More to Pseudomonads than the kiwifruit crisis….

By Siouxsie Wiles 14/11/2010

For anyone who has missed the news this past week, the NZ kiwifruit industry is potentially in big trouble with a growing number of orchards infected with Pseudomonas syringae pathovar* (pv) actinidiae (PSA). PSA causes kiwifruit canker**. The halo they produce on infected leaves is caused by a toxin whose production seems to be regulated by temperature.

I have to confess to having a bit of a love-hate relationship with the genus Pseudomonas. I spent many long hours in their company during my PhD studies. On the other hand, many of them are also naturally fluorescent***. Which is why I thought I’d tell you a little more about some members of this amazing group of microbes.

There are over 100 species in the genus Pseudomonas and they are ubiquitous in the environment. You will find them in soil, water, hospitals… Their success is mostly due to the fact that they are able to metabolise a wide variety of nutrients. I tend to think of them as  ‘Jacks of all trades’.

First up is Pseudomonas aeruginosa, which is an opportunistic pathogen (basically it requires some some of break-down in the immune system, perhaps from an injury, or as a result of an immunosuppressive treatment, genetic defect or infection). It can cause infections of the urinary tract, respiratory system, soft tissue, and bones. It is particularly good at infecting patients with severe burns, HIV, cancer and cystic fibrosis (CF).  A study in Lithuania put the mortality due to P. aeruginosa in the intensive care unit setting at over 50%.

Next is Pseudomonas putida – numerous strains are capable of degrading a wide range of carbon sources including benzene, toluene and napthalene. This makes them useful as bioremediation agents for cleaning up environmental contaminants.

And last but not least, Pseudomonas fluorescens. Some strains have been shown to protect plants from infection the when added to the soil (a process known as biocontrol). There are numerous mechanisms by which they can do this, including out-competing pathogens in the soil or producing antimicrobial compounds. Indeed, the antibiotic Mupirocin was originally isolated from a strain of P. fluorescens.

P. fluorescens strain SBW25 has also been used to investigate the real-time evolution of diversity by Paul Rainey and colleagues. They discovered that just five days incubation in a broth-filled tube was enough for the founding bug to diversify from looking round and smooth when grown on an agar plate into the wonderfully named wrinkly and fuzzy  spreaders. These experiments have been going on for years now and the wrinklies and fuzzies have been joined by cart wheels, fried eggs and gnome hats, to name just a few. If you know anyone who doesn’t believe in evolution, please point them to Paul’s work.

So there you have it. There is much more to Pseudomonas than kiwifruit canker.

* A pathovar is a bacterial strain or set of strains that can be differentiated into subspecies on the basis of distinctive pathogenicity to particular plant hosts. There are over 50 pathovars of P. syringae.

** A defined area of diseased tissue

*** Just to clarify, fluorescence is not the same as bioluminescence but close enough to keep me interested!

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