The bacterium disables one defense while hijacking another
An ancient bacterium that has shadowed human civilization since the time of Hippocrates has finally yielded one of its deepest secrets. Researchers in Germany have identified five proteins that allow Borrelia recurrentis — the spirochete behind louse-borne relapsing fever — to neutralize the body's immune defenses and weaponize its own biological machinery for deeper invasion. The disease, long dismissed as a relic of poverty and poor sanitation, resurged among European refugees as recently as 2015, reminding us that neglected pathogens do not disappear — they wait. In naming these Chi proteins and mapping their dual function, science has found a new foothold in a very old struggle.
- A bacterium that kills one in five untreated patients has been quietly evading human immunity for millennia by deploying a set of molecular disguises only now fully understood.
- The Chi proteins work on two fronts simultaneously — shutting down the complement system that would normally flag and destroy the invader, while hijacking the body's own plasmin to tunnel deeper into tissue.
- Louse-borne relapsing fever is not a historical curiosity: it re-emerged among refugees in Europe in 2015, exposing how quickly a neglected disease can cross borders when conditions of crowding and poverty persist.
- The research team at Universitätsmedizin Frankfurt and Justus Liebig University Giessen has now characterized all five Chi proteins, mapping the precise mechanisms the pathogen uses to survive and spread inside a human host.
- These findings open a concrete path toward faster diagnostic tests and therapeutic strategies that could disrupt the bacterium's evasion tactics — potentially reducing a 20% untreated fatality rate for the world's most vulnerable populations.
Borrelia recurrentis is among the oldest documented human pathogens — a spirochete bacterium transmitted by body lice that causes louse-borne relapsing fever, a disease Hippocrates described twenty-five centuries ago. Infection brings waves of high fever that break and return, sometimes repeatedly. Antibiotics resolve it. Without treatment, one in five patients dies, a toll borne almost entirely by people without reliable access to medical care.
For much of the twentieth century, relapsing fever erupted in major European epidemics before retreating to the Horn of Africa — Eritrea, Ethiopia, Somalia, South Sudan — where body lice thrive in crowded, under-resourced conditions. The disease seemed geographically contained until 2015, when cases appeared among refugees arriving in Europe, a sharp reminder that old diseases do not respect modern borders.
What has made this pathogen so durable is a set of five closely related proteins — now named Chi proteins — that researchers at Universitätsmedizin Frankfurt and Justus Liebig University Giessen, led by Professor Peter Kraiczy, have identified and characterized. These proteins function as a molecular disguise operating on two simultaneous fronts.
First, the Chi proteins block the complement system — one of the body's most fundamental immune defenses — preventing it from marking the bacterium for destruction. Second, they capture plasminogen from the bloodstream and convert it into active plasmin, which the bacterium then uses to bore through tissue barriers and spread deeper into the host. The pathogen disables a primary alarm system while commandeering the body's own tools to advance further.
Kraiczy described this dual mechanism as giving Borrelia recurrentis significant advantages in both surviving and disseminating within a human host. Understanding how the Chi proteins work now opens possibilities for faster diagnostic tests and therapeutic approaches that could interfere with the bacterium's evasion strategy — offering, for the first time in a very long history, a molecular foothold against this ancient and persistent disease.
Borrelia recurrentis is an ancient pathogen with a modern problem. The spirochete bacterium, transmitted by body lice, causes louse-borne relapsing fever—a disease so old that Hippocrates documented it twenty-five centuries ago. A person infected experiences a high fever that breaks after several days, then returns in waves, sometimes repeatedly. Antibiotics can stop it. Left untreated, the infection kills one in five patients, a toll that falls heaviest on people without access to reliable medical care.
For most of the twentieth century, relapsing fever was a European scourge, erupting in major epidemics. Today it lingers as what researchers call a poverty-related neglected disease, surfacing in sporadic outbreaks across the Horn of Africa—Eritrea, Ethiopia, Somalia, South Sudan—places where body lice thrive in conditions of crowding and poor sanitation. The bacterium had largely vanished from Europe until 2015, when a sudden cluster of cases appeared among refugees arriving in several European countries, a reminder that old diseases do not stay confined to old maps.
What makes Borrelia recurrentis so difficult for the human body to fight is a set of five closely related proteins that the bacterium has evolved to be nearly invisible to our immune defenses. Researchers at Universitätsmedizin Frankfurt and Justus Liebig University Giessen, led by Professor Peter Kraiczy, have now identified and characterized these proteins, called Chi proteins, and explained how they work as a kind of molecular disguise.
The Chi proteins operate on two fronts. First, they bind to proteins circulating in the blood and prevent activation of the complement system—one of the body's most fundamental immune defenses, a cascade of proteins that marks invading bacteria for destruction and punches holes in their membranes. By blocking this system, the Chi proteins allow Borrelia recurrentis to slip past a critical checkpoint. Second, the Chi proteins capture plasminogen, an enzyme precursor in the blood, and convert it into active plasmin. The bacterium then hijacks this plasmin to bore through tissue barriers and spread deeper into the body. The combination is potent: the pathogen disables one of the body's primary alarm systems while simultaneously commandeering the body's own machinery to invade further.
Kraiczy explained the significance of this dual mechanism: the ability to block complement activation, paired with the capacity to exploit plasmin, gives Borrelia recurrentis substantial advantages in both surviving and disseminating once it enters a human host. Understanding how these proteins work opens a door to new possibilities—diagnostic tests that could identify infection more quickly, and potentially therapeutic approaches that could interfere with the bacterium's evasion tactics. For a disease that has haunted human populations since ancient times, this molecular insight represents a foothold toward better tools for detection and treatment.
Citas Notables
The ability to block the complement system, combined with the capacity to exploit plasmin, gives Borrelia recurrentis significant advantages in surviving and spreading after entering the human body.— Professor Peter Kraiczy, Universitätsmedizin Frankfurt
La Conversación del Hearth Otra perspectiva de la historia
Why does this bacterium need five different Chi proteins if they all seem to do the same thing?
They're related—they evolved from a common ancestor—but they're not identical. Having multiple versions might let the bacterium hedge its bets, or adapt to different immune environments in different people. We don't fully know yet.
So the bacteria are basically using our own blood proteins against us?
Exactly. It's not inventing anything new. It's recognizing that plasmin is already there, already doing its job in the body, and saying: we can use that. It's parasitism at the molecular level.
If antibiotics work, why is this still a problem?
Because antibiotics only work if you can reach a clinic and get diagnosed. In refugee camps, in rural Ethiopia, in places where lice spread fastest—those are often the places furthest from treatment. And if you don't know you have it, you can't treat it.
What changes if we understand these Chi proteins better?
We could design tests that detect the proteins themselves, not just the bacteria. Faster diagnosis. We might also find ways to block the proteins, which could slow the infection while antibiotics do their work. It's about giving the immune system a fighting chance.
Is this bacterium becoming resistant to antibiotics?
Not that we know of yet. But understanding its immune evasion tactics is separate from antibiotic resistance. Both matter. One is about how it hides; the other is about whether drugs can kill it.