Candida species are group of opportunistic fungi and the most common fungi pathogens that affect human. Candida species takes advantage of the host’s debilitated conditions to gain access to the circulation and deep tissues.  

The genus candida is made up of a wide variety of candida species such as Candida albicans, Candida glabrata, Candida tropicalis, Candida parapsilosis, Candida Africana and Candida krusei. Candida albicans is the most common species causing candidiasis to human.

Candida albicans

• It is distributed worldwide and responsible for causing infection in 70-80% of patients with invasive candidiasis
• It is present as a part of normal flora already in the skin and mucous membrane of host it causes infection in the infected host; it is therefore not transmitted.
• Causes opportunistic infections in immunocompromised hosts. It forms the part of the normal flora of the mucous membrane of the gastrointestinal, genitourinary, and respiratory tract.

Properties of Candida albicans

• It is ovoid or spherical  yeast with a single bud
• Forms the part of the normal flora of the mucous membrane of the gastrointestinal, genitourinary, and respiratory tract.
• Produces elongated pseudohyphae in cultures and tissues.
• Readily grows on sabouraud’s dextrose agar and on bacteriological culture media.
• Produces creamy white and smooth colonies with a yeasty odor when cultured.
• Can be differentiated from other candida species by carbohydrate fermentation reaction and by growth characteristic properties.
• It is the only fungus to produces chlamydospores on cornmeal agar culture when incubated at 25°c.

Pathogenesis of Candida albicans

Under certain conditions, Candida gains access to systemic circulation from the oropharynx of the gastrointestinal tract. It then colonizes the mucocutaneous surface and causes invasion in human tissues through different routes. Disruption of the skin or mucosa allows the fungus to invade the blood stream and cause infections.

In immunocompromised individuals, candida may move to organs such as lungs, spleen, brain, liver and heart. It may also induce uveitis and cause endophthalmitis. Immunocompromised individual are at high risks of developing candida infection.

Host immunity to Candida infection

Cell mediated and humoral antibodies are actively involved in protection against candida infection in health individuals but cell mediated immunity is the most active. A change in cell mediated immunity may cause extensive superficial candidiasis especially in immunocompromised individual meanwhile humoral antibodies plays a minimal role in protection against the disease but a greater role of protection against candida infections in healthy individual. To break it down, cell mediated immunity is more active in immunocompromised individual while humoral antibodies is confess protection in healthy individual.

Clinical Syndromes of Candida infection

Candida causes a wide spectrum of clinical illnesses as follows:

Cutaneous candidiasis

This type of infections can affect any warm and moist part of the body. It causes infection of the nail, rectum and other skin folds.

Mucocutaneous candidiasis

This is the most common infection of candidiasis. Causes diseases such as oral thrust and perianal diseases. In immunocompromised patient such as HIV infected individual, it causes oropharyngeal and esophageal candidiasis that result in poor intake of food which can lead to malnutrition, wasting and early death.

Chronic mucocutaneous candidiasis

Characterized by chronic, treatment-resistant, superficial Candida infection of the skin, nails, and oropharynx. However, infected patient do not show any evidence of disseminated candidiasis

Systemic candidiasis

Systemic candidiasis is Candida infection in an otherwise sterile body fluid such as blood, urine, or cerebrospinal fluid. It is the most common fungal infection among hospitalized individual in high income countries. Diagnosis can be difficult, especially when the Candida is not found in the bloodstream.

Diseases caused by systemic candidiasis includes endocarditis, gastrointestinal tract candidiasis, respiratory tract candidiasis, genitourinary candidiasis, and hepatosplenic candidiasis. In patients with AIDS, oral thrush and Candida esophagitis are more common but not candidemia and disseminated candidiasis. Candida endophthalmitis and central nervous infection (CNS) infection due to Candida species are other complications of Candida infection

Disseminated candidiasis

This is increasingly becoming a problem in patients with serious hematologic malignancies that are treated with immunosuppressive drugs for over a long period of time. Severe neutropenia in these patients is the most important predisposing condition for life-threatening infection caused by Candida. In this condition, Candida usually spreads through the circulation and involves many organs, such as lungs, spleen, kidney, liver, heart, and brain.

However, disseminated candidiasis is not a major problem in patients with AIDS. In such patients, serious infection of the oropharynx and the upper gastrointestinal tract is the major problem. The development of these conditions in previously healthy individuals not receiving broad-spectrum antibiotic therapy should be strongly suspected for possibility of infection with HIV.

Laboratory Diagnosis of Candida albicans

Specimens

Specimen can be any biological sample such as:

• Exudates
• Tissues
• Skin and nails scrape
• Hair follicles
• Vaginal and urethral swab

Microscopy

• Sample can be directly observed using 10% KOH solution for pseudo hyphae or budding yeast cells of candida
• Gram-stained smear of the exudates or tissue shows Gram-positive, oval, budding yeast and pseudo hyphae.
• Since Candida is found as a part of normal flora on normal skin or mucosa, only the presence of large numbers of Candida is of significance.
• Demonstration of pseudo hyphae indicates infection, and tissue invasion is of more diagnostic value.

Culture

• When cultured on Sabouraud’s dextrose agar (SDA), it produces typical creamy white, smooth colonies.
• Different Candida species are identified by their growth characteristics, sugar fermentation, and assimilation tests.
• Germ tube is a rapid method for identification of C. albicans and Candida dubliniensis. This test depends on the ability of C. albicans to produce germ tube within 2 hours when incubated in human serum at 37°C. This phenomenon is called Reynold–Braude phenomenon.
• Chlamydospores are typically produced by C. albicans on cornmeal agar at 25°C, but not by other Candida species.
• Moreover, CHROM agar allows for the presumptive identification of several Candida species by using color reaction in specialized media, thereby showing different colors of the colonies depending on the Candida species.
• Different Candida species can also be identified with more accuracy by biochemical assays, such as AP120C and AP131C. These assays evaluate the assimilation of various sugars for identification of different fungal species.

Treatment of Candidiasis

Candida albicans infection is treated with antifungals such as

• Fluconazole
• Itraconazole
• Ketoconazole
• Nystatin
• amphotericin B

Prevention and Control

Antifungal prophylaxis is indicated for patients with invasive candidiasis who are at high risk of developing invasive candidiasis. There is no vaccine available against candidiasis.

Bibliography

• Textbook of Microbiology and Immunology, 2/e by Subhash Chandra Parija
• Fungal Infections, Infestations and Parasitic Infections in Neonates, K. Robin Carder, in Neonatal Dermatology (Second Edition), 2008