PRINCIPLES OF MICROBIAL METABOLISM 



373 



Growth of an organism can be continued indefinitely, when repeatedly- 

 transferred upon fresh media. In the same medium, however, there 

 is at first a decided increase in the numbers and activities of a micro- 

 organism, soon coming to a maximum and this is followed by a rapid 

 decrease. The limitations to further growth are the lack of nutrients 

 and the formation of injurious waste products. 



Kruse reports that, after two days of growth on agar cultures, cer- 

 tain bacteria are reduced to 10 per cent of the maximum; in three days, 

 to 1 per cent and, in seven days, they are all dead. The curve shown 

 in fig. II 8 illustrates this process. 



Fig. 11. Rate of increase in numbers of a bacterium (from Buchanan) 



Different organisms grow at a different rate of rapidity and also 

 disintegrate at a different rate. Growth of microorganisms in a limited 

 amount of medium soon reaches a maximum due to the exhaustion of 

 one or more nutrients in the medium or to the formation of injurious 

 by-products, such as acids, alkalies or some toxic substances. 9 While 

 the older parts die off, the younger may still continue to grow and 

 these may use the nutrients made available by the decomposition 

 (autolysis) of the older parts. 



A microbial cell thus passes through a period of youth, full develop- 

 ment and old age. These stages can be expressed by the autocatalytic 



8 Buchanan, R. E. Life phases in a bacterial culture. Jour. Inf. Dis., 23: 

 109-125. 1918. 



9 Chambers, W. H. Studies in the physiology of the fungi. XI. Bacterial 

 inhibition by metabolic products. Ann. Mo, Bot. Card., 7: 249-289. 1920. 



