2 LABORATORY MANUAL OF MICROBIOLOGY 



(P, K, S, Ca, Mg, Fe, etc.), which are required by the organisms 

 for the synthesis of their cell substance. Some organisms are 

 very specific in the requirements of these nutrients, while others 

 can derive their energy, their carbon, their nitrogen, and the min- 

 eral elements from a great variety of substances. 



Microorganisms are divided broadly into two large groups, on 

 the basis of their energy requirements: (1) The autotrophic 

 organisms, which can obtain the energy required for their 

 activities from the oxidation of inorganic elements or their 

 compounds or from simple compounds of carbon, their carbon 

 from carbon dioxide, and their nitrogen and other minerals 

 from inorganic compounds. In addition to those organisms 

 which can obtain their energy from the oxidation of inorganic 

 substances or simple compounds of carbon (chemosynthetic), 

 the chlorophyll-bearing plants, which obtain their energy photo- 

 synthetically are also classified with this group. (2) The 

 heterotrophic organisms which obtain their energy and carbon 

 from complex organic substances. 



Among the heterotrophic organisms, however, there is also 

 very considerable specificity. Some organisms can obtain their 

 energy and carbon only from celluloses, while the great majority 

 of microorganisms cannot attack celluloses and cannot utilize 

 them either as sources of energy or of carbon. Some organisms 

 can obtain their nitrogen from inorganic compounds, such as 

 ammonium salts or nitrates; others require only organic nitrogen 

 sources; still others can use even gaseous atmospheric nitrogen. 

 Some can grow at a wide range of reactions; for the activities of 

 others only a very limited range of hydrogen-ion concentration 

 exists. Some organisms are able to grow on a great variety of 

 media, liquid or solid; others will develop only on very specific 

 media, when the particular nutrients are available. 



In addition to the nature of the nutrients, it is also important 

 to keep in mind that a specific osmotic pressure is required; in 

 other words, the concentration of the nutrients must not vary 

 within too wide limits. Solid media are often required; for this 

 purpose either inorganic gels, largely silicic acid (also mag- 

 nesium-gypsum blocks), or organic gels of a carbohydrate 

 nature (agar-agar, etc.) or of a protein nature (gelatin, coagulated 

 egg albumen, coagulated blood serum) are employed. 



