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The Gram-negative bacterium Pseudomonas aeruginosa fascinates researchers, not only because it remains a dangerous pathogen to sufferers of cystic fibrosis, but also in light of its metabolic versatility and complex virulence mechanisms – this was reflected by the large and international Pseudomonas community that gathered in Lausanne, Switzerland for the 14th international meeting dedicated entirely to the genus.
My PhD research focuses on the mechanisms by which transcription of the Type-III secretion system (T3SS) master switch, HrpL, is regulated in the plant pathogen Pseudomonas syringae. My motivation for attending a conference more focused on P. aeruginosa, for which I’m very grateful to the BSPP Travel Fund for making possible, was to take advantage of a related but better characterised bacterial model especially in the developing areas of cyclic-di-GMP signalling, lifestyle switching, biofilm formation and secretion mechanisms.
It only took until the second keyonote lecture for P. syringae to feature! David Guttman of the University of Toronto presented an ingenious yet conceptually simple workflow for identifying novel PAMPs in the DC3000 pathovar and subsequently their corresponding receptors (PRRs) in Arabidopsis thaliana. The ‘PAMP paradox’, that PAMPs are essential, conserved proteins (subject to general negative selection against modification) that are able to evolve to evade recognition by the host plant (further subject to positive selection at specific sites), suggests that candidate PAMPs are most likely to be found within the subset of highly conserved genes that also exhibit signatures of positive selection. By comparing the genomes of over 50 bacterial phytopathogens, Guttman identified approximately 3800 conserved genes which are in turn subject to the Ka/Ks test for positive selection. During a high-throughput assay to identify which candidate PAMPs can elicit the innate immune response and suppress growth of DC3000 applied subsequently, A. thaliana leaf discs are challenged with specific peptides exhibiting signatures of positive selection. This method, which confirmed a number of novel PAMPs, was utilised further to screen a library of A. thaliana mutants for corresponding PRRs, which are revealed when candidate peptides can no longer elicit the innate immune response. One peculiar finding is that a receptor associated with the CLV3 pathway for apical meristem development also moonlights as a PRR.
Also of interest, Jacob Malone from the John Innes Centre studied the role of cyclic-di-GMP signalling in rhizosphere colonisation by the biocontrol agent Pseudomonas fluorescens and presented his focus on the RccA GGDEF/EAL domain protein and its transcriptional response regulator RccR.
Christoph Keel from the University of Lausanne has discovered that certain strains of root-colonising P. fluorescens and P. protogens also exhibit strong pathogenicity against herbivorous insects. He showed that the potent Fit toxin is specifically activated within the insect gut, most effectively in aphids and Lepidoptera, due to activation of a signal receptor of the DctB family. These both plant-beneficial and insect pathogenic strains therefore offer great agricultural promise.
A particular highlight that would interest all microbiologists alike was Lynne Turnbull’s super-resolution video footage of explosive biofilm cell lysis in real-time, which results in the release of extracellular DNA and formation of membrane vesicles. In close second was Jan Roelf van der Meer’s account of the mechanism by which mobile ‘integrated and conjugative elements’ (ICE) are activated for conjugation. As part of a bistable population response, these are excised from the chromosome in only a few ‘donor’ cells during stationary phase.
We were also encouraged by a number of speakers, in particular Marvin Whitely from Texas, to abandon the general preconception that laboratory monocultures are representative of natural environments and instead embrace the fact that the vast majority are home to polymicrobial communities engaged in complex synsergistic or competing relationships.
Finally a session focused on clinical care offered an opportunity for truancy into the rolling, vineyard terraces of Lavaux, just along the shore of Lake Geneva. However, even this excursion offered a lesson in plant pathology – the rose trees that frame the vines are not there only for their aesthetic beauty, but primarily as a hyper-sensitive host that forewarns of the approach of the powdery mildew fungus, a plague of the grapevine!
Christopher Waite Imperial College London