Modulation of quorum sensing in Pseudomonas aeruginosa through alteration of membrane properties

Abstract

Changes in the cellular envelope are major physiological adaptations that occur when micro-organisms encounter extreme environmental conditions. An appropriate degree of membrane fluidity is crucial for survival, and alteration of membrane lipids is an essential adaptive response. Emerging data suggest that microbial cells may recognize alterations in their membrane viscosity resulting from certain environmental changes as a trigger for adaptive cellular responses. InPseudomonas aeruginosa, the quorum-sensing (QS) system involves a complex regulatory circuitry that coordinates the expression of genes according to a critical population density. Interestingly, it has been shown that the QS system ofP. aeruginosacan also be activated by nutritional stress, independently of the cell density, and therefore may be part of a more general adaptive response to stressful environmental conditions. In order to examine the proposed link between membrane properties and stress signalling, the effects of genetically engineered alterations of the membrane phospholipid composition ofP. aeruginosaPAO1 on the activation of the stringent response and the QS system were examined. ThelptAgene encoding a functional homologue of PlsC, anEscherichia colienzyme that catalyses the second step of the phospholipid biosynthesis pathway, was identified and disrupted. Inactivation oflptAaltered the fatty acid profile of phospholipids and the membrane properties, resulting in decreased membrane fluidity. This resulted in a premature production of the QS signalsN-butanoyl- andN-hexanoyl-homoserine lactone (C4-HSL and C6-HSL) and a repression of 2-heptyl-3-hydroxy-4-quinolone (PQS) synthesis at later growth phases. The effects on C4- and C6-HSL depended upon the expression ofrelA, encoding the (p)ppGpp alarmone synthase, which was increased in thelptAmutant. Together, the findings support the concept that alterations in membrane properties can act as a trigger for stress-related gene expression.