Table of Contents
Yeasts are a diverse group of unicellular fungi. Many species are harmless or useful (for example in baking, brewing, and winemaking), but some can cause disease in humans, especially when the immune system is weakened or the normal microbial balance is disturbed. In this chapter, we focus on pathogenic yeasts and the infections they cause.
General Features of Pathogenic Yeasts
Pathogenic yeasts share some common biological characteristics:
- They are unicellular fungi that reproduce mainly by budding.
- They possess a cell wall (rich in polysaccharides like glucans and mannans) and a cell membrane containing ergosterol (a key target of many antifungal drugs).
- Many are part of the normal human microbiota (especially on skin, mucous membranes, and in the gut) and only cause disease under certain conditions (opportunistic pathogens).
- They can switch between different growth forms (for example, round yeast cells and elongated pseudohyphae), which contributes to their ability to invade tissues.
The most important pathogenic yeast in humans is the genus Candida, particularly Candida albicans. Another clinically relevant group is the encapsulated yeast Cryptococcus.
Candida Infections (Candidiasis)
The Organism
Candida species (especially C. albicans, but also C. glabrata, C. tropicalis, C. parapsilosis and others) are common inhabitants of:
- Oral cavity
- Gastrointestinal tract
- Vagina
- Skin folds
Under normal circumstances, they are kept in check by:
- The host’s immune system
- The physical barriers of the skin and mucous membranes
- Competition from bacteria and other microbes
When these control mechanisms fail or are disturbed, Candida can overgrow and cause disease (candidiasis).
Predisposing Factors
Typical situations that favor candidiasis include:
- Antibiotic therapy: broad-spectrum antibiotics kill many bacteria of the normal flora, allowing yeasts to multiply more freely.
- Diabetes mellitus: elevated blood glucose encourages fungal growth; immune function may be impaired.
- Immunosuppression:
- HIV infection/AIDS
- Chemotherapy or radiation therapy
- Immunosuppressive drugs after organ transplantation
- Hormonal changes: pregnancy, oral contraceptives can influence vaginal flora and pH.
- Moisture and maceration of the skin: tight clothing, diapers, obesity (skin folds), prolonged occlusion.
- Medical devices: catheters, intravenous lines, prostheses provide surfaces for yeast adhesion and biofilm formation.
Forms of Candidiasis
Candidiasis can affect surface tissues (mucous membranes and skin) or become systemic (invasive).
1. Oral Candidiasis (Oral Thrush)
Typical features:
- White, creamy plaques on tongue, inner cheeks, palate, or throat.
- Plaques can often be wiped off, leaving a red, sometimes bleeding surface.
- Burning or pain in the mouth, difficulty swallowing if the infection extends into the esophagus.
Common in:
- Infants
- Elderly
- Patients using inhaled corticosteroids (for example, asthma medications) without rinsing the mouth afterward
- People with weakened immunity (for example, HIV infection)
2. Vaginal Candidiasis
Also called “yeast infection” of the vagina.
Main symptoms:
- Intense itching (pruritus)
- Redness and swelling of the vulva and vaginal mucosa
- Thick, white, “cottage cheese–like” discharge
- Burning, especially during urination or sexual intercourse
Risk factors often include antibiotic use, pregnancy, diabetes, and hormonal contraceptives. The vaginal pH is usually still acidic, which helps distinguish it from some bacterial infections.
3. Cutaneous Candidiasis
Yeast infections of the skin occur mainly in warm, moist areas:
- Skin folds (under breasts, groin, armpits, between fingers or toes)
- Diaper area in infants (diaper rash complicated by Candida)
Typical appearance:
- Red, moist, often shiny patches with sharply defined borders.
- Often with small “satellite” pustules or papules around the main lesion.
- Itching and burning are common.
4. Onychomycosis (Nail Infections)
Candida can infect nails and surrounding nail folds, especially:
- In people whose hands are frequently wet (for example, dishwashers, cleaners, bartenders).
- In those with chronic skin conditions or diabetes.
Signs include:
- Swelling and redness of the nail fold (paronychia).
- Thickened, discolored, and brittle nails.
5. Invasive Candidiasis (Systemic Infection)
When Candida enters the bloodstream or deeper tissues, it can cause severe, often life-threatening infections:
- Candidemia: presence of Candida in the blood; may lead to sepsis.
- Spread to internal organs: kidneys, liver, spleen, heart (endocarditis), eyes, brain, etc.
Patients at highest risk:
- Intensive care unit (ICU) patients
- Those with central venous catheters or long-term intravenous lines
- People receiving total parenteral nutrition (intravenous feeding)
- Patients after major surgery, particularly abdominal surgery
- Severely immunocompromised patients (for example, after stem cell transplantation)
Clinical signs are often non-specific (fever, chills, low blood pressure), making diagnosis challenging. Early treatment with antifungals is crucial.
Virulence Factors of Candida (Overview)
Candida possesses several properties that help it cause disease:
- Adhesins: surface molecules that enable adherence to epithelial cells and foreign materials (e.g., catheters).
- Morphological switching: ability to form pseudohyphae or hyphae that can penetrate tissues.
- Enzymes: proteases, phospholipases that damage host cells.
- Biofilm formation: complex communities attached to surfaces and embedded in a protective matrix; biofilms are more resistant to antifungal drugs and immune defenses.
These characteristics are especially relevant in persistent mucosal infections and in infections related to medical devices.
Diagnosis and Treatment (Overview)
For beginner level, it is enough to know that:
- Diagnosis is based on:
- Clinical appearance.
- Microscopic examination (yeast cells, budding cells, pseudohyphae).
- Culture from swabs or blood.
- Treatment uses antifungal agents that target fungal-specific structures:
- Topical drugs (creams, gels, suppositories) for localized infections.
- Systemic agents (tablets or intravenous) for severe or invasive disease.
Common drug groups include azoles (inhibit ergosterol synthesis), polyenes (bind ergosterol), and echinocandins (inhibit cell wall glucan synthesis). Choice depends on site and severity of infection, and on the species and resistance profile.
Preventive measures often focus on:
- Avoiding unnecessary broad-spectrum antibiotic use.
- Controlling diabetes and other underlying conditions.
- Proper care and timely removal of catheters and other invasive devices.
- Keeping skin folds dry and clean.
Cryptococcosis: An Encapsulated Yeast Infection
Another important yeast pathogen is Cryptococcus neoformans (and related species). This yeast is:
- Encapsulated: surrounded by a thick polysaccharide capsule that helps it resist phagocytosis.
- Found in the environment, especially associated with bird droppings (for example, pigeons) and in soil.
- Inhaled as small particles that reach the lungs.
Clinical Manifestations
In many healthy people, infection is asymptomatic or mild. In individuals with markedly weakened immune systems (especially AIDS patients), Cryptococcus can spread from the lungs to the central nervous system and cause:
- Cryptococcal meningitis: a serious inflammation of the meninges (membranes covering the brain and spinal cord).
Symptoms may include:
- Headache, fever
- Neck stiffness
- Sensitivity to light
- Confusion or changes in behavior
- In severe cases, coma and death
Cryptococcal meningitis requires long-term systemic antifungal therapy and management of intracranial pressure.
Yeasts as Opportunistic Pathogens
A key concept in yeast infections is opportunism:
- Many disease-causing yeasts are normally harmless commensals (such as Candida in the gut or vagina).
- Environmental yeasts (such as Cryptococcus) are usually no problem for immunocompetent individuals.
- Disease emerges when:
- The host’s immune defenses are impaired.
- The normal microbiota is disturbed.
- Physical barriers (skin, mucous membranes) are breached by trauma or medical procedures.
Thus, yeast infections often signal underlying conditions (for example, HIV, poorly controlled diabetes, or severe immunosuppression). Recognizing and treating the underlying cause is as important as treating the infection itself.
Summary
- Yeasts are unicellular fungi; most are harmless, but some (especially Candida and Cryptococcus) can cause disease.
- Candidiasis ranges from superficial infections (oral thrush, vaginal yeast infections, skin and nail involvement) to life-threatening systemic disease.
- Risk factors include antibiotic use, diabetes, immunosuppression, moisture, and invasive medical devices.
- Cryptococcus causes serious infections, particularly meningitis, mostly in people with severely weakened immune systems.
- Many yeast infections are opportunistic: they exploit weakened defenses or disturbed microbial balance rather than attacking healthy hosts outright.