Molecular Machinery Responsible for Graphene Oxide’s Distinct Inhibitory Effects toward Pseudomonas aeruginosa and Staphylococcus aureus Pathogens

Abstract

Graphene oxide flakes are considered as potential inhibitors for different pathogenic bacteria. However, the efficacy of inhibition changes for different types and strains of bacteria. In this work, we examine Pseudomonas aeruginosa and Staphylococcus aureus, two common hospital-acquired infections, which react quite differently to graphene oxide flakes. The minimum inhibitory tests yield two distinct outcomes: stopped proliferation for S. aureus versus almost no effect for P. aeruginosa. Integrating our experimental evidence with molecular dynamics simulations, we elucidate the molecular machinery involved, explaining the behavior we see in scanning electron microscopy images. According to our simulations, the peptidoglycan network, the outermost layer of S. aureus, is completely entangled with the flakes, acting as a hunting ground, which consequently results in the inhibition of the pathogen itself. Lipopolysaccharides, the outermost layer of P. aeruginosa, on the other hand, resist interacting with the flakes. Lipopolysaccharides make no effective contacts, and thus, no effective inhibition of the pathogen takes place. Likewise, the electron microscopy images show a complete coverage and wrapping of S. aureus. In contrast, for P. aeruginosa, barely any bacteria are spotted with any flakes on top except for some loosely half-covered cases. As we did not observe any damaged bacteria in our images, we exclude the knife-cutting inhibition mechanism and suggest the wrapping and trapping mechanism for S. aureus for our flakes’ rather large size (an average area of 0.05 μm2). The molecular machinery suggested in this work can be used for molecular engineering and functionalizing graphene flakes to inhibit different pathogens.