Medical Fungi and Their Classification

Medical Important Fungi

• Fungi are saprophytic (obtaining nourishment from dead organic matter) and parasitic eukaryotic organisms.
• Although previously classified with plants, fungi are now categorized in their own kingdom, Mycota.
• Fungi can be aerobic or facultatively anaerobic, and are chemoheterotrophic.
• Approximately 200,000 fungal species exist, although only around 100 are significant pathogens in humans.
• A small fraction of fungal species cause the majority of medically relevant infections.

Structure and Classification of Fungi

• Fungi can be unicellular or multicellular.
• Some fungi can be seen with the naked eye (e.g., mushrooms) whereas others cannot be seen (e.g., yeasts).
• Inappropriate storage of food items can lead to mold growth, which can cause foodborne diseases and allergic reactions.
• Some molds produce toxins that can cause various health issues, including cancer.
• Molds are often visible, filamentous structures.
• Yeasts are unicellular microorganisms often observed as spherical or oval-shaped cells.

Overview of Fungi

• Fungi derive their sustenance through a process of absorption.
• They acquire their nutrients through absorption.
• Fungi can live without oxygen.
• Some fungi are aerobic, meaning they need oxygen, and others are facultative anaerobes, which means they can survive with or without oxygen.
• They are chemoheterotrophic, meaning they obtain their energy and carbon from other organisms.

Mycoses

• Human fungal diseases are classified by their location in the body.
• Cutaneous mycoses affect the outermost layers of the skin, hair, and nails.
• Subcutaneous mycoses affect the tissue below the skin.
• Systemic mycoses spread profoundly through the internal parts of the body.
• Mycoses can be either opportunistic or true pathogenic.

Fungal Infection Types

• Superficial infections affect the epidermal layer of the skin.
• Cutaneous infections involve the keratinized layers of skin, hair, and nails.
• Subcutaneous infections penetrate deeper into the skin and underlying tissues.
• Systemic infections affect various organs and tissues in different parts of the body.

Fungal Growth

• Fungi can exist either in filamentous (mold-like) or unicellular (yeast-like) forms.
• Some types are known as dimorphic, and they shift between the mold-like and yeast-like forms based on environmental factors like temperature.
• Filamentous fungi have a mycelium made of thread-like hyphae.

Yeast-like Fungi

• Yeasts are typically single-celled and reproduce via budding.
• Some unicellular fungi reproduce by cell division (fission).
• Some yeasts can switch between unicellular (yeast-like) and multicellular (mold-like) growth forms depending on environmental conditions.

Dimorphic Fungi

• Dimorphic fungi display different forms in distinct environmental conditions.
• Fungi's shape often changes depending on environmental conditions like temperature and carbon dioxide.

Fungal Life Cycle

• Fungi reproduce through both sexual and asexual methods.
• Asexual reproduction involves the formation of spores or conidia.
• Sexual reproduction comprises several stages, typically starting with plasmogamy.

Cutaneous Mycoses

• Cutaneous mycoses are infections of the keratinized layers of the skin, hair, or nails.
• Dermatophytoses are a group of common fungal diseases classified into three genera.

Subcutaneous Mycoses

• Subcutaneous mycoses are deep-seated fungal infections.
• They are usually caused by fungi residing in the soil or decaying vegetation, entering deeper tissues by traumatic injuries.

General Fungal Properties

• Subcutaneous infections occur mostly via traumatic injuries or pinpricks.
• These types of mycoses are more commonly found in individuals who spend time in the soil and vegetation areas.

Sporotrichosis

• It is a subcutaneous mycosis, and the causal organism is Sporothrix schenckii.
• It is initially recognized by a localized granuloma with the potential for regional lymphadenopathy.

Chromomycosis

• This is a rare subcutaneous mycosis caused by various species of dematiaceous fungi.
• It usually involves an infection in the limbs.

Mycetoma

• Mycetoma is a subcutaneous mycosis with characterized by a chronic, localized inflammatory lesion.
• This cutaneous fungal infection is known to affect and erode bones. Often involves swelling and pus formation in infected tissue.

Systemic Mycoses

• Some fungal organisms are naturally pathogenic.
• Systemic mycoses happen commonly in the immunocompromised population.
• Other fungal infections are considered opportunistic, and they impact immunocompromised patients.
• Coccidioidomycosis, histoplasmosis, and blastomycosis are notable examples of systemic mycoses.

Opportunistic Mycoses

• Opportunistic mycoses often affect individuals with weakened immune systems or with medical conditions that compromise immune function.
• Conditions such as cancer and organ transplant recipients are at higher risk.
• The use of immunosuppressive drugs for various treatments has resulted in the spread of candidiasis, histoplasmosis, aspergillosis, and other opportunistic mycoses.

Nosocomial Infections

• Nosocomial infections are infections acquired in a hospital or healthcare setting.
• Roughly 15% of hospital acquired infections are caused by fungal organisms.
• Candida species are common nosocomial fungal pathogens.

Candidiasis

• Candidiasis is a common fungal infection caused by the yeast Candida albicans.
• It can occur in different body parts like skin, mouth, vagina, and intestines.
• Candida species are commonly part of normal body flora.

Cryptococcosis

• Cryptococcosis, a fungal infection, is associated with Cryptococcus neoformans.
• This fungus thrives in environments containing bird droppings.
• A major risk associated with cryptococcosis is its capability to lead to meningitis.

Aspergillosis

• Aspergillosis is caused by Aspergillus fumigatus, often seen in immunocompromised patients.
• This fungus can cause serious lung infections in vulnerable individuals.

Pneumocystis jiroveci

• Pneumocystis jiroveci pneumonia is an infection specifically linked to immunocompromised patients.
• This yeast-like fungus primarily affects the lungs.

References

• Several highly regarded texts on microbiology were used as references for this topic.