Peptide Utilization in Escherichia coli

Many bacteria have the capacity to utilize for their nutrition not only free amino acids, but also peptides cont,aining these amino acids. Often this capacity can be attributed to the ability of some bacteria to form and release into their environment proteolytic enzymes (1). However, evidence in recent years indicates that some peptides can penetrate the bacterial membrane (2, 3). Hence, another route for peptide utilization exists in which intracellular enzymes are responsible for the cleavage of the peptide bond. Therefore, for any particular microorganism the array of peptides that can be nutritionally effective depends upon the ability of the microorganisms to form intracellular and extracellular peptidases, their specificity, and the properties of those sites in the bacterial membrane that can admit peptides to the cell interior. Bacterial mutants, lacking the ability to form individual amino acids, provide a facile means to study the nutritional response to various peptides. In these organisms utilization of peptides containing the requisite amino acid provides an easily measured growth response (4). In such organisms, a positive response in the absence of extracellular peptidases indicates that the means exist for transporting the peptide into the cell and hydrolyzing it. The work of Stewart and Stahmann (5) and Sober (6) provided a means of readily obtaining the members of a homologous series of charged peptides. With the p ossibility of an intrinsically more systematic variation of parameters when dealing with a homologous series, it became of interest to test the utilization of such peptides in the above system. The response to oligolysine peptides was examined with an Escherichia coli lysine auxotroph, strain M-26-26 (7) ; to oligoarginine peptides with the arginine-requiring strain 39-A-23 (8) ; and to oligoglutamic peptides with strain M-22-64 (9), which requires glutamic acid. Some modifications of the oligolysine and oligoarginine peptides were carried out by acetylation and the resultant products were tested. It was possible to conclude that in E. coli size and presence of an unsubstituted a-amino group are critical factors in the ability of a peptide to be utilized.