The influence of membrane components on the uptake of peptides by gram-negative bacteria

The number and specificities of peptide transport systems in Escherichia coli are investigated using direct and sensitive fluorescence assays and transport-deficient mutants isolated as resistant to peptide mimetic antibiotics. It is shown that there are three peptide permeases of which the oligopeptide permease (Opp) is confirmed as being capable of transporting a wide range of di- and oligopeptides and the dipeptide permease (Dpp) and third peptide permease (Opt) are both shown to have wider specificities than has been previously reported. The Opt is shown to be energised directly by phosphate bond energy and is coded for by a gene mapping between 84 and 88 min. The applicability of monitoring the kinetics of peptide uptake as a means of measuring outer membrane permeability is studied. Using this method with wild type and outer membrane protein deficient strains, the porins are shown to be the major route for peptide diffusion across the outer membrane. The advantages of using this approach, and the possible mechanisms of the decrease in permeability caused by loss of OmpA, are discussed.