Bacterial pathogenicity and the evolution of pathogenicity
Pathogens have, by definition, evolved to target specific hosts to gain nutrients and enable replication. A comprehensive understanding of how pathogens do this is a major goal around the world. We work with Pseudomonas syringae pathogens of plants. These bacteria infect a wide range of plants and are sub-divided into pathogenic varieties (pathovars) depending on the host they were first isolated from and cause disease in. The major focus in this field is a specific protein secretion system (the type III system) that transfers effector (virulence) proteins from the pathogen cell into the cells of the plant host. However, it is very easy to lose sight of the fact that these pathogens do more than just use this secretion system and indeed the process of pathogenicity should consider pathogen ecology as well as examine infection and pathogenicity. To this end, Federico Dorati is developing an in vivo expression technology (IVET) system for Pseudomonas syringae pv. phaseolicola strain 1448A to identify the molecular basis of bacterial colonisation of bean plants. This includes the root environment (rhizosphere), the leaf and shoot surfaces (phyllosphere), the inside of the leaf (apoplast) and the seed. By examining gene expression in these different niches we hope to gain a holistic picture of the strategy used by the pathogen to colonise and infect plants and then identify potential ways that pathogen spread and infection can be controlled.
Plant pathogens are effectively in an arms race with plant hosts with each evolving strategies to overcome the other. We are interested in how bacterial pathogens evolve to overcome plant resistance. We collaborate with Dawn Arnold (UWE, Bristol) and John Mansfield (Imperial College) on pathogenicity island (see comparative figure below) genomic island biology and the role they play in shaping bacterial pathogenicity. We are also investigating effector integrons in Pseudomonas in collaboration with Steve Dorus (University of Bath).
Bacterial form and function
All organisms undergo phenotypic acclimation to various stimuli, i.e. they reversibly alter gene and protein expression to cope with changes in their environment.
Phenotypic acclimation is evident in bacteria when they colonise plants, in that they specifically upregulate genes that leads to the expression of traits that favour plant colonisation. Outside of the plant, many gene systems are downregulated. We are interested in understanding the systems used by bacteria to colonise plants, to understand how these systems (and the traits they control) contribute to ecological success (fitness – how well they grow and replicate) in the plant environment and how the expression of these systems are regulated. Although small in size, it is clear that bacterial phenotypic acclimation is anything but simple. In a recent study, we used a technique, SPyVET, to identify several regulatory genes that controlled expression of plant environment inducible genes in the saprophytic bacterium Pseudomonas fluorescens. Furthermore, we were able to make a link between regulator, inducible gene system and phenotypes expressed by the bacterium (see figure, left).
One very interesting outcome of the SPyVET analysis was the identification of a 7-regulator hierarchy controlling expression of the wss gene cellulose synthase operon. Cellulose is an extracellular polysaccharide that is important for plant colonisation and is also a major component of Pseudomonas fluorescens biofilm matrix. Of particular interest are the regulators FleQ, AlgR and AmrZ as they appear to be involved in the regulation of multiple gene systems (flagellum genes, cellulose genes, hyrdrogen peroxide resistance genes) and traits (flagellum biosynthesis, swimming and swarming motility, and oxidative stress). One unusual feature of the fleQ knockout mutant is the change in swarming motility from flagellum-dependent swarming in a uniform fashion to a flagellum-independent non-uniform, “spidery spreader” phenotype (see figure, right). Abdullah Alsohim is using a clean fleQ knockout mutant (created by our collaborators Jenna Gallie, Xue-Xian Zhang and Paul Rainey) in a suppressor analysis to identify the molecular mechanisms underlying this phenomenon. He will further use his mutants to analyse the role in bacterial ecological success.
Bacterial survival in the plant environment – how to survive animal ingestion
The lifecycle for many bacteria is such that they live in the big, wide world and thus have to cope with the associated abiotic and biotic stresses. Plant associated bacteria may be disseminated within leaf litter to the soil, water and air systems and only streetwise bacteria will cope with the various stresses encountered in these environments. One major problem that bacteria face is ingestion by animals and protozoa. This may take the form of nematodes and protozoa specifically targeting bacterial cells, earthworms that feed a la basking sharks and eat clods of earth or leaf litter containing bacteria, or ingestion by insects that feed on plant tissue – a problem if you are a bacterium and happen to be attached to the piece of plant tissue that is eaten. It makes sense therefore that bacteria may have evolved strategies to avoid being eaten or survive the ingestion process. In collaboration with Nick Waterfield, Maria Sanchez-Contreras (University of Bath) and Dawn Arnold (University of the West of England), Abdullah Alsohim and Federico Dorati are examining Pseudomonas bacteria to determine how they interact with various predators and to identify the molecular mechanisms involved.
Bleeding canker disease of Horse Chestnut
We have been studying Pseduomonas syringae pv. aesculi, which causes bleeding canker in Horse Chestnut trees. This disease is starting to emerge as major problem within the UK and Forest Research has estimated that 50% of the nations Horse Chestnut trees are affected by the pathogen. Since the disease can eventually lead to death, then this pathogen may have a major consequence on the Horse Chestnut population akin to that seen for Dutch Elm disease. Federico Dorati has been analysing strains provided by Richard Thwaites (Central Science Laboratory) for pathogenicity and host specificity. The genome sequences of four strains are being studied in collaboration with David Studholme (University of Exeter), Sophien Kamoun (The Sainsbury Laboratory) and Sarah Green (Forest Research).
Escherichia coli 0157 in the plant rhizosphere
Escherichia coli 0157 is found in the gut of farm animals, especially cattle and is spread onto pastures via defecation. Although E. coli is usually regarded as a gut pathogen that cannot easily exist outside of an animal body, E. coli 0157 can survive within the plant environment, but very little is known about how it does this. In collaboration with Liz Shaw, Simon Andrews, Penny Hirsch and Tim Mauchline, we are using gene discovery methods to this bacterium to gain insight to the strategies used to survive in the plant environment. Additionally, a new studentship will begin in October 2010 in collaboration with Simon Andrews, Nicola Holden and Ian Toth to examine the transcriptome of E. coli 0157 during plant colonisation.