# Effect of Salt on Synthetic Cationic Antimicrobial Polymer–Cell Interactions

## TL;DR

This study shows how salt affects the action of a cationic antimicrobial polymer on bacteria, reducing its effectiveness but still inhibiting bacterial reproduction.

## Contribution

The novel contribution is identifying how salt concentration reversibly alters polymer-cell interactions and antimicrobial efficacy.

## Key findings

- At 0.15 M NaCl or higher, PDADMAC stops killing E. coli but prevents cell reproduction.
- Salt reduces PDADMAC adsorption and membrane rigidification in model systems.
- Cell growth can restart after PDADMAC removal, indicating a reversible effect.

## Abstract

Cationic antiseptics
are deployed in a variety of settings, where
salinity ranges from almost pure water to hypertonic salt. Here, we
examine how dissolved NaCl affects the antimicrobial action of a model
antimicrobial, polydiallyldimethylammonium chloride (PDADMAC) to the
bacterium Escherichia coli (E. coli). Fluorescence microscopy is used to measure
the time course of both the adsorption of PDADMAC to E. coli and the cell viability. NaCl decreases the
density of adsorbed PDADMAC and diminishes its efficacy. At NaCl concentrations
at or above 0.15 M, PDADMAC no longer kills bacteria but still prevents
reproduction by halting the growth in cell length. Reproduction can
be restarted if PDADMAC is removed. Fluorescence depolarization measurements
show that PDADMAC rigidifies model membranes, but salt reduces the
rigidity. We therefore attribute the halt in cell growth to reversible
bridging by the polymer on the cell surface that prevents expansion
of the cell membrane.