Fungi That Abandon Biofilms Gain a Hidden Edge Inside the Human Body

For decades, the ability of disease-causing fungi to build biofilms has been treated as a textbook hallmark of virulence. Biofilms, the dense communities of microbial cells that coat catheters, heart valves, and other medical devices, are wrapped in a protective extracellular matrix of sugars, proteins, and other molecules that shields the enclosed cells from both antifungal drugs and attacking immune cells. Infections involving these structures are notoriously stubborn, and fungal biofilms alone are estimated to impose a global healthcare burden exceeding 40 billion dollars each year. Against this backdrop, a new study of the human fungal pathogen Candida parapsilosis, published in PLOS Biology and examined in an accompanying Primer by Shannon Esher Righi, delivers a genuinely surprising result: strains that produce little or no biofilm appear to be better adapted than their biofilm-producing counterparts for evading the immune system and persisting inside the host. The finding challenges a long-standing assumption about what makes a fungus dangerous and reframes the way scientists think about the evolutionary pressures shaping hospital outbreaks.

Candida parapsilosis has become an increasingly troubling player in hospital-associated infections worldwide. Like other Candida species, it lives harmlessly as a commensal on human skin for much of the time, but it can also cause disease ranging from superficial mucosal infections to life-threatening invasive illness, often seeded by its capacity to colonize indwelling medical devices such as central lines used to deliver intravenous fluids or medications. Outbreak isolates of the species increasingly carry resistance to fluconazole, a first-line antifungal drug, yet resistance alone does not explain why these strains spread so effectively through clinical settings and persist in patients. The new research, led by Daneshnia and colleagues, tracked a striking phenotype shared across a collection of global fluconazole-resistant outbreak isolates: markedly low biofilm production. Rather than treating this as a defect, the investigators asked what advantages these fungi might gain by trading in one of their most celebrated virulence traits.

To appreciate the significance of that question, it helps to understand how biofilms are built. Formation generally unfolds in four phases: initial colonization, in which cells adhere to a surface; proliferation coupled with the onset of matrix production; maturation of the three-dimensional community structure; and finally dispersal, in which cells are released to colonize new sites. In Candida species, these stages are governed by a complex transcriptional network that is evolutionarily young, as evidenced by the presence of many lineage-specific genes found only in closely related species. These so-called young genes stand in contrast to ancient, broadly conserved genes that perform core cellular functions shared across the tree of life. When Daneshnia and colleagues profiled gene expression in their low biofilm-producing isolates, they found widespread transcriptional reprogramming characterized by the upregulation of evolutionarily old genes involved in core metabolic functions, a molecular signature suggesting a reversion toward more ancient, conserved cellular processes.

That reversion makes energetic sense. Biofilm construction is an extraordinarily expensive undertaking, demanding the activation of specialized genetic networks, rapid cellular proliferation, and the synthesis of numerous extracellular matrix components. Long-term survival within a host, by contrast, rewards metabolic flexibility, and previous work has shown that the capacity for metabolic adaptation can outweigh the loss of conventional virulence factors. Consistent with this logic, the low biofilm-producing isolates grew robustly across a variety of both stress and non-stress conditions and routinely outcompeted biofilm-producing isolates in laboratory culture, demonstrating a clear fitness advantage. The pattern echoes observations from experimental bacterial populations, where matrix-producing strains can be outcompeted by infiltrating non-producing strains that avoid the energetic cost of making matrix material. In effect, the low biofilm fungi behave like cheaters in a cooperative system, reaping the benefits of a leaner metabolic budget while their biofilm-producing relatives shoulder the construction costs.

The most consequential findings, however, concern the fungal cell wall, the first point of contact between the pathogen and the host immune system. The cell wall polysaccharide beta-glucan is a prominent pathogen-associated molecular pattern, a molecular flag that immune cells use to recognize fungal invaders. To control this recognition, fungi routinely mask beta-glucan beneath an outer fibrillar layer of mannan, a process that is tightly regulated and highly responsive to nutritional signals in the environment. Daneshnia and colleagues found that their low biofilm-producing isolates exhibited elevated beta-glucan masking, with increased mannan exposure and correspondingly decreased beta-glucan exposure at the cell surface. As predicted, this architectural change translated directly into immune outcomes: the low biofilm isolates were phagocytosed and killed less efficiently by innate immune cells than biofilm producers with higher levels of exposed beta-glucan, indicating that the low biofilm phenotype is better equipped to evade immune detection in vivo.

One result might at first appear paradoxical. When the researchers transcriptionally profiled macrophages infected with the different isolate types, they found that the low biofilm-producing strains induced a heightened proinflammatory state rather than a dampened one. Yet this observation fits coherently with the rest of the data. Evading initial detection is only one survival strategy; being able to persist even after inflammatory activation is another, and it complements the enhanced stress tolerance the isolates displayed in vitro. The decisive evidence came from a mouse model of candidiasis, in which the low biofilm-producing isolates showed increased survival in vivo, particularly in immune cell-rich organs. In other words, these fungi do not merely hide from the immune system; they are also intrinsically better at withstanding life inside the very cells and tissues that mount the anti-fungal response, tolerating conditions that would prove lethal to their biofilm-producing relatives.

Together, the findings describe an evolutionary trade-off between biofilm formation and fitness within host tissues, and the parallel with bacterial pathogens is striking. Among pathogenic Salmonella strains, biofilm formation is conserved in lineages that colonize the intestines of multiple host species and cause localized gastroenteritis, but it is lost in strains that have become adapted to and restricted by a single host, where they cause systemic and invasive disease. Similar patterns have been documented in comparisons of commensal and invasive Escherichia coli strains. Candida parapsilosis, which has been isolated from a variety of environmental niches and remains a frequent commensal colonizer of human skin, has shown a rapid rise in both incidence and disease severity in recent years. The question raised by the new work is whether the emergence of these immunoevasive, low biofilm-producing isolates represents a shift toward a more invasive lifestyle. Changes in selective pressure toward immune avoidance and long-term persistence are thought to have driven biofilm loss in host-restricted Salmonella, and Daneshnia and colleagues propose a similar explanation for the fungus, pointing to the host environment itself as the main driver of strain evolution during these outbreaks.

Substantial questions remain open. From a genetics standpoint, the specific genes or mutations responsible for the low biofilm phenotype have not yet been identified, and it is unclear whether the phenotype and its underlying transcriptional rewiring are shared by other outbreak-associated Candida species. Defining the connections between biofilm formation, metabolism, and cell wall biogenesis promises insights into fungal physiology that extend well beyond a single species. On the immunology side, understanding how and why the low biofilm isolates simultaneously heighten macrophage activation and thrive in immune cell-rich organs is a rich avenue for further study. All of these threads converge on a single central question: is biofilm attenuation a cause or a consequence of host adaptation? The answer will shape how scientists model the transition from commensal to pathogen and how they interpret the molecular signatures of emerging fungal threats.

The practical implications are equally significant. Therapeutically, the study suggests that drugs targeting traditional virulence factors such as biofilm formation may impose selective pressures that inadvertently favor strains better suited to long-term persistence in the host, a consideration that should inform the design of future antifungal strategies. More broadly, the work arrives at a moment when new fungal threats are expected to emerge, driven by changing environmental conditions and by medical advances that produce ever-growing populations of immunosuppressed patients. If the forces that turned Candida parapsilosis outbreak isolates into leaner, more immunoevasive organisms are understood in detail, clinicians and researchers may be better positioned to anticipate which traits the next wave of fungal pathogens will carry, and to intervene before those traits translate into wider outbreaks.

Subject of Research: Evolutionary trade-off between biofilm formation and host persistence in Candida parapsilosis outbreak isolates

Article Title: A trade-off between fungal biofilm formation and persistence in the host

Article References: Righi, S. E. (2026). A trade-off between fungal biofilm formation and persistence in the host. PLOS Biology, 24(9), e3004027. https://doi.org/10.1371/journal.pbio.3004027

Image Credits: AI Generated

DOI: 10.1371/journal.pbio.3004027

Keywords: Candida parapsilosis, fungal biofilms, immune evasion, beta-glucan masking, antifungal resistance, hospital outbreaks, virulence, metabolic flexibility, macrophages, host-pathogen interaction, PLOS Biology, evolutionary trade-off