria are particularly 

 liable to undergo this form of degeneration especially the lactic 

 acid organisms generally as the result of over-nutrition ; an 

 analogous instance is seen in fatty degeneration in animals. 



Temperature is an important factor in determining the form of 



