C. albicans incorporates methaemoglobin into surface attached and aggregate biofilms that exhibit increased virulence and antifungal resistance

Inside the human body, a constant, quiet battle takes place between our immune defenses and the microscopic invaders that seek to exploit us. Among the most successful of these invaders is a fungus called Candida albicans. It lives harmlessly in many people, but when the body's defenses are weakened or when medical devices are inserted, it can turn dangerous, causing infections that are difficult to treat. A key reason for its success is its ability to form biofilms. Think of a biofilm as a fortified city built by the microbes; they stick to surfaces and wrap themselves in a protective slime that shields them from medicines and the immune system. For decades, scientists have known that these fungal cities are a major cause of persistent illness, but they have not fully understood how the body's own fluids sometimes help the fungus build them. Specifically, the role of blood proteins in this process has remained a mystery, leaving a gap in our understanding of how these infections take hold in the bloodstream.

Researchers at the University of Kent and the University of Adelaide have now uncovered a surprising mechanism where the fungus uses a specific form of a blood protein to rapidly construct these protective structures. The story begins with hemoglobin, the molecule in red blood cells that carries oxygen. When red blood cells break open, a process known as hemolysis, this hemoglobin is released. Under normal conditions, most of it is in a state ready to carry oxygen. However, when exposed to the environment outside the cell, it often oxidizes, changing into a form called methemoglobin. This oxidized version cannot carry oxygen. The researchers discovered that Candida albicans does not just tolerate this oxidized protein; it actively seeks it out and incorporates it into its own structure. When the fungus encounters methemoglobin, it does not simply eat it for food. Instead, it uses the protein as a building block, binding to it and pulling it into massive clumps. These clumps are not just random piles of cells; they are highly organized, floating communities that function exactly like biofilms, complete with the same defenses and dangers as those stuck to medical devices.

The team demonstrated that this process is driven by the fungus's ability to change its shape. Candida can exist as a single round cell or as a long, thread-like structure called a hypha. The researchers found that the fungus must grow these long threads to effectively grab onto the methemoglobin and weave it into a large aggregate. If the fungus is prevented from growing these threads, the clumps fail to form. Furthermore, the fungus uses specific molecular "hands," known as adhesins, to grab onto the protein. The study identified two of these hands, called Als1 and Als3, as essential for the process. Without them, the fungus cannot bind to the methemoglobin effectively, and the protective clumps do not assemble. This interaction happens incredibly fast; within an hour of meeting the protein, the fungus begins to stick to it, and within a few hours, large, visible aggregates have formed.

What makes this discovery particularly concerning is the strength of these new structures. The researchers tested how well these methemoglobin-fueled clumps could survive attacks from common antifungal drugs. They found that the clumps were significantly harder to kill than normal fungal cells. The drugs could not penetrate the dense mass of protein and cells, allowing a portion of the population to survive. Crucially, this resistance was not a permanent genetic change; it was a temporary shield provided by the physical structure of the clump. Once the drugs were removed and the cells were grown again, they became sensitive to treatment once more. This suggests that the presence of methemoglobin creates a temporary safe haven where the fungus can hide until the threat passes. In experiments using moth larvae as a model for infection, the researchers observed that the fungus grown with methemoglobin was much more deadly than the fungus grown without it, confirming that these aggregates are more virulent.

The study also looked at what happens inside the fungus during this rapid assembly. By analyzing the genes the fungus turned on and off, the researchers saw a clear shift in strategy. Initially, the fungus activated genes to grab onto the protein and scavenge for iron. But as the clump formed, the fungus switched its metabolism. It began to act as if it were in a low-oxygen environment, slowing down its energy production and entering a dormant state. This shift to a slower, more conservative lifestyle likely contributes to its ability to survive antifungal attacks, as many drugs work best on fast-growing cells. The researchers used computer simulations to model how these interactions work, and the models confirmed that the long, thread-like growth of the fungus is the primary force that knits the small protein clumps together into the large, dangerous aggregates seen in the lab.

This work suggests that the body's own response to injury or infection might inadvertently help the fungus. When red blood cells are damaged, whether by infection, surgery, or the insertion of medical devices, methemoglobin is released. If the levels of this protein rise high enough, it can overwhelm the body's natural cleanup systems, which usually bind and remove free hemoglobin. In these conditions, the fungus can seize the opportunity to build these drug-resistant fortresses. The researchers noted that this phenomenon has been observed in patients with severe infections and blood clots, where the fungus is often found associated with blood. While the study was conducted in controlled laboratory settings, the findings raise the possibility that methemoglobin plays a significant, yet previously overlooked, role in how these infections establish themselves and resist treatment. The study does not claim to have solved the problem of fungal infections, but it has identified a new way the fungus builds its defenses, offering a fresh perspective on why some infections are so hard to clear and suggesting that the state of the blood itself may be a critical factor in the outcome of the battle.