Multivalent antimicrobial polymer nanoparticles target mycobacteria and gram-negative bacteria by distinct mechanisms

Due to the emergence of antimicrobial resistance to traditional small molecule drugs, cationic antimicrobial polymers are appealing targets. Mycobacterium tuberculosis is a particular problem, with multi- and total drug resistance spreading and more than a billion latent infections globally. This study reports nanoparticles bearing variable densities of poly(dimethylaminoethyl methacrylate) and the unexpected and distinct mechanisms of action this multivalent presentation imparts against Escherichia coli verses Mycobacterium smegmatis (model of M. tuberculosis), leading to killing or growth inhibition respectively. A convergent ‘grafting to’ synthetic strategy was used to assemble a 50-member nanoparticle library and using a high- throughput screen identified that only the smallest (2 nm) particles were stable in both saline and complex cell media. Compared to the linear polymers, the nanoparticles displayed 2- and 8-fold enhancements in antimicrobial activity against M. smegmatis and E. coli respectively. Mechanistic studies demonstrated that the antimicrobial particles were bactericidal against E. coli, due to rapid disruption of the cell membranes. Conversely, against M. smegmatis the particles did not lyse the cell membrane but rather had a bacteriostatic effect. These results demonstrate that to develop new polymeric anti-tuberculars the widely assumed, broad spectrum, membrane-disrupting mechanism of polycations must be re-evaluated. It is clear that synthetic nanomaterials can engage in more complex interactions with mycobacteria, which we hypothesise is due to the unique cell envelope at the surface of these bacteria