Population Dynamics of Antimicrobial Peptides are Driven by Single-cell Heterogeneities and Retention of Peptides in Dead Cells*

Antimicrobial peptides (AMPs) are natural antibiotics of multicellular systems that utilize electrostatics to target bacteria selectively. Like most antibiotics, AMPs need a minimum concentration to inhibit the growth of a bacterial culture. Unlike other antibiotics, AMPs’ distribution and kinetics in the culture is dictated by electrostatics. In this talk, I present our recent data mapping a quantitative picture of these dynamics. Surprisingly, we evidence 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 is due to absorption of a significant number of AMPs in individual cells, which can reduce the effective concentration of AMPs in the culture. To investigate this hypothesis, we utilized a single-cell imaging platform to track dye-tagged AMPs and the time evolution of their translocation into the bacteria. Our single-cell analysis confirms that bacteria not only absorb a significant fraction of AMPs, but also retain them after cell death, which sequesters AMPs’ availability for attacking more cells. Further, we developed a theoretical model of which recapitulates experimental behavior using AMP retention hypothesis.