Fungal pathogens are fungi that cause diseases in humans, animals, and plants. While most of the 144,000 classified fungal species are harmless, about 200 species cause illness. These pathogenic fungi are found worldwide and exist as single-celled yeasts or multicellular, filamentous molds.
How Fungi Cause Disease
Fungal pathogens use several mechanisms to infect a host. The first step is adhesion, where the fungus attaches to host cells. This process is mediated by proteins on the fungal surface that bind to molecules on host cells, ensuring a secure attachment.
Once attached, the fungus must breach physical barriers like skin or mucous membranes. Many fungi secrete enzymes, such as proteases and lipases, that break down host tissues to create an entry point. Some can also use physical force with specialized structures to penetrate cells directly. This invasion is aided by the fungus’s ability to change its shape, a process called morphological transition.
An example of morphological transition is the switch from a single-celled yeast to a filamentous hyphal form. This change allows the fungus to spread through tissues and makes it harder for the host’s immune system to eliminate. The formation of biofilms, dense communities of fungal cells in a protective matrix, also helps the pathogen persist. Biofilms shield fungi from immune cells and antifungal drugs, contributing to chronic infections.
To establish infection, some fungi produce mycotoxins, which are toxic metabolites that damage host cells and disrupt physiological processes. Fungi also have ways to evade the host’s immune system. These strategies include altering their surface molecules to avoid recognition, producing substances that suppress the immune response, or hiding within the host’s own cells.
Key Fungal Pathogens Affecting Humans
Fungal infections in humans, or mycoses, are categorized as superficial, cutaneous, subcutaneous, or systemic. Superficial and cutaneous infections are the most common, affecting the outer layers of the skin, hair, and nails. Dermatophytes are a primary cause of these infections, leading to conditions like ringworm and athlete’s foot. Another common superficial fungus is Malassezia, a yeast that can cause skin conditions like pityriasis versicolor.
Subcutaneous mycoses involve the deeper layers of the skin, muscle, and connective tissue, often occurring when a fungus enters through a cut or wound. An example is sporotrichosis, caused by Sporothrix schenckii, a fungus found in soil and on plants. The infection can cause nodules and ulcers that spread along lymphatic vessels.
Systemic fungal infections are the most serious, affecting internal organs. They often occur in individuals with weakened immune systems, making them opportunistic pathogens. Candida species are a frequent cause of mucosal infections like thrush and bloodstream infections (candidemia). Aspergillus species, inhaled from the environment, can cause illnesses ranging from allergic reactions to invasive aspergillosis in immunocompromised individuals.
Other systemic pathogens include Cryptococcus neoformans, which can cause meningitis in people with HIV/AIDS, and Pneumocystis jirovecii, a cause of pneumonia in those with compromised immunity. Some fungi, known as primary pathogens, can cause systemic disease in healthy individuals. These include Histoplasma, Blastomyces, and Coccidioides, which are found in specific geographic regions and cause lung infections when their spores are inhaled.
Impact of Fungal Pathogens on Plants and Animals
Fungal pathogens also pose threats to agriculture and wildlife. In the plant kingdom, fungi are the most prevalent disease-causing organisms, with over 20,000 species known to be parasitic on plants. These pathogens can cause diseases in crops, leading to economic losses and threatening food security. Examples include rusts, smuts, and powdery mildews.
Magnaporthe oryzae, the cause of rice blast disease, threatens rice production worldwide. Fusarium species can infect grains like wheat and corn, reducing yields and contaminating them with mycotoxins. The historical impact of plant fungi is shown by events like the Irish potato famine, caused by Phytophthora infestans, and the destruction of American chestnut trees by Cryphonectria parasitica.
Fungal pathogens also affect animal populations. White-Nose Syndrome, caused by the fungus Pseudogymnoascus destructans, has killed millions of hibernating bats in North America. The fungus grows on the bats’ muzzles and wings, disrupting hibernation and leading to death. Another example is chytridiomycosis, caused by Batrachochytrium dendrobatidis, which has been implicated in the decline of amphibian species globally.
Identifying and Combating Fungal Infections
Diagnostic methods for fungal infections depend on the type and location of the infection. For skin, hair, or nail infections, a common technique is the direct microscopic examination of a sample treated with potassium hydroxide (KOH). Fungal culture, where a sample is grown on special media to isolate the fungus, is another diagnostic tool, though it can be slow.
For systemic infections, diagnosis is more complex. Imaging techniques like X-rays or CT scans can help visualize fungal masses, such as an aspergilloma in the lungs. Molecular methods like the polymerase chain reaction (PCR) can detect fungal DNA in patient samples for rapid diagnosis. Serological tests, which detect fungal antigens or antibodies in the blood, are also used for diagnosing certain systemic mycoses.
The treatment of fungal infections relies on several classes of antifungal drugs.
- Azoles inhibit an enzyme needed to produce ergosterol, a component of the fungal cell membrane.
- Polyenes, like amphotericin B, bind directly to ergosterol, creating pores in the membrane that cause the fungal cell to die.
- Echinocandins target the fungal cell wall by inhibiting the synthesis of β-glucan, a component not found in human cells.
- Allylamines are often used topically for skin infections.
A challenge in combating fungal infections is the rise of antifungal resistance. Like bacteria, fungi can evolve mechanisms to evade these drugs, making infections harder to treat. The potential for side effects from some antifungal medications also complicates treatment. Preventive measures include good hygiene for skin infections and prophylactic antifungal therapy for high-risk, immunocompromised patients.