Student - April 2, 2009


Pseudomonas bacteria defend themselves against hungry attackers by excreting soapy substances. This is one of the results of phytopathologist Dr. Irene de Bruijn’s PhD research.

In her thesis, De Bruijn focused on two closely related ‘cyclic lipopeptides’ produced by the Pseudomonas fluorescens bacteria. Numerous single-celled organisms make use of this kind of soapy substance to move, to penetrate other organisms, or to attach themselves and form a biofilm. They are of interest to the Phytopathology Laboratory chiefly because of their protective properties for food crops. For example, thanks to its excretory products, P. fluorescens keeps the phytophtora plant disease away from tomato plants.

De Bruijn investigated which genes the bacteria use to produce the lipopeptides massetolide and viscosin, and which genes are involved in regulating this production. ‘It is important to identify the molecular mechanism to open up the possibility of developing the bacteria into an organic pesticide’, says De Bruijn.

Her research led to the discovery of a series of new genes. She also succeeded in transferring genetic material for forming lipopeptides from one pseudomonas strain to the other, which means it might be possible in future to cause bacteria to produce lipopeptides with extra protective properties.

Molecular genetic research has also revealed that the bacteria dose the production of the soapy connective substance in response to outside stimuli. One of these, De Bruijn and her American colleagues discovered, is the presence of single-celled ‘scavengers’, protozoa which ‘graze’ on colonies of pseudomonas. This predation leads to increased soap production – which signal is responsible for this is not yet clear – causing the protozoa to burst open. The effectiveness of this means of defence was revealed by a test with pseudomonas bacteria which were robbed of the genes to produce viscosin and massetolide. Both in the lab and in soil enriched with protozoa, these bacteria suffered far more from grazing and were present in far lower densities than the bacteria that were armed against the protozoa. / Rik Nijland

Irene de Bruijn received her PhD on Tuesday 31 March from Pierre de Wit, Professor of Phytopathology