E. coli as a Model for the Description of the Antimicrobial Mechanism of a Cationic Polymer Surface: Cellular Target and Bacterial Contrast Response

In this study we use E. coli as a model to investigate the antimicrobial mechanism of a film made of a copolymer based on mPEG, MMA and DMAEMA and whose surface is active towards Gram-negative and Gram-positive bacteria. The polymer contains not quaternized amino groups able to generate a charged surface by protonation when in contact with water. For the purpose, we adopted a dual strategy based on the analysis of cell damage caused by contact with the polymer surface, and on the evaluation of the cell response to the surface toxic action. The lithic effect on the protoplasts of E. coli showed that polymer surface can affect the structure of cytoplasmic membranes while assays of calcein leakage from LUVs at different phospholipids composition indicated that action on membranes does not need a functionally active cell. On the other hand, the significant increase in sensitivity to actinomycin D demonstrates that polymer interferes also with the structure of the outer membrane, modifying its permeability. The study on gene expression, based on the analysis of the transcripts in a temporal window where the contact with the polymer is not lethal and the damage is reversible, showed that some key genes of the synthesis and maintenance of OM structure (fabR, fadR, fabA, waaA, waaC, kdsA, pldA, pagP), as well as regulators of cellular response to oxidative stress (soxS), are more expressed when bacteria are exposed to polymer surface. All together these results identified the outer membrane as the main cellular target of antimicrobial surface and indicated a specific cellular response to damage, providing more information on the antimicrobial mechanism. In perspective, data here reported could play a pivotal role in a microbial growth control strategy based not only on the structural improvements of the materials, but also on the possibility of intervening on the cellular pathways involved in the contrast reaction to these and other polymers with similar mechanisms.