Introduction to Fungi PDF 2024

Introduction to Fungi Dr Roxanne Rule Clinical Microbiologist Department of Medical Microbiology, National Health Laboratory Service and University of Pretoria [email protected] ...

Introduction to Fungi Dr Roxanne Rule Clinical Microbiologist Department of Medical Microbiology, National Health Laboratory Service and University of Pretoria [email protected]  Introduction Mycology = The study of fungi Eukaryotic organisms Many different species exist Most relevant to dentistry = Candida species Oral mycoses = fungal infections of the oral cavity  Morphology Fungal forms Dimorphic Yeast Mould Fungi  Morphology YEASTS MOULDS Unicellular Multicellular Spherical/ovoid cells Multiple specialised structures that perform specific functions - Hyphae: Thread-like tubes containing fungal cytoplasm and organelles - Septa: Cross walls that divide the hyphae into unit cells - Conidiophores/sporangiophores: Stalk that bears the conidium/sporangium - Conidium/sporangium: An enclosure in which conidia/spores are formed - Conidia: Asexual reproductive units - Spores: Sexual or asexual reproductive units  Reproduction Fungi exhibit both sexual and asexual forms of reproduction  Pathogenicity In general, medically important fungi do not possess the virulent attributes of bacteria such as exotoxins and endotoxins Therefore, they cause slowly progressive, chronic infections Candida species Adhere to host tissues → form biofilm Able to switch and modify surface antigens → immune evasion Able to form hyphae → allows for invasion Produces enzymes (phospholipase, proteinase, haemolysin) → tissue destruction  Human Mycoses Human fungal infections Subcutaneous Superficial mycoses Systemic mycoses mycoses  Superficial Mycoses Involves the mucosal surfaces and keratin containing structures of the body (skin, nails and hair) Relatively common infections Not life threatening Pose more of a cosmetic problem than anything else Can be caused by: Candida species (candidiasis) Dermatophytes (tinea/ring worm) Superficial Mycoses - Candidiasis  Superficial Mycoses - Dermatophytoses Result in skin, hair and nail infections Commonly referred to as tinea or ringworm Caused by slow growing moulds of the species: Trichophyton species Epidermophyton species Microsporum species These organisms use keratin as a source of nutrition Don’t invade deeper layers of the skin  Subcutaneous Mycoses Infection of the subcutaneous tissue Usually the result of traumatic implantation of environmental fungi leading to chronic, progressive disease, tissue destruction and sinus formation Examples: Sporotrichosis Eumycetoma  Systemic Mycoses Most serious, often fatal form of mycoses Involve the internal organ systems of the body Commonly seen in immunocompromised patients Organ transplant patients Prosthetic devices ICU patients HIV/AIDS patients Long term steroid/immunosuppressive therapy Malignancies Histoplasmosis, blastomycosis and coccidioidomycosis may occur in otherwise healthy individuals  Systemic Mycoses DISEASE ORGANISM Cryptococcosis Cryptococcus neoformans Histoplasmosis Histoplasma capsulatum Invasive candidiasis Candida species Invasive aspergillosis Aspergillus species  Opportunistic Mycoses Opportunistic pathogens = When fungi (such as Candida albicans) that are generally innocuous for healthy humans cause disease in compromised patient groups These organisms are usually part of the normal skin, GIT or genital flora Predisposing factors: Reduced cell mediated immunity Altered skin and mucus membrane integrity Suppression of normal flora by antibiotics  Risk Factors for Fungal Infections HIV/AIDS (especially advanced stages of disease) Organ transplant patients Patients with malignancies Patients with auto-immune disorders (systemic lupus erythematosus (SLE)) Diabetes mellitus Use of immunosuppressive agents (cytotoxic agents, corticosteroids) Prolonged antibiotic use Travellers/inhabitants of regions with endemic mycoses  Laboratory Diagnosis Accurate diagnosis of human mycoses requires appropriate specimen collection and transport to the microbiology lab Because most fungi are slow growing, any delay in transport compromises specimen quality and decreases the probability of isolating the causative fungal species as a result of overgrowth by contaminants Common specimens: Respiratory tract specimens Hair, skin, nails Blood CSF Aspirates Tissue  Laboratory Diagnosis - Microscopy Upon receipt of the specimen in the lab, direct microscopy is done to identify fungal elements Gram stain KOH  Laboratory Diagnosis - Culture Medically important fungi require different cultural and growth requirements to bacteria Vast majority of fungi grow aerobically Sabouraud dextrose agar is commonly used to isolate medically important fungi This media has a high carbohydrate content as well as low pH which is inhibitory to most bacteria Sabouraud agar may also be supplemented with antibiotics to further suppress bacterial growth  Prevention of Fungal Infections Prevention is always better than cure! Ways to prevent fungal infections: Correct any underlying immunosuppression Prophylactic therapy for immunocompromised patients Infection prevention and control measures Hand hygiene Isolation of select patients (Candida auris) Antifungal Agents Antifungal Agents  Azoles - Fluconazole Mechanism of action: Blocks ergosterol synthesis by inhibiting 14-α demethylase Indications: Oropharyngeal, vaginal and oesophageal candidiasis Systemic candidiasis Prophylaxis for invasive candidiasis Treatment and prophylaxis of cryptococcal meningitis  Polyenes – Amphotericin B Mechanism of action: Forms a complex with ergosterol and disrupts the fungal plasma membrane, leading to leakage of cytoplasmic contents and cell death Indications: Severe systemic infections Systemic candidiasis Cryptococcal meningitis Blastomycosis, histoplasmosis and coccidiomycosis Nephrotoxic  Echinocandins - Micafungin Mechanism of action: Inhibit fungal cell wall synthesis by inhibiting 1,3 β D-glucan synthase Indications: Severe systemic infections Systemic candidiasis Ineffective against Cryptococcosis, moulds