Population Dynamics of Antimicrobial Peptides and Bacteria

Abstract

Antimicrobial peptides (AMPs) are broad-spectrum antibiotics that utilize electrostatics to target bacteria selectively. Like most antibiotics, AMPs need a minimum concentration to inhibit the growth of a bacterial population. Despite our knowledge of the molecular structures and membrane interactions of AMPs, we poorly understand AMP's dynamics at the cellular and the population level. Here we demonstrate that the minimum inhibitory concentration (MIC) of AMPs is strongly dependent on the cell density, even in dilute cultures where direct cell-to-cell interactions are minimal. We hypothesize that this dependence arises because individual cell absorbs a significant number of AMPs which considerably reduces the effective concentration of AMPs in the culture. To investigate this hypothesis, we used a live single-cell imaging platform to track fluorescently tagged AMPs and the time evolution of their translocation into bacteria. We also developed a chemical reaction-like model to elucidate the kinetics of interacting bacteria and AMPs. Our single-cell analysis shows that bacteria not only absorb a significant fraction of AMPs from the culture but also retain them even after cell death, which sequesters AMP's availability for attacking more cells.