Communication Breakdown: Into the Molecular Mechanism of Biofilm Inhibition by CeO₂ Nanocrystal Enzyme Mimics and How It Can Be Exploited

Abstract

Bacterial biofilm formation is a huge problem in industry and medicine. Therefore, the discovery of anti-biofilm agents may hold great promise. Biofilm formation is usually a consequence of bacterial cell-cell communication, a process called quorum sensing (QS). CeO₂ nanocrystals (NCs) have been established as haloperoxidase (HPO) mimics and ecologically beneficial biofilm inhibitors. They were suggested to interfere with QS, a mechanism termed quorum quenching (QQ), but their molecular mechanism remained elusive. We show that CeO₂ NCs are effective QQ agents, inactivating QS signals by bromination. Catalytic bromination of 3-oxo-C₁₂-AHL a QS signaling compound used by <i>Pseudomonas aeruginosa</i>, was detected in the presence of CeO₂ NCs, bromide ions, and hydrogen peroxide. Brominated acyl-homoserine lactones (AHLs) no longer act as QS signals but were not detected in the bacterial cultures. Externally added brominated AHLs also disappeared in <i>P. aeruginosa</i> cultures within minutes of their addition, indicating that they are rapidly degraded by the bacteria. Moreover, we detected the catalytic bromination of 2-heptyl-1-hydroxyquinolin-4(1<i>H</i>)-one (HQNO), a multifunctional non-AHL QS signal from <i>P. aeruginosa</i> with antibacterial and algicidal properties controlling the expression of many virulence genes. Brominated HQNO was not degraded by the bacteria <i>in vivo.</i> The repression of the <i>Pseudomonas</i> quinolone signal (PQS) production and biofilm formation in <i>P. aeruginosa</i> through the catalytic formation of Br-HQNO on surfaces with coatings containing CeO₂ enzyme mimics validates the non-toxic strategy for the development of anti-infectives.