When you think of a deadly fungus, you might imagine its hyphae spreading through tissue. But what if the real culprit was hidden inside the fungus all along?
This is the story of a puzzling scientific discovery that challenged our understanding of what makes a pathogen dangerous.
For years, scientists have known that the fungus Rhizopus microsporus can cause a devastating disease in rice seedlings. The cause wasn't the fungus itself, but a toxin called rhizoxin produced by bacteria living inside the fungal cells.
This remarkable example of nature's complexity where one organism lives inside another in a relationship called endosymbiosis 8 raised urgent questions for medical researchers.
If bacteria living inside farm-infecting fungi produce toxins, could the same be true for fungi that cause deadly human infections?
Mucormycosis is a life-threatening fungal infection, primarily affecting immunocompromised patients, with mortality rates exceeding 50% in many cases 7 . Rhizopus species are responsible for the majority of these infections 1 7 .
Given the known partnership between Rhizopus microsporus and toxin-producing bacteria in plants, researchers naturally wondered whether similar bacterial partners might be contributing to human disease.
Mortality rate for mucormycosis in many cases 7
In 2008, a crucial study directly addressed this question. Researchers investigated eight clinical Rhizopus isolates from human infections using three independent methods 1 .
| Research Method | What Was Tested | Result |
|---|---|---|
| Metabolic Analysis | Presence of bacterial toxins | No toxins detected |
| PCR Amplification | Bacterial 16S rDNA | No bacterial DNA found |
| Fluorescence Microscopy | Visual presence of bacteria inside fungal cells | No bacteria observed |
To definitively determine whether endosymbiotic bacteria contribute to the virulence of human-pathogenic Rhizopus strains, researchers designed a comprehensive set of experiments using clinical isolates obtained from patients with mucormycosis.
Researchers first used universal primers targeting bacterial 16S rDNA to check for bacterial DNA in 28 clinical isolates of Zygomycetes 7 .
For fungi that tested positive for bacteria, scientists used ciprofloxacin antibiotics to create "cured" fungal strains free of their bacterial partners 7 .
The critical step involved comparing the disease-causing ability of original fungi with their bacteria-free counterparts using multiple models 7 .
15 of 28 clinical isolates contained endosymbiotic bacteria, with approximately one-third closely related to known Burkholderia species 7 .
High-performance liquid chromatography confirmed that fungi with bacteria produced rhizoxin, while cured fungi did not 7 .
| Experimental Model | Comparison | Outcome |
|---|---|---|
| Endothelial Cell Injury | Fungi with vs. without endosymbionts | No difference in damage caused |
| Mouse Infection Model | Original vs. bacteria-free fungi | No reduction in virulence |
| Fly Infection Model | Original vs. bacteria-free fungi | No reduction in virulence |
Understanding the relationship between fungi and their bacterial inhabitants requires specialized laboratory methods and reagents:
Primary Function: Antibiotic elimination of bacteria
Application: Creating bacteria-free fungal strains for comparison studies 7
Primary Function: DNA staining
Application: Visualizing bacteria inside fungal hyphae using microscopy 3
Primary Function: Chemical separation and detection
Application: Identifying and measuring bacterial toxins like rhizoxin 7
Primary Function: Precision bacterial implantation
Application: Artificially introducing bacteria into fungi to study symbiosis formation 9
While the 2008 study provided convincing evidence that toxin-producing bacteria aren't essential for human Rhizopus infections, more recent research has revealed that the story is more nuanced than initially thought.
A groundbreaking 2022 study discovered that a different type of bacterial endosymbiont—Ralstonia pickettii—in a clinical Rhizopus microsporus isolate does significantly enhance fungal virulence through a completely different mechanism 4 .
Rather than producing plant toxins, this bacterium helps the fungus evade immune system cells
The bacterium secretes factors that block phagocytosis—the process where immune cells engulf and destroy invaders 4 .
This protective effect works against both environmental predators (soil amoebas) and human immune cells (macrophages), suggesting an evolutionary advantage 4 .
The initial investigation into toxin-producing bacteria in clinical Rhizopus isolates revealed an important truth: the strategies that make fungi dangerous to plants differ from those that make them dangerous to humans. While the search for toxin-producing endosymbionts in human-pathogenic fungi came up empty, this research opened the door to discovering more sophisticated and diverse symbiotic relationships.
The journey from wondering about bacterial toxins in human fungi to discovering completely different virulence mechanisms demonstrates how scientific understanding evolves through careful experimentation.
As one recent editorial noted, basic research continues to reveal "interesting developments in our understanding of the interactions between Rhizopus microsporus and endosymbiotic bacteria" 2 —assuring us that this fascinating story is far from over.