STUDIES IN MICROBIOLOGY (ly 



Bacterial Growth; A Bacterial Enzymc; 

 Comparative Biochemistry 



(Readings: R. Y. Stanier, M. Doudoroff, and E. A. Adelberg, The Microbial 

 World, Prentice-Hall, 1957, pp. 26-37, 225-239, and 255-256. K. V. Thimann, 

 Life of Bacteria, Macmillan, 1955, pp. 3-31 and 550-560. Weisz, pp. 173-175. 

 S.P.T., pp. 484^88. Villee, pp. 131-138.) 



Bacteria are single-celled organisms, much 

 smaller than the yeasts, algae, and protozoa, 

 examples of which we have already seen. Many 

 kinds of bacteria can be distinguished on the 

 basis of such characteristics as shape, color, 

 nutritional requirements, and biochemical con- 

 stitution. We shall first study the bacterium 

 Serratia marcescens. It is rodlike in shape, red 

 in color, and requires a source of animal or 

 vegetable material for growth. 



As the bacterium takes in nutrient substances 

 from its surroundings and converts them into 

 its own proteins, it grows up to a point at which 

 it splits into two cells, each of which continues 

 to grow in the same manner. That is, it repro- 

 duces by fission. We shall follow the growth of 

 Serratia by counting the number of cells present 

 before and after letting the bacteria grow for 

 two hours. 



*A detailed discussion of preparations for the 

 microbiological experiments in Exercises VI through 

 IX will be found in Appendix A (pp. 135-140). 



In order that growth occur rapidly, the bac- 

 teria will be suspended in broth, aerated in order 

 to allow the cells to respire freely, and incubated 

 at 37°C. Furthermore, our source of bacteria 

 will be a young culture, that is, one in which 

 the cells are growing rapidly. In an old culture 

 the cells have used up all the nutrients and are 

 no longer growing. To start growing again 

 when transferred to fresh medium, the cells 

 have to reorganize their machinery, and this 

 results in a delay — a lag period — before growth 

 begins. 



The number of cells present in a bacterial 

 suspension can be counted by spreading a 

 dilute sample on the surface of agar (a jelly- 

 like material) to which nutrients are added. 

 Each cell grows into a colony, and one counts 

 the colonies. 



With the help of the enzymes they contain, 

 microorganisms can carry out numerous chemi- 

 cal reactions. Serratia marcescens contains the 

 enzyme catalase which breaks down hydrogen 

 peroxide to water and oxygen (2H2O2 — * 2H2O 



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