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Chapter 21 : Signal Trafficking with Bacterial Outer Membrane Vesicles
Category: Microbial Genetics and Molecular Biology; Bacterial Pathogenesis
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The gram-negative bacterium Pseudomonas aeruginosa is a ubiquitous opportunistic pathogen that causes infection in immunocompromised individuals, including those with the heritable disease cystic fibrosis (CF). Quorum sensing (QS) has been proposed to be important for colonization of the CF lung, and virulence studies indicate that inactivation of QS in P. aeruginosa significantly reduces virulence in mammalian, plant, and insect models. Pseudomonas quinolone signal (PQS) biosynthesis proceeds through a head-to- head condensation of anthranilic acid and β-keto-decanoic acid to form the immediate precursor of PQS, 2-heptyl-4-quinolone (HHQ). This chapter focuses on membrane vesicles (MVs), including their potential use as trafficking vehicles for a variety of cargo, including cell-cell signals. To be utilized as trafficking vehicles, MVs must (i) have the ability to deliver their cargo to other cells and (ii) possess physiologically relevant cargo, necessitating transfer between cells. MVs isolated from P. aeruginosa have significant antimicrobial activity, particularly against gram-positive bacteria. This antimicrobial activity is multifaceted, including both small molecule and protein components. Recent studies using thin sectioning and transmission electron microscopy revealed that MVs were consistently present in the biofilm EPS matrix.
A simplified model for P. aeruginosa QS. Each system utilizes a distinct chemical signal that is sensed by a corresponding transcriptional regulator. The signal/transcriptional regulator complex binds DNA and elicits changes in gene expression. Approximately 5% of all P. aeruginosa genes are regulated by QS ( 47 , 50 ).
Transmission electron micrograph of negatively stained P. aeruginosa MVs. Diameter of a single MV is shown for scale.
Structures of relevant quinolones produced by P. aeruginosa.
P. aeruginosa MVs have antimicrobial activity against gram-positive bacteria. P. aeruginosa cells (left) and P. aeruginosa MVs (right) were spotted onto a confluent lawn of Staphylococcus epidermidis. S. epidermidis killing is indicated by zones of lysis.
Proposed mechanisms of MV formation. (Model 1) In areas lacking peptidoglycan-outer membrane protein linkages, MVs form when the outer membrane grows faster than the underlying peptidoglycan layer. This causes the outer membrane to bulge and eventually “pinch off” to form MVs. (Model 2) During normal peptidoglycan turnover, soluble low-molecular-weight peptidoglycan fragments are not internalized efficiently by the cell, resulting in accumulation of these fragments in the periplasm. Accumulation of these small fragments exerts turgor pressure on the outer membrane, causing it to swell and form MVs. (Model 3) Ionic interactions between PQS and Mg2+ in the P. aeruginosa outer membrane enhances anionic repulsion between LPS molecules, resulting in membrane blebbing. LPS, lipopolysaccharide; OM, outer membrane; PG, peptidoglycan; IM, inner membrane. Figure reprinted from reference 36 with permission from Blackwell Publishing.