Peptides and Bacterial Growth

Lactobacillus casei grows in the absence of vitamin Ba, provided that D-alanine and an unidentified peptide are added to a medium complete in other nutrients (1, 2). A partial hydrolysate of protein contained several chromatographically distinct fractions that possessed this “peptide factor” activity (3). It appeared likely that a single amino acid linked in a variety of combinations was responsible for these growth effects. An active, purified fraction was identified tentatively as a mixture of alanyl and tyrosyl dipeptides of valine, leucine, and isoleucine (3). An expeditious solution of the problem thus appeared possible by testing synthetic peptides containing these amino acids. Such studies, described below, revealed that dipeptides containing L-alanine in either the carboxyl or amino position possessed growth-promoting activity; L-alanine was inactive under the same conditions, as were peptides of tyrosine. Examination of the reasons for this enhanced activity of L-alanine peptides revealed that n-alanine, which is itself essential for growth in the absence of vitamin Ba, strongly inhibited utilization of L-alanine, but did not inhibit utilization of peptides of this amino acid. Despite this fact, resting cells of L. casei hydrolyzed each of the L-alanine peptides utilized for growth. Certain implications of these and related findings are discussed briefly.