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Schizophrenic antifungal bacteria


Dutch researcher Daniël van den Broek investigated bacteria which fight fungal infections in plants. Spontaneous variations in the phase of these bacteria reduce the anti-fungal protective function but increase the bacteria’s competitive advantage and with this their chances of survival.

Scientists worldwide are looking for biological alternatives to chemical pesticides in order to protect crops against pathogens. One of these is the use of bacteria which protect plants, for example, by producing antifungal substances. Daniël van den Broek investigated how to improve the reliability of crop protection methods that use these microorganisms.

Van den Broek studied Pseudomonas bacteria. The molecular biologist first of all isolated and described these bacteria, which are found in or on the roots of maize. Of the 214 strains isolated, 46 were found to suppress the growth of pathogenic fungi, for example those which cause black root rot. In total 43 of these 46 medicinal strains spontaneously switched between the two phases.

Van den Broek discovered that these switches were caused by the spontaneous mutation of certain genes. As these mutations are reversible, the bacteria can switch back and forth between the two phases. Spontaneous mutants no longer produced drugs against the fungi, but they grew faster than their fungal-controlling alter egos.


The phase variation mechanism in these so-called biocontrol bacteria is reversible. Therefore switching between the two phases enables the entire population to respond to changes more quickly. This is a clear competitive advantage. Stressful conditions result in a switch to the non-medicinal form, which increases the competitive and survival chances of the bacteria. If the balance between the medicinal and the non-medicinal forms tips too far the wrong way, the bacteria can no longer adequately control the fungal infections.

Although more than 100 crop protection products based on microorganisms are available throughout the world, bacteria are still not widely used in these as they are not reliable enough in the field situation. Research needs to come up with methods to improve the efficiency and reliability of the bacteria, for example, by controlling the phase. Furthermore the phenomenon of phase variation has a negative effect on industrial processes such as the production of vaccines or enzymes. A better understanding of this phenomenon could contribute to improvements in vaccine development.

Daniël van den Broek’s research was funded by Technology Foundation STW.

Dr Daniël van den Broek | alfa
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