THE NUTRITION OF BACTEKIA 5 



individual bacteria touch each other ? Is the intervening space 

 always equal ? Examine the surface of a hay infusion for a 

 zoogloea mass. 



Exercise 6. Comparison of the Shapes of Yeasts and Mold Fungi 

 with Bacteria 



Put a piece of baker's yeast into a 3 per cent cane-sugar 

 solution, and place the dish for a few hours in a warm place 

 (25-30 C.). Examine a drop of the solution containing yeast 

 cells, comparing their size, form, and structure with that of the 

 bacteria studied above. Examine microscopically such mold 

 fungi as Mucor, Aspergillus, or Penicillium. How do these or- 

 ganisms compare with bacteria in respect to size and complexity 

 of structure ? 



SECTION II 

 THE NUTRITION OF BACTERIA 



Bacteria require for their nourishment compounds of carbon, 

 nitrogen, hydrogen, sulphur, oxygen, phosphorus, potassium, and 

 magnesium. For some organisms iron and calcium compounds 

 are also needed. A few bacteria may use carbon dioxide in 

 gaseous form, and several use elementary nitrogen, but most 

 bacteria require compounds of the above elements in suitable 

 form. In nature, bacteria are found growing upon more or less 

 complex organic substances. Unlike the chlorophyll-containing 

 plants, most bacteria are unable to lay hold of the atmospheric 

 carbon dioxide, but must derive their carbon compounds from 

 other plants or animals, or from excreta from these organisms. 



A proper amount of moisture is required in order that certain 

 of the compounds may be dissolved. If water is too abundant, 

 the food supply will be too dilute to supply the needs of the 

 bacteria; on the other hand, if water is scantily supplied, the 

 concentration of solutes is too high to permit of absorption, if 

 indeed they are dissolved at all. 



In making artificial culture media the general principle holds 

 that the medium should approximate as closely as possible that 



