ENERGY SUPPLY OF THE CELL 159 



The study of food selection with microorganisms was 

 started by Pasteur's observation (1860) that Penicillium 

 used d-tartrates in preference to the 1-form. Duclaux 

 (1889) and Pfeffer (1895) studied the choice which 

 Aspergillus and Penicillium made from different sources 

 of energy offered, such as salts of organic acids, glycerol 

 and sugars. They found that, as a rule, both foods 

 were attacked, but usually the one more completely 

 than the other. Acetates were used more readily than 

 either butyrates or tartrates. Dextrose was used in 

 preference to glycerol. Pfeffer found that in a 4.5% 

 peptone solution, glycerol was not touched by Aspergillus. 



A very highly specialized case of food selection is that 

 of Penicillium palitans which, according to Stokoe 

 (1928) destroys practically all the caprylic acid of coconut 

 oil while comparatively little of the other fatty acids 

 had disappeared. The caprylic acid is oxidized at the 

 /3-carbon as usual, but then, instead of complete breakage 

 of the carbon chain at this point, the keto-acid is decar- 

 boxylized leaving amyl methyl ketone which is the cause 

 of the rancid odor of coconut oil. 



C5HnCH2-CH2-COOH + 02 = CsHn-CO-CHg-COOH = 



Caprylic acid Keto acid 



CsHnCOCHs + CO2 



Amyl methyl ketone 



While molds have been used very extensively for such 

 experiments, we know a good deal less about the pref- 

 erences of bacteria with regard to their sources of energy. 

 The most typical and best known case is the protection 

 of proteins by sugar. There is much less protein decom- 

 position in media containing sugar (Literature and 

 explanations, see Berman and Rettger, 1918b). 



Most of these facts lack a good explanation. It does 

 not seem that the available energy of a food compound 

 determines the preference. Some compounds are simply 



