Candida albicans forms azole-resistant liquid-suspension biofilms with distinct characteristics and a potential to aid in dispersal

In the microscopic world of human infection, a single-celled fungus called Candida albicans is a formidable adversary. It is a common resident of the human body, but when the immune system falters or medical devices like catheters are introduced, it can turn deadly. For decades, scientists have understood how this fungus survives by building protective cities known as biofilms. Imagine a fortress: the fungus cells stick to a surface, such as a plastic catheter or a tooth, and build a thick, slimy shield around themselves. This shield, made of their own secretions, protects them from antibiotics and the body's immune defenses. This process of sticking to a surface and building a fortress has been the standard model for how Candida causes trouble. However, this model assumes the fungus always needs a solid place to stick. In the complex, fluid environments of the human body, such as the bloodstream or the space between tissues, a solid surface is not always available. The question remained: if there is no wall to cling to, how does this dangerous fungus organize itself, survive, and spread?

Researchers at Monash University have now discovered that Candida albicans does not simply float aimlessly when it cannot find a surface. Instead, it builds a different kind of fortress entirely, one that floats freely in liquid. The team found that when the fungus is placed in a liquid environment where it cannot easily stick to the bottom, or when a surface-based fortress becomes too crowded, the cells spontaneously form dense, three-dimensional clusters that hang suspended in the fluid. These are not just loose clumps of cells; they are highly organized structures, which the researchers call liquid-suspension biofilms. Inside these floating cities, the fungus cells arrange themselves into a complex web. Some cells look like round balls, while others stretch out into long, tube-like shapes. These different forms are all connected by a vast network of long, thread-like filaments that act as the structural beams holding the entire floating city together.

To understand how these floating structures form, the scientists grew the fungus in a laboratory setting that mimicked a surface-free environment. They used a special type of gel that the fungus could not stick to, forcing it to grow in the liquid above. They observed that the fungus ignored the gel and instead formed dense, irregular balls of growth floating in the liquid. These floating structures were surprisingly large, reaching hundreds of micrometers in size, and were packed with cells. When the researchers looked closely at the internal architecture, they saw that the cells were not just bunched together randomly. They were woven together by a massive network of hyphae, which are the long, thread-like extensions the fungus uses to grow. This structure is distinct from the flat, sheet-like biofilms that grow on surfaces, yet it shares many of the same protective qualities.

The most critical finding of this study is that these floating biofilms are just as dangerous as the ones stuck to surfaces, particularly when it comes to resisting medicine. The researchers tested the floating clusters against fluconazole, a common antifungal drug used as a first-line treatment for fungal infections. They found that the floating biofilms were completely resistant to the drug, just like the surface-adhered biofilms. In contrast, the individual, floating cells that had not formed these structures were easily killed by the same drug. This suggests that the act of forming the floating cluster itself provides a shield against treatment. The researchers also discovered that these floating structures are not a random accident but a regulated behavior. They found that the same master control genes that tell the fungus to build a surface fortress also tell it to build a floating one. When the researchers disabled these control genes, the fungus lost its ability to form the floating clusters, proving that this is a deliberate, biological strategy rather than a passive reaction.

The study also revealed how these floating biofilms might help the infection spread. The researchers observed that when a surface-based biofilm grows for a long time and runs out of space, it begins to produce these floating clusters. These clusters can then break away and travel through the body's fluids. When the researchers took these floating clusters and placed them onto a new, clean surface, they successfully started new surface-based biofilms. This indicates that the floating biofilm acts as a vehicle for dispersal, carrying the infection to new locations in the body. Unlike the traditional view where infections spread by releasing single, weak cells into the bloodstream, this new mechanism suggests the fungus can travel as a protected, drug-resistant unit.

The researchers also looked at the genetic activity of these floating biofilms to see how they differ from their surface-bound cousins. They found that while the two types of biofilms share a very similar genetic program, the floating ones have a unique focus on scavenging for iron. Iron is a vital nutrient for the fungus, but it is often scarce in the human body. The floating biofilms turned on genes specifically designed to hunt down and capture iron more aggressively than the surface biofilms did. This suggests that the floating lifestyle places different nutritional demands on the fungus, perhaps because it is surrounded by fluid rather than a solid surface. Despite these differences, the overall genetic profile of the floating biofilm converges with the surface biofilm over time, showing that they are two sides of the same coin, adapted to different physical environments.

This discovery changes the way we think about fungal infections. For years, the focus has been on how the fungus sticks to medical devices. Now, we know that even in the absence of a surface, or when a surface is full, the fungus has a backup plan. It can build a floating city that is just as tough and drug-resistant as the one stuck to a wall. This floating mode of growth could explain why some infections are so hard to treat and why they seem to spread to new parts of the body even when the original site is treated. The study suggests that the fungus is far more adaptable than previously thought, capable of switching between a surface-bound fortress and a mobile, floating colony depending on the conditions it faces.

The implications of this finding extend beyond the laboratory. In patients with urinary catheters or those suffering from bloodstream infections, doctors often see clumps of cells and white blood cells in the urine or blood. These clumps have been difficult to interpret, sometimes dismissed as simple debris. This new research suggests that these clumps could be the floating biofilms described in the study. If these floating structures are indeed a common feature of human infection, they represent a hidden reservoir of drug-resistant cells that can seed new infections anywhere in the body. Understanding that the fungus can build these floating fortresses opens up new avenues for research. Scientists can now ask how to break down these floating structures or prevent them from forming in the first place.

The researchers confirmed that this behavior is not limited to a specific type of liquid or a single strain of fungus. They observed the formation of these floating biofilms in various growth media, including those that mimic the conditions inside the human body. They also showed that the process is reversible; the fungus can transition from a surface biofilm to a floating one and back again, depending on the availability of space. This flexibility highlights the resilience of the organism. It is not just a passive victim of its environment but an active architect that reshapes its own structure to survive.

In the end, this paper provides a clearer picture of how Candida albicans survives in the human body. It shows that the fungus does not rely on a single strategy. It can build a fortress on a wall, but if the wall is gone or full, it builds a fortress in the air. This floating fortress is complex, organized, and highly resistant to the drugs we use to fight it. By uncovering this hidden mode of growth, the researchers have identified a new target for understanding and treating fungal infections. The next step is to determine how often these floating biofilms occur in real patients and whether targeting them can improve outcomes for those suffering from these difficult infections. The study does not offer an immediate cure, but it provides a crucial piece of the puzzle, revealing a previously unknown way that a deadly pathogen organizes itself to survive and spread.