Some fungi are more versatile than others; for example, some fungi are used in medicine, while others are toxic. Consider the dimorphic fungi, which can exist in two different forms. At room temperature, they usually appear as molds. But, when exposed to body temperature or grown in enriched media, they transform into a yeast-like substance, or sometimes spherules.
This shape-shifting ability is triggered by temperature, which is why it’s called thermal dimorphism. This highlights their ability to withstand varying temperatures, although they can still be affected by cold as discussed in this article about how fungi survive freezing temperatures.
While this is a fascinating trait, it also has important implications for human health. Several types of dimorphic fungi are pathogens, meaning they can cause disease in humans. In fact, these fungi are responsible for a significant number of systemic mycoses, which are infections that can spread throughout the body. The CDC estimates that Onygenales dimorphic fungi cause over 650,000 new infections each year in the United States alone.
This article will take a closer look at some of the key dimorphic fungal pathogens, including:
- Blastomyces
- Coccidioides
- Histoplasma
- Paracoccidioides
We’ll explore their unique characteristics, how they are identified, and how their evolutionary adaptations contribute to their virulence, or ability to cause disease.
Morphological and Environmental Characteristics of Key Dimorphic Fungi
Dimorphic fungi are fascinating because they can switch between two forms depending on their environment. Here’s a look at some important species:
Blastomyces dermatitidis
This fungus is most often found in North America, especially around the Mississippi and Ohio River valleys. The mycelial form, which is the mold-like phase, produces small structures called conidia, typically 2-10 micrometers in diameter. The yeast phase, on the other hand, has large yeast cells with broad bases that bud off. These cells can range from 8-15 micrometers, and sometimes can even get up to 30 micrometers.
Coccidioides immitis/posadasii
You’ll find these fungi in the dry, semi-dry areas of the southwestern United States, as well as parts of Central and South America. In tissues, they form something called spherules, which contain endospores. In the mycelial phase, they produce arthroconidia, which measure about 2.5-4 x 3-6 micrometers. Keep in mind that the “spherule phase” is not something you’d see in routine lab procedures.
Histoplasma capsulatum
This one has a worldwide distribution, but it’s also common in the Mississippi and Ohio River valleys. The yeast cells are small, only about 3-4 x 2-3 micrometers, and you’ll often find them inside macrophages (a type of immune cell). The mycelial form produces tuberculate macroconidia, which are larger, ranging from 8-14 micrometers in diameter.
Paracoccidioides brasiliensis/lutzii
These fungi are found in South America, especially Brazil. The yeast cells are quite large, ranging from 20-60 micrometers, and have multiple buds, giving them the appearance of a “pilot’s wheel.”
How are dimorphic fungi identified in the lab?
Figuring out which dimorphic fungus is causing an illness requires careful testing in a laboratory. Here’s an overview of the methods scientists use:
Traditional Methods
These tried-and-true methods have been used for decades:
- Microscopic examination: Looking at clinical samples like tissue or sputum under a microscope. Special stains can highlight the fungi and help identify their unique shapes.
- Culture and temperature conversion: Growing the fungus in a lab dish at different temperatures. This is key to observing the dimorphic conversion – the change in form from mold to yeast (or vice versa). It’s worth noting that working with these cultures can be risky and requires strict safety measures.
Modern Molecular Diagnostics
These cutting-edge techniques offer faster and more accurate identification:
- Exoantigen testing: Detecting specific fungal antigens (proteins) released into culture.
- DNA probe assays: Using labeled DNA fragments to find specific DNA sequences unique to each fungus.
- PCR and Real-time PCR: Amplifying fungal DNA to rapidly and sensitively detect the presence of a specific fungus. For example, real-time PCR can target the BAD1 gene to identify Blastomyces dermatitidis.
- DNA sequencing and ITS region analysis: Sequencing a specific region of the fungal DNA called the ITS region. This is like scanning a barcode to identify the exact species of fungus.
Histopathology
This involves examining tissue samples under a microscope to observe the fungi directly within the infected tissue. Pathologists look for characteristic appearances like:
- The spherules (round structures) of Coccidioides
- The yeast cells of Histoplasma nestled inside macrophages (immune cells)
Combining these traditional and modern methods gives doctors the best chance to accurately diagnose and treat infections caused by dimorphic fungi.
Evolutionary Adaptations and Virulence Factors
Dimorphic fungi are fascinating organisms, and understanding how they’ve evolved to thrive in different environments is key to understanding their ability to cause disease.
Genomic Comparisons within Ajellomycetaceae
Comparing the genomes of different fungi, especially within the Ajellomycetaceae family, can tell us a lot about how these organisms adapted to infect humans. Genome sequencing can reveal the genetic differences between pathogenic (disease-causing) and non-pathogenic species.
One important area of study is how gene families have expanded or contracted over time. These changes in gene content can be linked to the evolution of fungal virulence – their ability to cause disease.
Shift in Nutrient Acquisition
One major adaptation involves how these fungi get their food. Pathogenic species have often lost genes that allow them to break down plant material. Instead, they’ve gained genes that help them utilize animal-based substrates, like proteases, which are enzymes that break down proteins.
Carbohydrate Metabolism and Protein Catabolism
Pathogenic Ajellomycetaceae species tend to have fewer carbohydrate-active enzymes compared to their saprophytic (decay-feeding) relatives. This suggests a decreased reliance on carbohydrates as an energy source. Interestingly, the number of peptidases (enzymes that break down proteins) remains relatively similar between pathogenic and non-pathogenic species.
Transporter Families
Another adaptation involves the loss of transporters that are associated with plant cell wall substrates in pathogenic species. These transporters are responsible for bringing specific nutrients into the fungal cell, and their loss reflects the shift away from plant-based nutrition.
Mating Type Locus Evolution
The mating system of these fungi has also evolved. Non-pathogenic species in Ajellomycetaceae often exhibit homothallic mating systems, meaning they can reproduce sexually with themselves. In contrast, most pathogenic species are heterothallic, requiring two different mating types to reproduce sexually.
Clinical Manifestations and Treatment Considerations
Dimorphic fungi can cause a range of infections, and how they show up in your body and how they’re treated depends on a few key things.
Common Sites of Infection
The lungs are often the first place these fungi set up shop. Inhaling fungal spores can lead to a pulmonary infection, which is like a lung infection. From there, the infection can potentially spread to other organs, leading to a more widespread, systemic disease.
Factors Influencing Disease Severity
How sick you get from a dimorphic fungal infection depends a lot on your immune system. If your immune system is strong, you might not even know you’ve been exposed, or you might have mild symptoms. But if your immune system is weakened, the infection can be more severe. The amount of fungal spores you’re exposed to also plays a role. A higher dose can increase the chances of developing a more serious infection.
Treatment Options
Antifungal medications are the main way to fight these infections. Azoles like itraconazole and fluconazole, as well as amphotericin B, are commonly used. These drugs work by targeting the fungal cells and stopping them from growing or killing them outright.
Determining which antifungal medication will be most effective can be tricky. Antifungal susceptibility testing, which checks how well a particular drug works against a specific fungus, isn’t usually done for dimorphic fungi. That’s because there’s not a lot of data available, and there aren’t standardized testing methods for fungi such as Blastomyces dermatitidis, Coccidioides immitis/posadasii, Histoplasma capsulatum, and Paracoccidioides brasiliensis/lutzii. Because of these limitations, doctors usually rely on their experience and knowledge of the different antifungal medications to choose the best treatment option.
Frequently Asked Questions
What are some infections caused by dimorphic fungi?
Dimorphic fungi can cause a range of infections, often affecting the lungs. Common examples include histoplasmosis (caused by Histoplasma capsulatum), blastomycosis (caused by Blastomyces dermatitidis), coccidioidomycosis (caused by Coccidioides immitis and Coccidioides posadasii), and paracoccidioidomycosis (caused by Paracoccidioides brasiliensis). These infections can range from mild to severe, depending on the individual’s immune system and the specific fungus involved.
Is Candida albicans a dimorphic fungus?
Yes, Candida albicans is considered a dimorphic fungus. It can exist in both yeast and hyphal (filamentous) forms, and its ability to switch between these forms is important for its virulence and ability to cause infections, such as thrush and yeast infections.
How do you treat dimorphic fungi infections?
Treatment for dimorphic fungi infections typically involves antifungal medications. The specific medication and duration of treatment depend on the type of infection, its severity, and the patient’s overall health. Common antifungal drugs used include azoles (like itraconazole and fluconazole) and amphotericin B.
What is the difference between yeast, mold, and dimorphic fungi?
Yeast are single-celled fungi, while molds are multicellular filamentous fungi. Dimorphic fungi are unique because they can exist in both yeast and mold forms, depending on environmental conditions like temperature. They typically grow as molds in the environment and as yeasts in the body.
In Conclusion
Dimorphic fungi can be hard to diagnose because they can take different forms depending on their environment. Identifying them accurately requires a careful combination of looking at them under a microscope, culturing them, and using molecular methods.
It’s important to diagnose these infections quickly and correctly so that the right antifungal treatment can be started right away. This can make a big difference in how well a patient recovers.
Ongoing research into the genes and virulence factors that make dimorphic fungi harmful is essential. As we learn more about these fungi, we can develop better ways to diagnose and treat the infections they cause. This will lead to improved diagnostic tools and therapeutic strategies in the future, ultimately improving patient outcomes.