A Layer of Dead Cells at the Periphery Protects Biofilms from Antimicrobial Peptides

Abstract

Under stress or sub-optimal growth conditions, some bacterial species are able to aggregate into defensive structures known as biofilm. In biofilms, cells not only compete for resources, but also are able to benefit one another. To date, we do not have direct evidence on how cells in biofilm protect each other against antimicrobial peptides (AMPs). We have utilized a live microscopy platform to monitor the time-course of the attack of AMPs on Bacillus subtilis biofilms. Consistent with the recent discoveries of the absorption of AMPs into dead cells, we observe that cells on the periphery of a biofilm protect the interior cells by blocking the flux of AMP towards the interior of the biofilm. Our results demonstrate the dynamics of AMP activity on a dense microbial community where the growth and survival of the population is coordinated with various cell-to-cell communication processes. The experimental procedure and quantitative framework of our approach provides novel insight on how AMPs can be optimized for targeting bacterial populations rather than individual cells.