The World of Microfungi & Microfungal Diseases

 

juboo lim 

Let us now go into the world of microfungi, providing the same level of detail we’ve covered for viruses, bacteria, and parasites earlier in this blog. 

Origin of Microfungi

Microfungi belong to the kingdom Fungi, a group of eukaryotic organisms distinct from plants, animals, and bacteria. Fungi appeared approximately 1.5 billion years ago, evolving from early eukaryotic ancestors. Fungi are crucial to Earth's ecosystems, as they break down organic material, cycle nutrients, and interact with plants and animals.

Microfungi evolved from early fungal species, developing diverse forms and life strategies. They exist in a range of environments, from soil and decaying matter to living hosts (humans, plants, and animals). Fungi’s symbiotic and pathogenic relationships with other organisms have driven their evolution.

Classification of Microfungi

Microfungi are a subset of fungi, characterized by their microscopic size. They include moulds, yeasts, and other fungi. Micro fungi can be classified based on morphology, reproductive structures, and ecological roles:

Their morphology is the following:

Moulds are filamentous fungi, characterized by thread-like structures called hyphae. The hyphae form a network called a mycelium (e.g., Aspergillus).

Yeasts are unicellular fungi that reproduce by budding or binary fission (e.g., Candida albicans).

Dimorphic fungi can exist as both moulds and yeasts depending on environmental conditions (e.g., Histoplasma capsulatum). Their reproductive structures are described below:

Ascomycota means they produce spores in sac-like structures called asci (e.g., Penicillium).

Basidiomycota means they produce spores on club-like structures called basidia (e.g., Cryptococcus).

Zygomycota means they produce zygospores through sexual reproduction (e.g., Rhizopus).

Chytridiomycota means they produce motile spores with flagella, mostly aquatic fungi.

Let’s now look at their ecological roles:

 Saprophytic fungi decompose dead organic matter (e.g., Penicillium). Parasitic fungi infect living organisms (e.g., Candida albicans). Mutualistic fungi form beneficial symbiotic relationships, such as mycorrhizae with plants.

Pathogenicity of Microfungi:

Many microfungi are opportunistic pathogens. They cause disease primarily in immunocompromised individuals or when environmental conditions favour fungal growth. Their pathogenicity arises from several factors such as: 

1. Invasion: Some fungi invade tissues and organs, causing direct damage. For example, Candida species can invade the bloodstream and internal organs.
2. Toxin Production: Fungi may produce mycotoxins, which are toxic compounds that can affect humans and animals when inhaled or ingested (e.g., Aspergillus flavus produces aflatoxins).
3. Immune Evasion: Some fungi evade the immune system by modifying their surface proteins, allowing them to persist within the host.

Examples of Pathogenic Microfungi and Diseases

1. Candida albicans (causes candidiasis)

Classification: Ascomycota, yeast

Disease they cause are oral thrush, vaginal yeast infections, systemic candidiasis

Symptoms they show are white patches in the mouth, itching, discomfort, systemic infections in immunocompromised individuals. They are treatable with antifungal drugs like fluconazole; systemic infections require stronger antifungal treatment.

2. Aspergillus fumigatus (causes aspergillosis)

Classification: Ascomycota, mould. The diseases they cause are pulmonary aspergillosis, allergic bronchopulmonary aspergillosis (ABPA). Symptoms presented are, cough, fever, shortness of breath, lung infections. In terms of curability, they are treatable with antifungal drugs like voriconazole or amphotericin B.

3. Cryptococcus neoformans (causes cryptococcosis)

Classification: Basidiomycota, yeast. The diseases they cause are cryptococcal meningitis, primarily in immunocompromised individuals. The symptoms for cryptococcal meningitis are, headache, fever, neck stiffness, neurological symptoms. As for curability this condition is treatable with antifungal drugs (amphotericin B and flucytosine); long-term therapy required for severe cases.

4. Histoplasma capsulatum (causes histoplasmosis)

Classification: Ascomycota, dimorphic fungus. Here the disease is lung infection (histoplasmosis). Symptoms shown are fever, cough, fatigue, chest pain. Histoplasmosis can be treated with antifungal medications like itraconazole; severe cases may require amphotericin B.

5. Pneumocystis jirovecii (causes pneumocystis pneumonia).

The classification is Ascomycota, a yeast-like fungus. The disease cause is pneumonia in immunocompromised individuals (e.g., HIV/AIDS patients). Symptom-wise, it is presented with cough, fever, difficulty breathing. It is treatable with antibiotics (trimethoprim-sulfamethoxazole) rather than antifungals, as Pneumocystis lacks ergosterol in its cell membrane.

What about non-pathogenic microfungi? Many microfungi are harmless or beneficial such as Penicillium. This microfungus is responsible for the discovery of penicillin, the first antibiotic. Also used in food production (e.g., in cheese fermentation). There is also the Saccharomyces cerevisiae, commonly known as baker’s yeast, used in baking, brewing, and fermentation.

Why are some microfungi disease-causing (pathogenic)? There are at least 3 reasons, we know, namely:

1. Opportunistic Nature: Most pathogenic fungi are opportunistic, meaning they cause disease only in individuals with weakened immune systems (e.g., people with HIV/AIDS, cancer, or those taking immunosuppressive drugs).
2. Environmental Exposure: Inhalation of fungal spores, especially from soil or decaying organic matter, can lead to infections in humans.
3. Fungal Virulence Factors: Pathogenic fungi possess virulence factors such as the ability to evade the immune system, survive within host cells, and produce toxins.

Fungal Toxins (Mycotoxins)

Certain fungi produce mycotoxins, toxic secondary metabolites that can cause serious health issues such as

Aflatoxins produced by Aspergillus flavus, can contaminate crops like peanuts and corn, leading to liver damage or cancer. The consumption of mouldy peanuts or its sauce, seen or unseen can cause this

Ochratoxins produced by Aspergillus and Penicillium species, can lead to kidney damage.

Trichothecenes produced by Fusarium species, associated with immunosuppression and digestive disorders.

Fungi vs. Bacteria.

Firstly, their size.  Fungi are generally larger than bacteria and are eukaryotic, meaning they have a defined nucleus and organelles. Bacteria are prokaryotic and lack a nucleus.

Secondly, their method of reproduction.  Fungi reproduce through spores, which can be asexual or sexual. Bacteria reproduce asexually by binary fission.

Thirdly, their cell structure.  Fungi have chitin in their cell walls, while bacteria have peptidoglycan.

Curability of Fungal Infections? 

Most fungal infections are treatable with antifungal medications such as azoles (e.g., fluconazole), echinocandins (e.g., caspofungin), and polyenes (e.g., amphotericin B). However, some fungal infections, particularly in immunocompromised patients, can be severe or even fatal if not treated promptly.

To sum up, microfungi, like bacteria and viruses, play essential roles in the environment, but certain species can become pathogenic, especially in individuals with weakened immune systems. Understanding their biology, classification, and pathogenicity helps in the treatment and prevention of fungal diseases.

This ends our series on infectious diseases caused by viruses, bacteria, parasites and micofungi. Hope they are  educational and informative.