Introduction to Fungi in Mycology

Introduction to Fungi

• Mycology is the study of fungi.
• Fungi are eukaryotic organisms.
• Many different types of fungi exist.
• Candida species are most relevant to dentistry.
• Oral mycoses are fungal infections of the oral cavity.

Morphology

• Fungi can exist in various forms including yeast, dimorphic fungi, and mould.
• Yeasts are unicellular, spherical/ovoid cells.
• Molds are multicellular, with thread-like structures called hyphae.
• Hyphae may have cross-walls called septa.
• Fungal structures such as conidiophores and sporangiophores produce conidia/spores (asexual reproductive units) or spores (sexual or asexual).

Reproduction

• Fungi reproduce both sexually and asexually.
• Asexual reproduction involves spore-producing structures.
• Sexual reproduction involves fusion of cytoplasm and nuclei.

Pathogenicity

• Medically important fungi generally cause slow-progressing, chronic infections.
• Candida species adhere to host tissues, form biofilms, and modify surface antigens, enabling immune evasion and tissue destruction.
• They produce enzymes like phospholipase, proteinase, and haemolysin to cause tissue destruction.

Human Mycoses

• Human fungal infections are categorized as superficial, subcutaneous, and systemic.

Superficial Mycoses

• Affect mucosal surfaces and keratin-containing structures (skin, nails, hair).
• These infections are relatively common and are often cosmetic problems, not life-threatening.
• Common causes include Candida species and dermatophytes (tinea/ringworm).

Subcutaneous Mycoses

• Infection of the subcutaneous tissue.
• Often result from environmental fungi entering the tissue through trauma.
• May cause chronic, progressive disease, tissue destruction, and sinus formation.
• Examples are sporotrichosis and eumycetoma.

Systemic Mycoses

• The most serious and often fatal form of mycoses.
• Involve the body's internal organ systems.
• Common among immunocompromised patients—patients with organ transplants, prosthetic devices, ICU patients, HIV/AIDS, long-term steroid use, or malignancies.
• Examples include cryptococcosis, histoplasmosis, invasive candidiasis, and invasive aspergillosis.

Opportunistic Mycoses

• Opportunistic pathogens—fungi that are normally harmless—cause disease in immunocompromised patients.
• These fungi are typically found in the body's normal flora.
• Predisposing factors include reduced cell-mediated immunity, altered skin/mucus membrane integrity, or suppression of normal flora by antibiotics.

Risk Factors for Fungal Infections

• HIV/AIDS (especially advanced stages)
• Organ transplant patients
• Patients with malignancies
• Patients with autoimmune disorders (like SLE)
• Diabetes mellitus
• Use of immunosuppressive agents (cytotoxic agents, corticosteroids)
• Prolonged antibiotic use
• Travelers/inhabitants of regions with endemic mycoses

Laboratory Diagnosis

• Accurate diagnosis requires proper collection and transport of specimens to the microbiology lab.
• Fungi grow slowly, thus delays in transport lead to overgrowth and inaccurate isolation.
• Common specimens include respiratory tract, hair, skin, nails, blood, CSF, aspirates, and tissue.

Laboratory Diagnosis - Microscopy

• Direct microscopy is performed to identify fungal elements in collected specimens.
• Common stains include Gram stain and KOH.

Laboratory Diagnosis - Culture

• Medically important fungi have varied growth requirements compared to bacteria.
• Sabouraud dextrose agar is often used to isolate fungi.
• This medium has a high carbohydrate content and low pH, inhibiting bacterial growth.
• Antibiotics may be added to further suppress bacterial growth.

Prevention of Fungal Infections

• Prevention is better than cure.
• Methods to prevent fungal infections include treatment of underlying immunosuppression, prophylactic therapy for immunocompromised patients, and infection prevention/control measures such as hand hygiene or isolation of selected patients.

Antifungal Agents

• Antifungals target different mechanisms to stop fungal growth and include polyenes, azoles, echnocandins, allylamines, and other agents.
• Mechanism of action varies, such as targeting cell membrane damage or inhibiting cell wall synthesis.

Antifungal Agents - Specific Example (Fluconazole)

• Blocks ergosterol synthesis by inhibiting 14-α-demethylase.
• Used for oropharyngeal, vaginal, oesophageal candidiasis; systemic candidiasis, prophylaxis for invasive candidiasis; and treatment/prophylaxis of cryptococcal meningitis.

Antifungal Agents - Specific Example (Amphotericin B)

• Forms complexes with ergosterol, disrupting the fungal cell membrane, causing cytoplasmic leakage, and cell death.
• Used to treat serious systemic infections like systemic candidiasis, cryptococcal meningitis, blastomycosis, histoplasmosis, and coccidioidomycosis.

Antifungal Agents - Specific Example (Micafungin)

• Inhibits fungal cell wall synthesis by inhibiting 1,3 beta-D-glucan synthase.
• Used for systemic candidiasis and severe systemic infections, but not effective against Cryptococcosis and molds.

This quiz covers the fundamentals of fungi, including their morphology, reproduction, and pathogenicity. It explores various types of fungi, with a focus on those relevant to dental health, such as Candida species. Test your knowledge on these essential aspects of mycology and fungal infections.