Agr-Mediated Dispersal of Staphylococcus aureus Biofilms

Blaise R Boles,Alexander R Horswill,Pascale Cossart

doi:10.1371/journal.ppat.1000052

Abstract

The agr quorum-sensing system of Staphylococcus aureus modulates the expression of virulence factors in response to autoinducing peptides (AIPs). Recent studies have suggested a role for the agr system in S. aureus biofilm development, as agr mutants exhibit a high propensity to form biofilms, and cells dispersing from a biofilm have been observed displaying an active agr system. Here, we report that repression of agr is necessary to form a biofilm and that reactivation of agr in established biofilms through AIP addition or glucose depletion triggers detachment. Inhibitory AIP molecules did not induce detachment and an agr mutant was non-responsive, indicating a dependence on a functional, active agr system for dispersal. Biofilm detachment occurred in multiple S. aureus strains possessing divergent agr systems, suggesting it is a general S. aureus phenomenon. Importantly, detachment also restored sensitivity of the dispersed cells to the antibiotic rifampicin. Proteinase K inhibited biofilm formation and dispersed established biofilms, suggesting agr-mediated detachment occurred in an ica-independent manner. Consistent with a protease-mediated mechanism, increased levels of serine proteases were detected in detaching biofilm effluents, and the serine protease inhibitor PMSF reduced the degree of agr-mediated detachment. Through genetic analysis, a double mutant in the agr-regulated Aur metalloprotease and the SplABCDEF serine proteases displayed minimal extracellular protease activity, improved biofilm formation, and a strongly attenuated detachment phenotype. These findings indicate that induction of the agr system in established S. aureus biofilms detaches cells and demonstrate that the dispersal mechanism requires extracellular protease activity.

Highlights

Most bacteria have an inherent ability to form surface-attached communities of cells called biofilms [1]
The S. aureus quorum-sensing system is encoded by the accessory gene regulator locus and the communication molecule that it produces and senses is called an autoinducing peptide (AIP), which is an eight-residue peptide with the last five residues constrained in a cyclic thiolactone ring [15]
Biofilm-encased cells are protected from antibiotic therapy and host immune response, and these encased cells can develop into a chronic infection

Summary

Introduction

Most bacteria have an inherent ability to form surface-attached communities of cells called biofilms [1]. Studies on the opportunistic pathogen Pseudomonas aeruginosa indicate that cell-to-cell communication (often termed ‘‘quorumsensing’’) is required to make a robust biofilm under some growth conditions [12]. This study demonstrated that bacteria dispersing from biofilms displayed high levels of agr activity, while cells in a biofilm had predominantly repressed agr systems. These findings correlate well with prior data indicating that agr deficient S. aureus strains form more robust biofilms compared to wild type strains [13,14]

Methods

Discussion

Conclusion