# The Pseudomonas aeruginosa Cpx system provides a cyclic-di-GMP independent link between cell envelope stress and surface sensing

## TL;DR

This paper shows that the Cpx system in Pseudomonas aeruginosa helps the bacteria sense surfaces and form biofilms without relying on a key signaling molecule, expanding our understanding of how these bacteria cause infections.

## Contribution

The study identifies a novel Cpx signaling pathway in Pseudomonas aeruginosa that enables surface sensing independently of cyclic-di-GMP.

## Key findings

- The P. aeruginosa Cpx system includes two unique adaptor proteins, cpxM and cpxH.
- Cpx is activated by surface attachment through a mechanism independent of cyclic-di-GMP.
- Cpx influences gene expression related to antibiotic resistance, biofilm matrix production, and redox homeostasis.

## Abstract

Bacteria surveil their cell envelope through a network of envelope stress response systems (ESRs). Beyond regulation of envelope maintenance, ESRs influence expression of a range of virulence traits among pathogenic bacteria. The Cpx two-component system, a conserved ESR, responds to envelope stress generated by bacterial contact with a solid surface. This feature is particularly interesting in the context of bacterial biofilm formation, a key virulence trait of the opportunistic pathogen Pseudomonas aeruginosa, which initiates biofilm formation upon detecting envelope stress-related cues of surface adhesion. While a putative Cpx system is present in P. aeruginosa, it exhibits dissimilarities from orthologous systems and has not been evaluated for its roles in stress signaling and/or biofilm formation. Here, we found that the P. aeruginosa Cpx system includes two previously uncharacterized adaptor protein genes, cpxM (PA3203) and cpxH (PA3207), unique to the genus Pseudomonas. P. aeruginosa Cpx functions as an ESR and is responsive to stimuli related to outer membrane protein dysbiosis. Cpx is also activated upon surface attachment by a mechanism independent of the nucleotide second messenger cyclic-di-GMP (c-di-GMP), a global regulator of P. aeruginosa biofilm formation. We further show that the Cpx system influences gene expression related to antibiotic resistance, biofilm matrix production, iron acquisition, and redox homeostasis. These findings present an expanded view of envelope stress signaling in P. aeruginosa surface sensing, demonstrating that this biofilm-inducing stimulus is transmitted through both c-di-GMP-dependent and -independent signaling arms.

Pseudomonas aeruginosa is an opportunistic bacterial pathogen that causes chronic infection in humans by forming protective, multicellular structures called biofilms. The strong natural resistance of bacterial biofilms to antibiotic and immune clearance presents a major therapeutic obstacle in P. aeruginosa disease management. As these structures often assemble on surfaces, i.e. host tissues or indwelling medical devices, the ability of P. aeruginosa to sense and respond to surface contact is a key step in initiating biofilm formation. We report that the Cpx signaling system in P. aeruginosa is activated upon surface attachment and operates independently of other known surface-sensing systems. Cpx responds to cellular stress, particularly disruptions to cell-surface proteins, suggesting that stress generated by bacterial surface adhesion is a relevant biofilm-inducing signal. These findings expand knowledge of surface-sensing mechanisms in P. aeruginosa and link surface recognition to a variety of other disease-related cellular processes regulated by the Cpx system.