Structure, Pathogenesis, and Diagnosis of Candida albicans

Candida Species

Candida species belongs to yeast. There are medically eight important species of candida which can cause infection in humans.They are :

• Candida albicans
• Candida stellatoidea
• Candida tropicalis
• Candida krusei
• Candida guilliermondii
• Candida glabrata
• Candida parapsilosis

Out of these eight species of candida, candida Albicans is responsible for the most of the human infections( greater than 90%)

Candida albicans is an opportunistic pathogen that fails to induce disease in most immunocompetent persons but can do so in those with impaired host defense as in AIDS patients ,patients undergoing prolonged antibiotic therapy or in patients with invasive surgery. It is part of normal flora of mucous membrane of the upper respiratory tract (URT),gastrointestinal (GI) tract .

Morphology ofCandida albicans

Candida albicans is a small,oval yeast measuring 2-4 µm in diameter .In tissues,it may appear as yeast ,budding yeast,or pseudohyphae .Pseudohyphae are elongated yeast cells that visually resemble hyphae ,but not the true hyphae i.e. elongated yeast cells joined end to end resembling that of hyphae.

• Candidiasis is a fungal infection caused by yeasts that belong to the genus Candida.
• There are over 20 species ofCandida yeasts that can cause infection in humans.
• The most common of which is C. albicans.
• Candida yeasts normally live on the skin and mucous membranes without causing infection.
• however, overgrowth of these organisms can cause symptoms to develop.
• Symptoms of candidiasis vary depending on the area of the body that is infected.
• Candidiasis that develops in the mouth or throat is called "thrush" or oropharyngeal candidiasis.
• Candidiasis in the vagina is commonly referred to as a "yeast infection."
• Invasive candidiasis occurs when Candida species enter the bloodstream and spread throughout the body.

Virulence factors and pathogenesis

The pathogenesis of candida infection is extremely complex. However, adherence of candida to the epithelium of the gastrointestinal tract or genital tract is critically important .Candida sp commonly colonizes the mucosal surface and their ability to invade and cause infection is first dependent on a binding .Fibronectira a component of host extracellular matrix also play a role in the initiation and dissemination of candida infection.

Besides that ,3 distinct aspartyl-protease have been found in these yeast cells that play a major role in causing infection .Different hydrophobic molecule present on the surface of yeast cells also appear to be important in pathogenesis,as there is a strong co-relation between adhesion and surface hydrophobicity. Similarly , not least important is the immune-suppressive activity of yeast mannans that may contribute to the virulence of these yeasts.

Disease caused byCandida albicans

Disease caused by Candida albicans is commonly called candidosis may be superficial or systemic :

i)Superficial candidosis

The superficial forms of candidosis is usually an opportunistic endogenous infection. It may occur at different sites.

• Candidosis at mucous membrane

Overgrowth of c.albicans in mouth produces white patches called ‘thrust’ which occurs most frequently in infants and old aged people .The thrust is usually the pseudomembrane formed on the mucous membrane.The ‘thrust’ may appear on the mucous membrane of the female genital tract which appears with the symptoms like itching and discharge and especially seen in patients with diabetes or patient’s undergoing prolonged antibiotic therapy.

• Candidosis of skin

Infection of the skin usually occurs in sites that may become abnormally moist and warm such as axillae,groin, toe cleft, etc.

• Candidosis of nail

Fingers and nail become involved when repeatedly immersed in water. For example : in person’s employed as a dishwasher in restaurants and institutions. Infections are characterized by thickening and reddening at the injection site.

ii)Systemic candidosis (infection)

In immunocompromised or immunosuppressed individuals yeast may disseminate to many different organs and may cause systemic candidosis .C.albican is the most frequent cause of endocarditis following surgery of heart valves. Internal organs like lungs,kidneys,and many other organs may be secondarily invaded when pre-existing diseases such as TB,cancer persist.

Lab diagnosis

Specimens

Respiratory secretions,skin scraping,nail scrapping from the mucous membrane, vaginal secretions, CSF, blood etc.

1. Direct microscopy

Scrapings from the lesions of the skin, nail or mucous membranes are examined in a wet film in KOH. In this method, tissue specimens are mixed with few drop of 10% KOH solution and allow to stand for few minutes before observing the slide under a microscope.

Alternatively, as yeast shows gram positive reaction gram staining technique may be used for direct observation mostly in sterile clinical specimens where C. albicans may be observed as budding yeast cells or sometimes pseudohyphae may be seen. In direct microscopy, C. albicans can be seen as small, oval 2-4 um in diameter cells.

2. Culture

Candida albicans usually grow in the most bacterial as well as fungal culture medium on blood agar under 24-48 hrs incubation , the colonies are white opaque and convex . On the fungal culture medium like Sabouraud`s agar colonies are smooth, creamy white with yeasty odor.

3. Germ-tube test

Germ-tube test is very significant in identifying C. albicans as germ tube test is given by C. albicans but not by other candida species. In this test, a loopful of yeast colony from a cultured plant is inoculated into 0.5ml of human serum in a small test tube and the tube is incubated at 37 C for 2-3 hours. After incubation, the preparation is examined under a microscope for the observations of the germ tube. Germ tube appears at the hyphal-like extension from the yeast cell that is produced without a constriction at the point of origin from the yeast cell.

4. Corn meal agar morphology

This method is currently satisfactory for the definitive identification of C. albicans when the germ tube test is negative. Chlamydospore develops in a nutritionally poor medium such as com-meal agar at 28 C. Since, Chlamydospore is typically formed by C. albicans and not by other species of candida, it is a definitive identification of C. albicans.

5. Skin test

Candida skin test shows universal positivity and is useful as an indicator of competent CMI i.e. candida antigens are uniformly positive in immune competent adults. A person who does not respond to candida antigens in the skin tests us presumed to have deficient CMI.

Prevention and treatment

There is no vaccine available for candida infection, however, certain conidial infection of oral thrush can be prevented by the use of oral clotrimazole or nystatin.

The drug of choice of oropharyngeal or oesophagal thrush is fluconazole. Treatment of skin infections consists of topical antifungal drugs. Example: Clotrimazole and nystatin. Treatment of disseminated candidosis that is systemic infection consists of either amphotericin B or fluconazole. These two drugs can be used with or without fluorocytosine.

REFERENCE

Cheesbrough, M. Medical Laboratory Manual for Tropical Countries. Vol. Vol 2. ELBS London, 2007.

Tille, P. Diagnostic Microbiology. 13th. Elsevier, 2014.

D, Grenwood, Slack RCB, and Peutherer J. Medical Microbiology. Dunclude Livingstone: ELBS, 2001.