Science - August 21, 2019

Students combat feared Xylella bacteria

Albert Sikkema,Roelof Kleis

There's still no remedy for the feared Xylella bacteria. Students in Wageningen are trying to fight the bacteria with phage therapy.

The logo of Xylencer: a stylised bunch of grapes with a virus © Xylencer

Xylella fastidiosa is a bacteria that causes extensive damage to important crops such as grapes (California), olives (Italy) and citrus fruits (Brazil). But the bacteria isn’t choosy; it uses more than 400 different host plants. Fear of this bacteria is so widespread that it has a quarantine status in Europe. ‘If Xylella is found in your orchard, all of the plants within a radius of 100 meters have to be uprooted,’ explains Master’s student Hetty Huijs (Biotechnology).

Xylencer, a compound of Xyllella fatidiosa and silence, is the name of the project that Wageningen will enter into this year’s international competition for synthetic biology iGEM. The core of the approach developed by the students is to attack the bacteria with genetically modified bacteriophages. Bacteriophages (shorter form: phages) are viruses that use bacteria to multiply.

We release a protein that warns the plant that there is something wrong and that the immune system must be activated
Cleo Bagchus

The bacteria does its destructive work by settling in the xylem of the plant. The sap stream is disturbed, resulting in desiccation and death. There is no way to combat the disease. So this was a great challenge for the iGEM researchers, added Cleo Bagchus (Master’s Biotechnology and Molecular Life Sciences). ‘The problem has to be a serious one; that always helps in the competition.’

The students are focusing on grape production, where infection occurs via the leafhopper. And genetically modified phages, the natural enemy of Xylella, are used. The genome of the phages is expanded with the code for a couple of extra, specific proteins. One of those proteins helps to activate the immune system of the host plant so that the bacteria is noticed.  

‘Xylella is usually able to circumvent the plant’s immune system,’ says Bagchus. ‘We release a protein that warns the plant that something is wrong and that the immune system has to be activated. We give the immune system a push.’ Another protein ensures that the virus, just like the bacteria, binds easily to the leafhopper.’ We use the same binding protein so that not only the Xylella but also the phage is distributed.’

Handy trick
In other words, the solution is distributed just as easily along the same pathway (the leafhopper) as the disease. A handy trick, Huijs says. ‘It’s difficult to attach phages to trees, especially if you have to do that tree by tree. It’s difficult to reach all of the places where the bacteria may be. This is why we came up with this self-distributing phage.’ That’s the theory. Whether or not it will work in practice still has to be seen.

The Xylencer iGEM team consists of ten Master’s students and one Bachelor’s student in Wageningen. The group was formed this spring; in May the students began to work fulltime on their project, which will be credited as a Bachelor’s or Master’s thesis. In September a go/no go moment has been planned to see if the approach is ripe and successful enough to be presented at the final round in Boston in November. Up to now, Wageningen has never done better than second place.

The project involves costs, and some of them will be covered by crowdfunding. In a few weeks, the collection will start on WUR’s crowdfunding platform. More information about the project can be found on the website.