91 



A Study of the Action of Bacteria on Milk Proteins.* 



George Spitzer and H. M. Weeter, Purdue University. 



It is generally recognized that most bacteria have an action on or- 

 ganic food material which is characteristic for different species and is in- 

 fluenced by their previous environment and the kind and relative propor- 

 tion of the different foods in the media. As the food and water require- 

 ments of higher plant and animal life and of bacteria are similarly re- 

 lated, bacterial metabolism involves the change which the food materials 

 undergo by virtue of bacterial action and is determined by the properties 

 and composition of the end products. With the present chemical methods 

 of analysis it is possible to determine with considerable degree of ac- 

 curacy the initial composition of the bacterial foods, also the end prod- 

 ucts. Of what takes place within the organisms little is known. Infer- 

 ences can only be drawn from the changes in the medium and the nature 

 of the enzymes secreted by the bacteria. When bacteria are grown in a 

 medium containing both proteins and cai-bohydrates it has been found 

 that the cleavage products are modified, depending upon the source and 

 chemical complexity of the protein and carbohydrates. 



B. Coli, when grown in a nitrogenous medium in presence of easily 

 fermentable carbohydrates, fails to produce indol or the production of 

 indol is extremely rare, but when B. Coli is grown in a medium contain- 

 ing the same nitrogenous foods in presence of carbohydrates which do 

 not ferment readily indol is produced. The character of the proteins 

 likewise influences the growth and metabolism of bacteria and the cleav- 

 age products are not of the same kind and character. The proteins are 

 hydrolized by bacterial enzymes into simpler complexes, such as pro- 

 teoses, peptones, and possibly peptids and amino acids. 



There is a marked difference depending on the source of nitrogen, 

 and a still greater difference depending on the species of bacteria, in the 

 production of cleavage products. According to Taylor (Ztschr. f. 

 Physiol. Chem., Vol. 36), B. Coli digests casein mainly into proteoses and 

 peptones with the formation of only a small per cent, of amino acids, 



* "Contribution from Purdue University Agricultural Experiment Station, Depart- 

 ment of Dairy Husbandry." 



