Bacterium Transforms into Weapon

Photo: The bacterium Sodalis glossinidius

The bacterium Sodalis glossinidius,
now able to attack sleeping sick-
ness parasites inside tsetse
flies;© ITG

Application in the field is still a long way of, but the technique shows enough promise to gain Linda De Vooght. 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.; Source: Antwerp Institute of Tropical Medicine (ITG)