Identification and characterization of a novel inhibitor of alginate overproduction in Pseudomonas aeruginosa

Diversity of extracytoplasmic function sigma (σECF ) factor-dependent signaling in Pseudomonas

Pseudomonas bacteria are widespread and are found in soil and water, as well as pathogens of both plants and animals. The ability of Pseudomonas to colonize many different environments is facilitated by the multiple signaling systems these bacteria contain that allow Pseudomonas to adapt to changing circumstances by generating specific responses. Among others, signaling through extracytoplasmic function σ (σ^ECF ) factors is extensively present in Pseudomonas. σ^ECF factors trigger expression of functions required under particular conditions in response to specific signals. This manuscript reviews the phylogeny and biological roles of σ^ECF factors in Pseudomonas, and highlights the diversity of σ^ECF -signaling pathways of this genus in terms of function and activation. We show that Pseudomonas σ^ECF factors belong to 16 different phylogenetic groups. Most of them are included within the iron starvation group and are mainly involved in iron acquisition. The second most abundant group is formed by RpoE-like σ^ECF factors, which regulate the responses to cell envelope stress. Other groups controlling solvent tolerance, biofilm formation and the response to oxidative stress, among other functions, are present in lower frequency. The role of σ^ECF factors in the virulence of Pseudomonas pathogenic species is described.

Pseudomonas aeruginosa Regulated Intramembrane Proteolysis: Protease MucP Can Overcome Mutations in the AlgO Periplasmic Protease To Restore Alginate Production in Nonmucoid Revertants

The progression of cystic fibrosis (CF) from an acute to a chronic disease is often associated with the conversion of the opportunistic pathogen Pseudomonas aeruginosa from a nonmucoid form to a mucoid form in the lung. This conversion involves the constitutive synthesis of the exopolysaccharide alginate, whose production is under the control of the AlgT/U sigma factor. This factor is regulated posttranslationally by an extremely unstable process and has been commonly attributed to mutations in the algT (algU) gene. By exploiting this unstable phenotype, we isolated 34 spontaneous nonmucoid variants arising from the mucoid strain PDO300, a PAO1 derivative containing the mucA22 allele commonly found in mucoid CF isolates. Complementation analysis using a minimal tiling path cosmid library revealed that most of these mutants mapped to two protease-encoding genes, algO, also known as prc or PA3257, and mucP Interestingly, our algO mutations were complemented by both mucP and algO, leading us to delete, clone, and overexpress mucP, algO, mucE, and mucD in both wild-type PAO1 and PDO300 backgrounds to better understand the regulation of this complex regulatory mechanism. Our findings suggest that the regulatory proteases follow two pathways for regulated intramembrane proteolysis (RIP), where both the AlgO/MucP pathway and MucE/AlgW pathway are required in the wild-type strain but where the AlgO/MucP pathway can bypass the MucE/AlgW pathway in mucoid strains with membrane-associated forms of MucA with shortened C termini, such as the MucA22 variant. This work gives us a better understanding of how alginate production is regulated in the clinically important mucoid variants of Pseudomonas aeruginosaIMPORTANCE Infection by the opportunistic pathogen Pseudomonas aeruginosa is the leading cause of morbidity and mortality seen in CF patients. Poor patient prognosis correlates with the genotypic and phenotypic change of the bacteria from a typical nonmucoid to a mucoid form in the CF lung, characterized by the overproduction of alginate. The expression of this exopolysaccharide is under the control an alternate sigma factor, AlgT/U, that is regulated posttranslationally by a series of proteases. A better understanding of this regulatory phenomenon will help in the development of therapies targeting alginate production, ultimately leading to an increase in the length and quality of life for those suffering from CF.

Extracytoplasmic function sigma factors in Pseudomonas aeruginosa

The opportunistic pathogen Pseudomonas aeruginosa, like all members of the genus Pseudomonas, has the capacity to thrive in very different environments, ranging from water, plant roots, to animals, including humans to whom it can cause severe infections. This remarkable adaptability is reflected in the number of transcriptional regulators, including sigma factors in this bacterium. Among those, the 19 to 21 extracytoplasmic sigma factors (ECFσ) are endowed with different regulons and functions, including the iron starvation σ (PvdS, FpvI, HasI, FecI, FecI2 and others), the cell wall stress ECFσ AlgU, SigX and SbrI, and the unorthodox σ^VreI involved in the expression of virulence. Recently published data show that these ECFσ have separate regulons although presenting some cross-talk. We will present evidence that these different ECFσ are involved in the expression of different phenotypes, ranging from cell-wall stress response, production of extracellular polysaccharides, formation of biofilms, to iron acquisition.

σ Factor and Anti-σ Factor That Control Swarming Motility and Biofilm Formation in Pseudomonas aeruginosa

Pseudomonas aeruginosa is capable of causing a variety of acute and chronic infections. Here, we provide evidence that sbrR (PA2895), a gene previously identified as required during chronic P. aeruginosa respiratory infection, encodes an anti-σ factor that inhibits the activity of its cognate extracytoplasmic-function σ factor, SbrI (PA2896). Bacterial two-hybrid analysis identified an N-terminal region of SbrR that interacts directly with SbrI and that was sufficient for inhibition of SbrI-dependent gene expression. We show that SbrI associates with RNA polymerase in vivo and identify the SbrIR regulon. In cells lacking SbrR, the SbrI-dependent expression of muiA was found to inhibit swarming motility and promote biofilm formation. Our findings reveal SbrR and SbrI as a novel set of regulators of swarming motility and biofilm formation in P. aeruginosa that mediate their effects through muiA, a gene not previously known to influence surface-associated behaviors in this organism.

IMPORTANCE

This study characterizes a σ factor/anti-σ factor system that reciprocally regulates the surface-associated behaviors of swarming motility and biofilm formation in the opportunistic pathogen Pseudomonas aeruginosa. We present evidence that SbrR is an anti-σ factor specific for its cognate σ factor, SbrI, and identify the SbrIR regulon in P. aeruginosa. We find that cells lacking SbrR are severely defective in swarming motility and exhibit enhanced biofilm formation. Moreover, we identify muiA (PA1494) as the SbrI-dependent gene responsible for mediating these effects. SbrIR have been implicated in virulence and in responding to antimicrobial and cell envelope stress. SbrIR may therefore represent a stress response system that influences the surface behaviors of P. aeruginosa during infection.

σ Factor and Anti-σ Factor That Control Swarming Motility and Biofilm Formation in Pseudomonas aeruginosa

Pseudomonas aeruginosa is capable of causing a variety of acute and chronic infections. Here, we provide evidence that sbrR (PA2895), a gene previously identified as required during chronic P. aeruginosa respiratory infection, encodes an anti-σ factor that inhibits the activity of its cognate extracytoplasmic-function σ factor, SbrI (PA2896). Bacterial two-hybrid analysis identified an N-terminal region of SbrR that interacts directly with SbrI and that was sufficient for inhibition of SbrI-dependent gene expression. We show that SbrI associates with RNA polymerase in vivo and identify the SbrIR regulon. In cells lacking SbrR, the SbrI-dependent expression of muiA was found to inhibit swarming motility and promote biofilm formation. Our findings reveal SbrR and SbrI as a novel set of regulators of swarming motility and biofilm formation in P. aeruginosa that mediate their effects through muiA, a gene not previously known to influence surface-associated behaviors in this organism.

IMPORTANCE

This study characterizes a σ factor/anti-σ factor system that reciprocally regulates the surface-associated behaviors of swarming motility and biofilm formation in the opportunistic pathogen Pseudomonas aeruginosa. We present evidence that SbrR is an anti-σ factor specific for its cognate σ factor, SbrI, and identify the SbrIR regulon in P. aeruginosa. We find that cells lacking SbrR are severely defective in swarming motility and exhibit enhanced biofilm formation. Moreover, we identify muiA (PA1494) as the SbrI-dependent gene responsible for mediating these effects. SbrIR have been implicated in virulence and in responding to antimicrobial and cell envelope stress. SbrIR may therefore represent a stress response system that influences the surface behaviors of P. aeruginosa during infection.