GROWTH OF THE MICROBIAL CELL 13 



gen in natural substrates and is responsible for the continuous stream 

 of ammonia and nitrate forming the available sources of nitrogen that 

 makes possible the growth of higher plants. 



THE GROWTH OF THE MICROBIAL CELL IN 



PURE CULTURE AND IN MIXED 



POPULATIONS 



When nutrients are available in sufficient concentration and when the 

 environmental conditions are favorable for the development of the 

 microbial cell in pure culture, growth follows a definite sigmoid-shaped 

 curve. Slow multiplication is followed by rapid development, until a 

 certain maximum number of cells within a given volume of medium is 

 reached J the rate of growth then diminishes. The maximum population 

 of Aerobacter aero genes grown in a medium containing lactose and 

 ammonium tartrate increases at first in proportion to the concentrations 

 of these nutrients but later becomes independent of them. The onset of 

 the stationary phase may be due to several factors: exhaustion of sub- 

 stances necessary for growth, change in the reaction of the medium to 

 one unfavorable for further development, accumulation of toxic prod- 

 ucts. When the nutrients in the medium are exhausted, addition will 

 restore growth. When an unfavorable change in reaction has taken 

 place, the addition of acid or alkali will render the medium again favor- 

 able. The production of toxic substances in the medium can be counter- 

 acted usually by the use of heat or by treatment with charcoal, though 

 some of the injurious bodies may be heat-resistant. 



In the presence of other microorganisms, a certain organism may 

 show reactions markedly different from those obtained in pure culture: 

 it may produce substances that are either favorable or injurious to the 

 other cells, it may compete with the other organisms for the available 

 nutrients or it may render the medium more favorable for their de- 

 velopment. It has been shown (936), for example, that certain bacteria 

 like Bacillus cereus can attack native proteins but not amino acids, 

 whereas others like Pseudomonas fluorescens can attack amino acids but 

 not proteins J when these two organisms were placed together in the 

 same medium, their activities supplemented one another. Numerous 



