Fungi: General Characteristics

• Mycoses are fungal infections.
• Mycology is the the study of Fungi.
• A mycologist specializes in mycology.
• Most fungi are saprophytes meaning they get energy by decaying matter.
• Fungi are eukaryotic.
• There are over 50,000 species of fungi, but fewer the 100 are pathogenic to humans.
• Recently there are increased cases of opportunistic Mycoses in patients with underlying conditions such as diabetes, HIV, cancer or prolonged use of corticosteroids or antibiotics.

General Characteristics of Fungi

• Classification is by the form the fungi take.
• Yeasts are unicellular fungi, for example, Candida and Cryptococcus.
• Molds are multicellular filamentous fungi, for example, Aspergillus.
• Dimorphic fungi can exist as yeast or mold depending on temperature. Examples are Histoplasma and Coccidioides.
• They are thallophytes which possess true nuclei and heterotrophic members of the plant family that lack stems and roots.
• They do not have chlorophyll.
• Fungal cell walls have chitin.
• Ergosterol is in their cell membrane.
• Fungi reproduce sexually (spores after sexual fusion) and asexually(Conidia or budding).

Classification of Fungi

• Phylum Zygomycota includes Rhizopus, Absidia and Mucor
• Phylum Dikaryomycota
• Subphylum Ascomycotina includes Trichophyton, Histoplasma, and Blastomyces.
• Subphylum Basidiomycotina includes Cryptococcus.
• Deuteromycotina includes Candida, Epidermophyton, and Coccidioides.

Classification of Mycoses

• Fungal infections are classified based on their location and depth of involvement.

Superficial Mycoses

• Superficial Mycoses affect the outer layers of the skin and hair.
• An example is Tinea versicolor (caused by Malassezia), which is a superficial yeast infection that causes hypo/hyperpigmented patches.
• Pityriasis capitis (dandruff) is caused by Malassezia.

Cutaneous Mycoses

• Cutaneous Mycoses involve the outer skin layers, nails, and hair.
• They are caused by dermatophytes like Trichophyton, Microsporum, and Epidermophyton.

Subcutaneous Mycoses

• Subcutaneous Mycoses occur in deeper layers of the skin, after trauma.
• An example is Sporotrichosis (caused by Sporothrix schenckii), often acquired by gardeners through thorns.
• Another example is Chromoblastomycosis, caused by dematiaceous fungi, presenting as verrucous lesions.

Systemic Mycoses

• Systemic Mycoses are fungal infections that affect internal organs, usually in immunocompromised individuals.
• They can be due to primary or opportunistic pathogens.
• Superficial and cutaneous mycoses are usually mild.
• Systemic mycoses can be life threatening.
• Systemic mycoses due to primary pathogens originate primarily in the lungs and may spread to many organ systems.
• These Organisms are inherently virulent.
• Primary pathogens causing systemic mycoses are dimorphic.
• An example is Histoplasmosis caused by Histoplasma capsulatum, common in areas with bat or bird droppings, which affects the lungs.
• Coccidioidomycosis is caused by Coccidioides immitis, affects lungs, and can cause meningitis.
• Blastomycosis is caused by Blastomyces dermatitidis, found in soil and decaying organic matter, affecting the lungs and skin.

Systemic Mycoses due to Opportunistic Pathogens

• Often found in patients with immune deficiencies like AIDS, or alteration of normal flora by antibiotics.
• Examples are Candidiasis, Cryptococcosis and Aspergillosis.
• Systemic Mycoses are Caused by soil fungi.
• They are acquired by inhalation and mostly asymptomatic, surviving in the body.
• Some are part of the normal flora such as Candida and are innocuous unless defenses are compromised.
• In symptomatic disease dissemination of infection can occur in any organ.
• Systemic Mycoses tend to cause disease to certain persons in whom they are disseminated, and are often fatal infections.
• All fungi are dimorphic and have a unique morphological adaptation to exist in tissue or to growth at 37°C.

Pathogenic Species of Systemic Mycoses

• Coccidioides immitis
• Histoplasma capsulatum
• Blastomyces dermatitidis
• Paracoccidioides brasiliensis

Opportunistic Mycoses

• Candida and related yeasts
• Cryptococcus neoformans
• Aspergillosis
• Zygomycosis

Pathogenesis of Mycoses

• Fungi cause infections by adhering to host tissues, evading immune responses, and producing tissue-damaging enzymes
• Adherence occurs when fungal cell surface proteins bind to host cells.
• Invasionoccurs when fungis penetrate host tissues using enzymes (e.g., proteases, phospholipases).
• Immune Evasion: Many fungi can hide within host cells or alter their surface antigens to avoid immune recognition.
• Immunocompetent individuals typically have localized infections.
• Immunocompromised individuals are more susceptible to invasive infections, including disseminated infections (e.g., in cancer, diabetes, and HIV patients)

Clinical Manifestations of Mycoses

• Tinea versicolor presents as hyperpigmented or hypopigmented patches on the chest, back, and arms.
• Tinea capitis presents as scaly, bald patches on the scalp.
• Tinea pedis presents as red, itchy skin between the toes, sometimes with peeling.
• Tinea corporis presents as circular, raised lesions with central clearing ("ringworm").
• Tinea cruris as itchy, red lesions in the groin area.
• Sporotrichosis causes Painless ulcerative lesions that may spread along lymphatic pathways.
• Chromoblastomycosis causes Warty, raised, pigmented lesions.
• Histoplasmosis causes Fever, cough, chest pain, and potential disseminated disease.
• Coccidioidomycosis causes Pneumonia-like symptoms where 5-10% may develop hypersensitivity reactions (Erythema nodosum or erythema multiforme) - Valley Fever or Desert rheumatism or meningitis in severe cases.
• Blastomycosis causes Fever, cough, skin lesions, or bone involvement.
• Cryptococcosis causes Headache, fever, and symptoms of meningitis.

Diagnosis of Fungal Infections

• Includes Specimens such as Skin scraps, Sputum, Pus, CSF, biopsy and blood for serological diagnosis
• KOH Preparation is when Skin scrapings are treated with potassium hydroxide (KOH) to visualize fungal elements.
• India Ink: Used for detecting Cryptococcus neoformans in cerebrospinal fluid (CSF).
• Fungal cultures must be grown on specialized media (e.g., Sabouraud dextrose agar (SDA)).
• Mold cultures take weeks to grow, while yeast cultures grow faster.
• Dimorphic fungi can switch between yeast and mold form, requiring different culture conditions (e.g., 37°C for yeast, room temperature for mold). Caution should be observed as the spores are very infectious
• Treatment includes primary infection is self-limiting requiring only supportive treatment.
• For disseminated cases Amphotericin B (0.4-0.8mg/kg/d) for months is useful.
• Systemic miconazole and Ketoconazole are useful though show limited benefits in meningeal involvement
• Fluconazole is a modern antifungal useful both orally.

Opportunistic Fungal Infections

• Opportunistic fungi cause infections in immunocompromised individuals
• Immunocompromised Conditions: HIV/AIDS, cancer, organ transplantation, diabetes mellitus, chronic steroid therapy or use of drugs.
• Some common opportunistic fungi are Candida, Aspergillus, Cryptococcus

Candida Infections (Candidiasis)

• Caused by oval budding yeasts of the genus Candida, most commonly Candida albicans.
• Candida is a member of the normal flora of mucous membranes in the respiratory, gastrointestinal and female genital tracts

General Characteristics of Candida

• Candida appears as Gram-positive
• Candida is Oval in shape
• Candida Measuring 2-3 x 4-6µm
• Candida is Gram positive elongated budding cells(Pseudohyphae)
• Candida Ferments glucose and maltose producing both acid and gas.
• Candida Produces acid from sucrose
• Carbohydrate fermentations together with colonial differentiates C.albicans

Candidiasis Clinical Manifestations

• Oral Candidiasis (Thrush) causes White patches on the tongue, cheeks, or throat, common in immunocompromised individuals & in infants.
• Vaginal Candidiasis causes Itching, discharge, and redness in the vaginal area, that cause irritation discharge. Predisposing conditions include diabetes, pregnancy, and antibiotic medications.
• Cutaneous Candidiasis causes a Red, itchy rash, most common in obese and diabetics.
• Systemic Candidiasis can lead to invasive disease, affecting the bloodstream (candidemia), internal organs, and endocarditis.

Diagnosing of Candidiasis

• Cultures are Grown on blood cultures, throat swabs, vaginal cultures on SDA & Nutrient agar.
• Microscopy is preformed after taking a KOH prep for skin or mucosal lesions.
• Antigen testing, also be preformed a carbohydrate extract of Candida gives positive precipitin reactions .
• Treatment of candidiasis includes topical antifungals such as nystatin, clotrimazole.
• Gentian violet(GV) can also be used for application for oral thrush,2% miconazole for vaginitis.
• Nystatin suppresses intestinal and vaginal candidiasis.
• Systemic antifungals are used for invasive candidiasis.
• Candida infection is not communicable and under normal situation most individuals harbor organisms.

Aspergillosis

• Caused by filamentous fungi of the genus Aspergillus.

Clinical Manifestations

• Pulmonary Aspergillosis is the The most common presentation, typically in patients with lung disease.
• Invasive Aspergillosis can cause fever, cough, hemoptysis, and respiratory distress.
• Diagnosis of Aspergillosis is done by CT scans looking for, "halo" sign.
• Microscopy and Culture Sputum or biopsy specimens may show septate hyphae.
• Galactomannan antigen test is used to evaluate fluid samples and blood.
• For treatment Voriconazole is the first-line treatment.
• Amphotericin B and echinocandins are used in severe cases.

Cryptococcosis

• Caused by Cryptococcus neoformans and Cryptococcus gattii, found in pigeon droppings.
• Clinical Manifestations
• Meningitis is the most common form of cryptococcal infection, particularly in immunocompromised patients.
• Pulmonary cryptococcosis pulmonary involvement with symptoms such as cough, chest pain, and shortness of breath particularly in immunocompromised patients.
• Meningitis is typically diagnoses via CSF analysis where you will look for capsules.
• Blood, CSF or sputum cultures can also be taken.
• Cryptococcosis is treated with Amphotericin B and Fluconazole

Mucormycosis (Zygomycosis)

• Disease is caused by fungi from the order Mucorales.
• Rhinocerebral Mucormycosis is the most common presentation.
• Rhinerebral mucormycosis is characterized by fever, congestion especially in patients with diabetes.
• Can be diagnosed with broad non septate hyphae

Opportunistic Fungal Infections

• Prophylaxis using antifungals such as fluconazole can be used for patients with Candida.
• Environmental: In hospitals reduce fungal spores.

Important Facts

• Fungi are distinct from plants and animals.
• Grow as Filaments
• May be acquired from disease causing in the environment.
• Diseases are serious in immunocompromised individuals.