A first-person account of rewriting textbooks and challenging scientific dogma
"When I first began studying Candida albicans in the 1980s, the scientific community had a straightforward story about this common yeast: it was asexual, a mere commensal organism that occasionally caused troublesome infections in compromised patients."
When I first began studying Candida albicans in the 1980s, the scientific community had a straightforward story about this common yeast: it was asexual, a mere commensal organism that occasionally caused troublesome infections in compromised patients. But as I delved deeper into its genetic mysteries, I discovered there was far more to this microbe than anyone had imagined. My name is Alexander "Sandy" Johnson, and my journey with Candida has been one of rewriting textbooks and challenging long-held assumptions 6 .
Over my career at the University of California, San Francisco, where I've served as Professor and Vice Chair of the Department of Microbiology and Immunology, I've had the privilege of uncovering one of Candida's best-kept secrets: it has a sexual cycle, controlled by an elegant genetic switch that transforms the yeast between two distinct forms 6 . This discovery didn't just satisfy scientific curiosity—it opened new pathways for understanding how this opportunistic pathogen survives inside human hosts and causes disease. The story of this discovery exemplifies how science often reveals its greatest secrets when we question established dogmas and look beyond what we think we know.
Professor and Vice Chair, Department of Microbiology and Immunology, University of California, San Francisco
Fungal pathogenesis, genetic regulation, white-opaque switching, and mating in Candida albicans
Candida albicans is a fungal microorganism that naturally inhabits various parts of the human body, including the mouth, gut, and reproductive tract. For most people, it exists harmlessly as part of our natural microbiome. However, when the body's balance is disrupted—by antibiotics, immunosuppression, or other factors—Candida can proliferate excessively and cause infections ranging from simple thrush to life-threatening systemic conditions 6 .
What makes Candida particularly fascinating is its morphological flexibility. Unlike many microorganisms that maintain a single form, Candida can switch between different physical states, each with distinct properties and capabilities. This plasticity is key to its success as a pathogen, allowing it to adapt to different environments within the human host and evade immune responses.
At the heart of my research is the remarkable white-opaque switch—a biological phenomenon where individual Candida cells can transition between two stable, heritable forms:
This transition isn't merely a change in appearance; it represents a fundamental reprogramming of the cell's biology, affecting everything from its surface properties to its metabolic preferences and susceptibility to antifungal drugs. The switch is controlled by a master regulatory circuit centered on the mating-type locus homeodomain proteins, which act as genetic toggle that locks the cell in one state or the other 6 .
| Concept | Description | Significance |
|---|---|---|
| Candida albicans | A commensal yeast that can become an opportunistic pathogen | Causes infections in immunocompromised individuals; model for understanding fungal pathogenesis |
| White-opaque switch | A heritable transition between two distinct cell types | Enhances Candida's ability to adapt to different environments and niches within the host |
| Mating-type locus | A genetic region containing genes that control cell type and mating behavior | Regulates the switch between white and opaque phases; controls mating competence |
| Transcriptional repressor TUP1 | A protein that regulates gene expression by suppressing specific genetic programs | Controls filamentation and other virulence-related processes in Candida 6 |
The journey to discovering mating in Candida began with puzzling observations that didn't fit the established narrative. Despite the scientific consensus that this yeast was asexual, we kept finding genetic signatures that hinted at a more complex story. The breakthrough came when we decided to systematically investigate these anomalies through a series of interconnected experiments:
We began by sequencing and manipulating the genes in the mating-type locus (MTL) of Candida, which resembled similar regions in sexual fungi. Using gene knockout techniques, we disabled specific genes to observe the effects on cellular behavior 6 .
We noticed that opaque cells—previously considered a laboratory curiosity—expressed genes typically associated with mating in other fungi. We developed methods to track and quantify the transition between white and opaque states under different environmental conditions.
We designed experiments to test whether opaque cells could actually mate. This involved mixing opaque cells of different mating types under various conditions and using selection markers to identify successful mating events through genetic recombination.
Perhaps most crucially, we tested whether mating could occur not just in laboratory dishes but in a more relevant biological context—inside a mammalian host. We introduced different mating types of Candida into mice and then looked for evidence of recombination days later 6 .
The process was anything but linear. Each answer generated new questions, requiring us to continually refine our approaches and develop new methodologies to test emerging hypotheses.
The results of our experiments fundamentally changed our understanding of Candida biology. When we knocked out both copies of the MTLa1 gene, the cells converted en masse to the opaque form. Even more strikingly, when we mixed these a-type opaque cells with α-type opaque cells, we observed successful mating—producing genetic hybrids with characteristics from both parent strains 6 .
The most dramatic confirmation came from the in vivo experiments. After introducing different mating types into mice, we recovered recombinant Candida cells that carried genetic markers from both parents, providing incontrovertible evidence that mating was occurring in a mammalian host 6 . This discovery had profound implications:
Explained how Candida could generate genetic diversity despite its supposed asexuality
Revealed the biological function of the mysterious opaque cell type
Suggested that mating might occur during infection, potentially producing new strains with different pathogenic properties
| Experiment | Key Result | Interpretation |
|---|---|---|
| MTL gene knockouts | Deletion of both MTLa1 alleles caused conversion to opaque phase | The MTL locus contains genes that repress the opaque cell fate |
| In vitro mating assays | Opaque cells of opposite mating types produced hybrid offspring | Opaque cells are mating-competent; Candida has a complete sexual cycle |
| In vivo mating experiments | Recombinant strains were recovered from infected mice | Mating occurs in mammalian hosts, potentially during infection |
| Genetic analysis of hybrids | Progeny showed novel combinations of parental traits | Sexual reproduction generates diversity with potential clinical implications |
Unraveling Candida's secrets required more than just clever experiments—it depended on a collection of specialized research tools and reagents. These are some of the key materials that made our discoveries possible:
| Reagent/Tool | Function | Role in Johnson's Research |
|---|---|---|
| Gene knockout constructs | Specific DNA sequences designed to disrupt target genes | Used to determine functions of MTL genes and regulatory proteins like TUP1 6 |
| Selection markers | Genes that confer resistance to antibiotics or allow growth in selective media | Enabled identification of successful gene replacements and detection of mating products |
| Candida albicans mutant libraries | Collections of strains with specific genes deleted or modified | Allowed systematic screening of genes involved in morphogenetic switching and pathogenicity 6 |
| Reporter genes | Genes that produce easily detectable signals (e.g., fluorescence) when expressed | Permitted visualization of when and where specific genes were active during switching and mating |
| Animal infection models | Laboratory mice used to study microbial behavior in living hosts | Provided crucial evidence that mating occurs in mammalian environments, not just laboratory conditions 6 |
Each of these tools served as a window into different aspects of Candida biology. The gene knockout constructs, for instance, allowed us to move beyond correlation to causation—not just observing what genes were present, but determining what they actually did. The animal models bridged the gap between laboratory observations and biological reality, reminding us that microorganisms behave differently in their natural environments than they do in plastic dishes.
When I look back on the journey from thinking of Candida as a simple asexual yeast to understanding its sophisticated sexual cycle and genetic switching mechanism, I'm reminded why I fell in love with science. It's not about confirming what we already know, but about exploring what we don't understand. The white-opaque switch and mating discovery have opened entire new fields of inquiry—researchers are now investigating how these processes affect virulence, drug resistance, and transmission 6 .
"The most rewarding aspect hasn't been the individual discoveries themselves, but seeing how they've enabled other scientists to ask better questions."
The most rewarding aspect hasn't been the individual discoveries themselves, but seeing how they've enabled other scientists to ask better questions. My laboratory's subsequent work systematically mapping the relationships between morphogenetic switching and pathogenicity using homozygous deletion libraries has helped identify specific genes that might be targeted with new antifungal strategies 6 .
Science, at its best, is a collaborative and cumulative enterprise. What began as a curious observation about cell morphology has grown into a rich understanding of how simple organisms employ sophisticated strategies to survive and thrive. As we continue to probe Candida's secrets, I'm confident that future discoveries will be even more surprising than what we've uncovered so far. After all, if there's one thing my career has taught me, it's that nature always reserves a few surprises for those willing to look closely enough.
The discovery of Candida's sexual cycle was just the beginning. Today, researchers worldwide are building on this foundation to develop new approaches to combat fungal infections.