Shielding, a new pathogen defense mechanism against polymorphonuclear neutrophilic leukocytes (PMNs).

Microbiology. 2009 Sep 3; Cornelis PIn an article published in this issue, Alhede et al. demonstrate that the opportunistic pathogen Pseudomonas aeruginosa can react to the presence of PMNs by producing abundant quantities of rhamnose-containing glycolipid biosurfactants, the rhamnolipids. PMNs are phagocytic cells and are important players in the innate immune response since they produce a range of antimicrobial molecules able to kill pathogens. Rhamnolipids are known to cause fast lysis of amoebae such as Dictyostelium discoideum cells. Rhamnolipids have previously been shown by the same research group to cause lysis of several cellular components of the human immune system, e.g. monocyte-derived macrophages and PMNs. In P. aeruginosa, rhamnolipid synthesis is under control of the quorum sensing system (QS), particularly via the RhlI-RhlR C4-homoserine lactone synthase/LuxR regulator system. To demonstrate the importance of rhamnolipids as shield against PMNs, the authors tested the capacity of an rhlA mutant (unable to produce these biosurfactants) to survive in the lungs of infected mice and found a drastic reduction of colony forming units compared to the wild type. Like other pathogens, P. aeruginosa is capable of forming biofilms. This is particularly the case in the lungs of cystic fibrosis patients where P. aeruginosa forms biofilms in the thick mucus layer. In this study the authors measured the release of lactate dehydrogenase by PMNs in contact with in vitro P. aeruginosa biofilms. They demonstrated that a large fraction of the PMNs exposed to the wild type biofilm rapidly lysed while no PMNs lysed after exposure to the rhlA mutant, demonstrating the importance of rhamnolipids production as a strategy to aggressively respond to PMNs. However, Alhede et al. found that in vitro biofilms produce very little rhamnolipids, which at first seems to contradict the rapid lysis observed by the authors. To investigate this, Alhede et al. demonstrated that exposure to PMNs acts as a signal triggering the fast production of rhamnolipids which stick to the cell paste rather than being released in the surrounding fluid. This suggests that PMNs become eliminated by contact with the biofilm. Collectively, the results presented by Alhede et al. suggest that biofilm P. aeruginosa cells can sense the presence of PMNs and respond by producing the rhamnolipid shield. A transcriptome analysis of the response of biofilm P. aeruginosa cells to exposure to PMNs confirmed the up-regulation of QS-dependent genes (85% of up-regulated genes) among which were the rhlA and rhlB genes for rhamnolipids production. The response to PMNs seems to be orchestrated by another QS signal molecule, the Pseudomonas quinolone signal (PQS), responsible for the induction of different virulence factors. The biosynthetic pqs operon is indeed up-regulated during PMN exposure suggesting increase in PQS signal production. Confirming the importance of the PQS signal molecule in the induction of rhamnolipid production upon PMNs exposure, a pqsA PQS deficient mutant was unable to produce the shield response.