C. albicans is ovoid or spherical yeast with a single bud.
It forms the part of the normal flora of the mucous membrane
and in tissues. Pseudohyphae are elongated yeast that may resemble hyphae morphologically, but
Candida grows readily on Sabouraud’s dextrose agar and on bacteriological culture media. C.
albicans produces creamy white, smooth colonies with a yeasty odor .
It can be differentiated from other Candida species bycarbohydrate fermentation reaction
and by characteristicgrowth properties.
Only C. albicans produces chlamydospores on cornmeal agar culture at 25°C.
Section IV - Medical Mycology By Dr. Kareem Lilo
Candida spp. are usually present as part of normal flora on
healthy mucosal surface of the oral cavity, gastrointestinal
tract, and vagina. Candida shows colonization at these sites in
more than 80% of healthy people. The organism, however, is
rarely present on the surface of normal human skin, except
occasionally from certain intertriginous area, such as the groin.
Pathogenesis of Candida infection
Under certain conditions, Candida gains access to systemic circulation from the oropharynx of the
gastrointestinal tract. Colonization of the mucocutaneous surface is the first stage in the
pathogenesis of Candidal infection. The fungus causes invasion in human tissue through different
routes. Disruption of the skin or mucosa allows the organism access to the blood stream. Massive
colonization with large numbers of Candida
also permits the organism to pass directly into the blood stream, causing the infection. In
immunocompromised hosts, Candida may disseminate to many organs, such as lung, spleen, liver,
may induce inflammation of the eye, causing endophthalmitis and also may involve skin in 10–30%
of patients with disseminated infection. Deficiency in host defence mechanisms plays a significant
role in development of Candida infection.
Both cell-mediated and humoral antibodies confer protection
against Candida in healthy adults. Cell-mediated immunity (CMI) is, however, most important.
Alteration in CMI may cause extensive superficial candidiasis, despite having normal or elevated
humoral antibodies. The humoral antibodies appear to play minimal role in protection against the
disease. Humoral antibodies confer protection against Candida in healthy adults.
Candida causes a wide spectrum of clinical illnesses as follows:
Cutaneous candidiasis: Candida species in immunocompetent
host can cause infection of any warm and moist part of the body exposed to environment. It causes
infection of the nail, rectum, and other skin folds.
Mucocutaneous candidiasis: Mucocutaneous candidiasis (thrush, perianal disease, etc.) is the most
common manifestation of candidiasis, but usually does not cause any mortality.
In patients with advanced immunodeficiency due to HIV infection, Candida species can cause severe
oropharyngeal and esophageal candidiasis that result in poor intake of food, leading
Section IV - Medical Mycology By Dr. Kareem Lilo
candidiasis, genitourinary candidiasis, and hepatosplenic candidiasis. Systemic candidiasis may be
candidemia and disseminated 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
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
These include exudates or tissues for microscopy obtained from skin or nails examined by
microscope for demonstration of pseudohyphae or budding yeast cells of Candida.
Gram-stained smear of the exudates or tissue shows Gram positive,oval, budding yeast and
pseudohyphae . 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 pseudohyphae
indicates infection, and tissue invasion is of more diagnostic value.
Culture on Sabouraud’s dextrose agar (SDA) 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 (Figure 4-4).
Section IV - Medical Mycology By Dr. Kareem Lilo
o Nonculture Candida detection tests
(d) D-inositol assay, and (e) 1,3-beta-D-glucan assay. Beta-Dglucan assay is a broad-spectrum test
that detects Candida as well as Aspergillus, Fusarium, Acremonium, and Saccharomyces species. This
is a highly specific and sensitive test.
Antifungal therapy forms the mainstay of treatment of the infections caused by Candida. These agents
include azoles ( fluconazole, triazole, ketoconazole), nystatin, and amphotericin is becoming
increasingly important worldwide .
A broad spectrum of diseases in humans ranging from direct invasion to hypersensitive reactions are
caused by Aspergillus species. Although more than 100 species have been described, the majority of
human diseases are caused by Aspergillus fumigates and Aspergillus niger, and less frequently by
Aspergillus flavus and Aspergillus clavatus.
Figure(74-2)Candida albicans showing formation of the germ
Section IV - Medical Mycology By Dr. Kareem Lilo
Aspergillus species are molds.
They have septate hyphae that form V-shaped dichotomous branches (Figure 4-5). The
Aspergillus species are identified by (a) their morphological features, (b) the pattern of
conidiophores development, and (c) the color of the conidia.
The presence of septate hyphae that branch at 45° angles is the typical feature of Aspergillus
species hyphae. The hyphae in tissues are best demonstrated with silver stains. The walls of
the hyphae are more or less parallel, unlike those of Mucor and Rhizopus, which are more or
Aspergillus species rarely cause infections in immunocompetent individuals. They cause invasive
infections mostly in the patients who are immunocompromised either due to (a) use
of immunosuppressive drugs, (b) underlying lung diseases, or (c) immunodeficiency diseases, such
as HIV. In immunocompromised host, Aspergillus species cause invasion of the blood, thereby
causing infarction, hemorrhage, and necrosis of lung tissues. Aspergillus spp. also produces toxic
metabolites that inhibit macrophage and neutrophil phagocytosis, facilitating
dissemination of the infection.
Aspergillus species unlike Candida species do not form the part of normal flora of humans. They are
ubiquitous in the environment; hence transmission of infection is mostly exogenous.
In immunocompetent hosts, Aspergillus species may primarily affect the lungs, causing four main
syndromes including (a) allergic bronchopulmonary aspergillosis, (b) chronic necrotizing
aspergillus pneumonia, (c) aspergilloma, and (d) invasive aspergillosis.
Figure( 4-5) Aspergillus species with septate hyphae that form V-shaped
Section IV - Medical Mycology By Dr. Kareem Lilo
which have colonized in tracheobronchial tree. This condition occurs often in association with
Chronic necrotizing pulmonary aspergillosis: It is a subacute infection seen in patients with some
degree of immunosuppression. The condition occurs in conjunction with alcoholism, underlying
lung disease, or chronic corticosteroid therapy.
may have been caused earlier by tuberculosis, sarcoidosis, cystic fibrosis, and
emphysematous bullae. The condition is characterized by the presence of a ball of fungus within the
cavity. The fungus, however, does not invade the cavity. It may cause hemoptysis.
Invasive aspergillosis: It is a rapidly progressive infection in patients who are severely
immunocompromised. The condition is mostly fatal. In immunocompromised host, Aspergillus
organisms cause a disseminated disease, leading to endophthalmitis, endocarditis, and abscesses in
the viscera, such as liver, spleen, kidney, soft tissues, and bone.
Laboratory diagnosis of invasive aspergillosis or chronic necrotizing aspergillus pneumonia depends
on demonstration of Aspergillus in tissue by direct microscopy and culture.
Comments
Post a Comment
اكتب تعليق حول الموضوع