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Bacterium transforms into weapon against sleeping sickness
30 August 2012
Institute of Tropical Medicine Antwerp
Scientists of the Antwerp Institute of Tropical Medicine (ITG) opened a new front against the cause of sleeping sickness. This parasite is transmitted between humans by tsetse flies. The researchers learned a bacterium living in those flies how to produce antibodies against the parasite. Application in the field is still a long way of, but the technique shows quite some promise.
Sleeping sickness is caused by trypanosomes, parasites being transmitted by the bite of a tsetse fly.
The World Health Organization estimates the yearly death toll at between 10 000 and 20 000 people. On top of that, the parasite also infects cattle, causing considerable economic loss. Many small African farmers depend on their cattle.
Without treatment, an infection is irrevocably fatal. Unfortunately, many poor people present at the hospital only in a late stadium. At that time the Trypanosoma parasites have lodged themselves in the brain, behind the notorious blood-brain barrier that keeps most drugs out. Arsenic compounds can pass the barrier and kill the parasite, but they also kill five per cent of the patients. New drugs are not in the pipeline.
Besides the parasite, one may also attack its vector, the tsetse fly. But insecticides may be detrimental to the environment, certainly in the long run. Therefore scientists look for alternative strategies. For instance genetically modified insects that are incapable of being infected by the parasite, or do not transmit it. But germline transformation of tsetse flies is unfeasible. To do so, one must be able to handle the eggs, but tsetse flies do not lay eggs, they directly bring forth a larva.
Therefore, the Antwerp researchers took another road. Tsetse flies harbour, as is the case with many insects, resident bacteria. One of them, Sodalis glossinidius (literally: companion of the tsetse fly) exclusively lives in tsetse flies. And it can be cultivated in the lab.
De Vooght was the first to genetically modify the bacterium so it produces, and excretes, a very efficient type of antibody, called a nanobody. She identified two different secretory pathways that transported the nanobodies out of the bacterium. She also demonstrated that the bacterium was not hampered by its modification, so it can stand its ground amidst non-modified, ‘wild type’ congeners inside the fly.
Next, with antibiotics she cleared tsetse flies of their wild type bacteria and replaced those by the modified bacteria. These successfully colonized the flies and started producing nanobodies. The nanobodies also were present in the midgut, where the sleeping sickness parasite also is to be found.
De Vooght demonstrated the feasibility of the technique, but it still needs some development before it can be used to control sleeping sickness in the field. For instance, the antibodies now produced by the bacteria, are directed against a form of the parasite occurring in humans, not in flies. This is simply because this antibody was available, while the one against the fly form still has to be developed. De Vooght: “We wanted to demonstrate first that the technique works in principle. Now we have achieved that, we can tackle the technical details.”
To the scientists it is just as important that symbiotic bacteria producing all kinds of substances are a means to getting insight into the interactions between disease-causing organisms and their insect vectors. The Antwerp researchers already demonstrated that the sleeping sickness parasite interferes with the saliva production of tsetse flies, forcing them to bite more humans than they otherwise would do. Insight in that kind of interactions might be instrumental to opening new ways of attacking diseases.
The bacterium Sodalis glossinidius, now able to attack sleeping sickness parasites inside tsetse flies. © ITG
Linda De Vooght transmits Sodalis bacteria onto a special culture medium. © ITG