Inhibition of bacterial growth by leucine peptides.

In previous reports (I, 2), data were presented on the growth response of a leucineless mutant (strain 679-680)’ of Escherichia coli strain K-12 to various L-leucine peptides. Measurement of the extent of bacterial growth after 24 hours incubation showed that, at low concentrations, the peptides were almost as effective in the stimulation of growth as was L-leucine; at high concentrations, however, the peptides appeared to exert an inhibitory action on the growth of the mutant (I). When the growth response of the mutant to L-leucine and to glycyl-L-leucine (G-L-L) was studied as a function of time as well as of the concenkation of the test compound, it became evident that the dipeptide acted as an inhibitor of the initiation of growth, and that the duration of the lag phase of growth (i.e., the period preceding the initiation of rapid growth) increased with increasing concentrations of G-L-L. However, the extent of bacterial growth ultimately attained was approximately the same for equimolar concentrations of L-leucine and G-L-L (2). In the present study, a comparison has been made of the relative inhibitory action of a number of leucine peptides on t,he growth of the leucineless mutant. The methods used in these experiments have been described previously (2). Nearly all of the L-leucine present in a peptide becomes available to the organism for growth; for convenience, therefore, the concentration of the leucine peptides will be given in terms of the amount, in micromoles per 10 ml., of leucine which may be derived from these peptides. Thus, 0.05 pM of L-leucyl-L-leucine (L-L-L-L) will be given as 0.10 PM, and this may be compared directly with 0.10 FM of L-leucine or L-leucylglycine (L-LG). Peptides of L-Leucine As Inhibitors-In most cases, the growth curves for equivalent concentrations of L-leucine and of L-leucine peptides varied