The action of antimicrobial peptide LL37 is slow but effective against non-growing Escherichia coli cells

Antimicrobial peptides (AMPs) are amino-acid based antibiotics that primarily attack bacteria by perturbing their membranes. It has long been suggested that AMPs are effective against non-growing cells that tolerate conventional antibiotics. Despite recent advances in quantifying the action of AMPs at the single-cell level, we still do not have a clear picture of how this action is correlated with the physiology of target cells. Here we take complementary approaches, from single-cell and population-level experiments, to investigate the efficacy of human AMP LL37 against Escherichia coli cells at different growth phases. We first analyze time-lapse, single-cell data of the action of LL37 peptides on exponentially growing cells, which reveals that they act faster on long, dividing cells than on small, newborn cells. Next, we test the consequence of this cell-age dependence on the efficacy of LL37 against non-growing E. coli cells in stationary phase. We observe a consistent trend that the action of LL37 is, on average, {approx}5.0 minutes slower on non-growing cells as compared to that on exponentially growing cells. However, this difference in the rate of action is not reflected in the minimum bactericidal concentration (MBC) of LL37 peptides. Contrary to our expectation, the MBC measured for non-growing cells is smaller than that for exponentially growing cells, indicating that over a long period of time the LL37 peptides are more potent against non-growing cells. Author summaryAntibiotic treatments can fail due to regrowth of a bacterial subpopulation that start proliferation after the treatment is over. The regrowth is often from non-growing, dormant cells that persist the action of antibiotics without being resistant. In this work, we demonstrate that human antimicrobial peptide LL37 is potent against non-growing Escherichia coli cells.