360 METABOLISM OF MICROORGANISMS 



follow: hemin (Hemophilus injiuenzae) ; putrescine, NHo(CH2)4NH2 

 {H. parainfluenzae) ; a-lipoic acid, also called thioctic acid, p. 254 {Strep- 

 tococcus lactis and Tetrahymena geleii) ; coprogen, an organic-iron com- 

 pound (Philobolus kleinii) . 



Inorganic Elements. The requirement of K, Mg, Mn, Fe, S, and P 

 for the growth of microorganisms is well established, and numerous reports 

 indicating the need for Ca, Cu, Zn, Mo, and Co have appeared. The 

 reason for the uncertainty regarding the need for some of these elements 

 is the small amount that is required and the difficulty of removing traces 

 of these elements from the medium. An example of the difficulties that 

 exist is illustrated by a study of the vitamin B12 requirements of bacteria. 

 This vitamin is synthesized by many bacteria, and since it contains cobalt, 

 this element must have been present in the medium. While the vitamin 

 can be shown to have been formed, the presence of cobalt in the medium 

 is not easily demonstrated. The quantity of cobalt required for the 

 synthesis of all the vitamin needed by the bacteria is of the order of 

 0.4 m^ug Co per liter ^ of medium — a quantity that is not easily detected. 



On the other hand, microorganisms that synthesize large quantities 

 {e.g., 2-3 jug per milliliter) of vitamin B12 must be supplied with cobalt 

 salts if maximal yields of vitamin B12 are to be obtained. 



In practical work the needs of microorganisms for all the inorganic 

 elements are met by adding phosphate or sulfate salts of potassium, 

 magnesium, manganese, and iron to the medium. These salts usually 

 carry enough impurities to meet the needs, if any, of the microorganism 

 for other elements. 



Growth efficiency 



Cells differ greatly as to the efficiency with which they convert nutrients 

 into cell material. The comparison is best made on the basis of dry 

 matter of food converted into dry matter of cells, in order to eliminate 

 the effect of varying moisture contents on the calculations. If the 

 figures are expressed on a percentage basis, the efficiency of the cell is 

 obtained. The efficiency varies under different conditions, but if those 

 giving optimal values are taken, the conversion of nutrients into cell 

 dry matter is about as follows: 



Group Example Percent 



Aerobic bacteria Azolohacter vivelandii 14 



Anaerobic bacteria Clostridium acetobutylicum I 



Yeast Saccharomyces cerevisiae, aerobic growth 50 



Yeast Saccharomyces cerevisiae, anaerobic growth 5 



Molds Penicillium chrysogenum, in penicillin production. 40 



Molds Aspergillus niger, in citric acid production 5 



^ One milligram is equivalent to 1,000 micrograms (^,g) and one microgram is equiva- 

 lent to 1,000 millimicrograms (m|j,g). 



