MICROORGANISMS AND FERMENTATIONS 9 



sodium are only required in appreciable amounts by organisms 

 normally living in salt liquids, i.e., marine and many pathogenic 

 organisms. Like all other organisms, the microorganisms are 

 able to derive the necessary sulphur, phosphorus and metallic 

 elements from inorganic salts. These salts are found in the 

 required proportions in plant ashes, and formerly it was the 

 practice to add them to nutrient media in this form. If micro- 

 organisms are cultivated in milk or the extracts of meat or plants 

 they will generally be well supplied with the requisite inorganic 

 nutriment. 



Nitrogen. Microorganisms may be divided into two main 

 groups according to their ability or inability to assimilate all the 

 nitrogen they require from inorganic sources. A few species 

 belonging to the former group can assimilate nitrogen from the 

 air, and thus incidentally improve the soil for plant growth. The 

 great majority of these, however, require their nitrogen in the 

 form of ammonia or nitrates ; they are represented by the typical 

 water bacteria, acetic acid bacteria, and many yeasts and moulds. 

 Belonging to the group which cannot grow in the absence of 

 proteins or the immediate decomposition products of proteins 

 are many of the putrefactive bacteria and the true lactic acid 

 bacteria. 



Carbon. Microorganisms fall into two groups with respect to 

 carbon assimilation. A few species of soil bacteria which oxidise 

 ammonia to nitrates resemble the higher plants in being able to 

 utilise atmospheric carbon dioxide as their sole source of carbon. 

 Unlike the higher plants, however, they are best able to carry out 

 this process in darkness no bacteria tolerate direct sunlight. 

 Most microorganisms require organic sources of carbon. Some 

 putrefactive bacteria can obtain all the carbon they need from 

 proteins, thus presenting a broad analogy to the carnivorous 

 animals. As a rule, however, microorganisms require special 

 sources of carbon such as carbohydrates, alcohols and organic 

 acids, and often show marked preferences for certain substances ; 

 thus some species will only grow in presence of certain sugars, a 

 circumstance which is put to advantage in distinguishing closely 

 related species from one another. 



Oxygen. Oxygen and Hydrogen are assimilated from most of 

 the nutrient substances and from water. Water must be regarded 

 as the most important of the nutrient substances ; it constitutes 

 about four-fifths of the total substance of the microorganism, 

 besides which it is absolutely essential to life as a solvent and 

 distributing medium for all the other nutrient substances. Like 

 animal and plants, most microorganisms are capable of assimilating 

 oxygen direct from the atmosphere. The analogy is only super- 



