Parasites take an enormous toll on human and veterinary health. But researchers may have found a way for patients with brain disorders and a common brain parasite to become frenemies.
A new study published in Nature Microbiology has pioneered the use of a single-celled parasite, Toxoplasma gondii, to inject therapeutic proteins into brain cells. The brain is very picky about what it lets in, including many drugs, which limits treatment options for neurological conditions.
As a professor of microbiology, I’ve dedicated my career to finding ways to kill dangerous parasites such as Toxoplasma. I’m fascinated by the prospect that we may be able to use their weaponry to instead treat other maladies.
Microbes as medicine
Ever since scientists realized that microscopic organisms can cause illness – what’s called the 19th-century germ theory of disease – humanity has been on a quest to keep infectious agents out of our bodies. Many people’s understandable aversion to germs may make the idea of adapting these microbial adversaries for therapeutic purposes seem counterintuitive.
But preventing and treating disease by co-opting the very microbes that threaten us has a history that long predates germ theory. As early as the 1500s, people in the Middle East and Asia noted that those lucky enough to survive smallpox never got infected again. These observations led to the practice of purposefully exposing an uninfected person to the material from an infected person’s pus-filled sores – which unbeknownst to them contained weakened smallpox virus – to protect them from severe disease.
The concept of inoculation developed with smallpox outbreaks several centuries ago.
MS Japanese 63/Wellcome Collection, CC BY
This concept of inoculation has yielded a plethora of vaccines that have saved countless lives.
Viruses, bacteria and parasites have also evolved many tricks to penetrate organs such as the brain and could be retooled to deliver drugs into the body. Such uses could include viruses for gene therapy and intestinal bacteria to treat a gut infection known as C. diff.
Why can’t we just take a pill for brain diseases?
Pills offer a convenient and effective way to get medicine into the body. Chemical drugs such as aspirin or penicillin are small and easily absorbed from the gut into the bloodstream.
Biologic drugs such as insulin or semaglutide, on the other hand, are large and complex molecules that are vulnerable to breaking down in the stomach before they can be absorbed. They are also too big to pass through the intestinal wall into the bloodstream.
All drugs, especially biologics, have great difficulty penetrating the brain due to the blood-brain barrier. The blood-brain barrier is a layer of cells lining the brain’s blood vessels that acts like a gatekeeper to block germs and other unwanted substances from gaining access to neurons.
