APPENDIX 



I. THE SIZE OF MICROORGANISMS 



One of the common sources of error in studying 

 physiology of bacteria is a wrong conception of the 

 amounts or quantities of living matter or living proto- 

 plasm involved. A feeding experiment with pigs, record- 

 ing accurately the quantities of food used and the 

 amounts of CO2, urea, excreta, etc., given off, but with- 

 out a record of the weight of the pigs before and after 

 the experiment, would appear very unscientific. Yet, 

 that is the customary way of describing fermentation 

 experiments with bacteria. For this reason a computa- 

 tion of the size, weight, surface, and composition of an 

 average representative bacterium cell is given here. 

 Bad. coll has been chosen because it is probably the best 

 known micro-organism, and because it is of average size. 



The size of Bad. coU varies considerably with age, and also with 

 the kind and amount of food. But we can consider a length of 1.5/i 

 and a diameter of O.Sfj, as representing the average size of cells in a 

 well grown broth culture. For the simpUcity of calculation, we shall 

 suppose the cell to be a cylinder. Its volume is 



F = 0.4 X 0.4 X T X 1.5m^ = 0.75/i3 



The specific gravity is larger than 1, but only shghtly so, for a very 

 high speed of the supercentrifuge is necessary to precipitate the cells. 

 If we assume 1.07 as density of the cell (twice that of skim milk, 

 which contains half as many solids as bacteria), the weight of one cell 

 would be 



TF = 8.0 X 10-10 mg. 

 395 



