Pathogenic Fungi Overview

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Questions and Answers

What equipment is necessary to examine a slide in both 10X and 40X magnification?

A digital imaging system

A microscope with interchangeable objectives (correct)

A fluorescent microscope

An oil immersion lens

Why is potassium hydroxide (KOH) used in the tube test for fungal examination?

To create an alkaline environment for fungal growth

To dissolve tissue samples for microscopic observation (correct)

To preserve the samples during incubation

To stain the fungi for easier visibility

What is the optimal pH for the growth of fungi on Sabouraud’s dextrose agar?

4.5

5.6 (correct)

6.5

7.0

What is a common issue when examining nail clippings or biopsy materials?

Background artifacts can be misleading Signup and view all the answers

At what temperatures do many fungi grow best during primary isolation?

25°C to 30°C Signup and view all the answers

How long are fungal cultures typically incubated for effective growth?

7 to 14 days Signup and view all the answers

What method can be employed to minimize dehydration of agar during extended incubation?

Seal culture plates with parafilm Signup and view all the answers

What identifies yeasts during culture evaluation?

Biochemical tests for metabolic activities Signup and view all the answers

What is the primary advantage of using calcofluor white stain over potassium hydroxide (KOH) preparation in direct microscopic examination?

It provides a clearer visualization of fungal elements. Signup and view all the answers

Why is potassium hydroxide (KOH) used in the preparation of clinical specimens for fungal examination?

To digest protein debris and dissolve keratinized cell cement. Signup and view all the answers

What type of specimens can potassium hydroxide (KOH) preparation be used on?

Epidermal scales, nails, hair, and skin scrapings. Signup and view all the answers

What are hyphae and conidia typically examined for in a KOH preparation?

Identification of fungal infections. Signup and view all the answers

What is the purpose of gently heating the slide during the KOH preparation?

To speed up the digestion of cellular material. Signup and view all the answers

How should a slide prepared with KOH be examined to identify fungal elements?

In bright-field or fluorescent microscopy. Signup and view all the answers

What is typically the outcome of a KOH microscopic examination?

A report stating results as positive or negative for fungal elements. Signup and view all the answers

Which of the following statements about direct microscopic examination of fungal elements is true?

It relies on the identification of characteristic spores and hyphae. Signup and view all the answers

What is the primary reason to collect samples only from the leading edge of skin lesions?

To avoid sampling nonviable organisms Signup and view all the answers

Which of the following is a requirement for transporting vaginal samples to the laboratory?

They should be kept moist in sterile tubes Signup and view all the answers

How should urine samples be processed for fungal culture?

Samples should be centrifuged and the sediment cultured Signup and view all the answers

What methods are routinely performed in a mycology laboratory?

Conventional staining and culturing on agar media Signup and view all the answers

What is a key safety measure for handling mold cultures and clinical specimens?

Conducting all work in a class II biosafety cabinet Signup and view all the answers

Which approach is NOT included in the laboratory diagnosis of fungal diseases?

Immunofluorescence microscopy Signup and view all the answers

What is one of the common student misconceptions about blood cultures for fungi?

They are not important for diagnosis Signup and view all the answers

Which of the following is true regarding the incubation of vaginal cultures?

They require incubation at 30°C for 7 days Signup and view all the answers

Study Notes

General Properties, Virulence, Pathogenesis, and Classification of Pathogenic Fungi

Fungi are eukaryotes, with higher complexity than bacteria.
Fungi reproduce both sexually and asexually.
Fungi can be unicellular or have long, branching filaments (hyphae) to form multicellular structures.

Fungal Structure, Metabolism, and Reproduction

Fungi have a nucleus, nuclear membrane, linear chromosomes, actin microfilaments, tubulin-containing microtubules, ribosomes, and organelles (like mitochondria and endoplasmic reticulum).
The Golgi apparatus is also present.
The fungal cell wall is primarily composed of chitin.
Chitin is a polysaccharide made of N-acetylglucosamine.
The cell wall also contains other polysaccharides, such as β-glucan, which is a polymer of glucose. β-glucan is the site of action of some antifungal drugs.
Fungal cell membranes have ergosterol, differing from cholesterol in human cell membranes.
This difference is exploited by some antifungal drugs (e.g., amphotericin B, azoles).

Types of Fungi

Fungi can be yeasts or molds.
Yeasts are single-celled organisms that reproduce asexually by budding.
Molds grow as long filaments called hyphae to form a mat-like structure (mycelium).
Hyphae can be divided by cross-walls (septate) or not (coenocytic); nonseptate hyphae are multinucleated.
Some fungi are thermally dimorphic.
They change their structures (e.g., mold to yeast) depending on the temperature.

Fungal Growth and Reproduction

Some fungi reproduce sexually (zygospores, ascospores, basidiospores) and asexually by producing conidia.
Fungi, in general, require a preformed organic carbon source.
Most fungi are obligate aerobes—some are facultative anaerobes.

Virulence, Pathogenesis, and Classification

Approximately 80,000 fungi species have been described; however, only about 400 are medically important; less than 50 cause most infections.
A majority are beneficial to humans, enhancing quality of life and being essential for recycling organic matter (e.g., for food and bio-active molecules such as penicillin)
Fungal infections (mycoses) often have chronic or subacute relapses, unlike other infections.
Most pathogenic fungi originate from the environment.
Candida albicans and dermatophytes are frequently present in humans, are part of the normal microbiota, and are therefore opportunistic pathogens that cause infection in immunocompromised people.
Fungi are opportunists usually infecting immunocompromised individuals, with altered floral communities and excessive antibiotic intake.

Fungal Adherence

Several, especially yeasts, colonize mucosal surfaces.
Adherence is key for colonization and virulence.
Adherence depends on adhesins on the fungus binding to receptors on host cells (very often a surface mannoprotein).

Fungal Invasion

Invasion, passing through barriers (skin, membranes) is crucial to fungal infections.
Some fungi change morphology (e.g. from mold to yeast) to better invade host tissues.
Fungi may produce extracellular enzymes, like proteases and elastases, contributing to their ability to invade host tissues.

Fungal Injury

Fungi can produce mycotoxins in the environment.
The injury or damage from fungal infections is primarily due to the destructive effects of inflammation and immunologic responses, especially in those with immunocompromised systems (e.g. neutropenic patients suffering from invasive mold infections).

Host Defense and Fungal Resistance

Innate immunity for healthy individuals includes phagocytic cells (neutrophils, macrophages), the complement system, and Pattern Recognition Receptors (PRRs).
Immunity to fungal infections often depends on the cellular immune response (TH1-mediated immunity).
Impaired cellular immunity, often found in immunocompromised individuals, leads to more severe fungal infections

Humoral and Cellular Immunity

Antifungal antibodies can be detected in some fungal infections, but may not signify resistance.
Cellular immunity plays a significant role, while decreased cellular immunity contributes to more severe fungal infections.

Laboratory Diagnosis of Fungal Infections

Specimen collection and transport are crucial for successful fungal diagnosis.
Clinical specimens are screened for fungi using microscopy and culture.
KOH is often used in microscopy.

Laboratory Diagnosis and Management

Laboratory safety precautions are important for mycology laboratories.
Mold cultures and specimens must be handled in appropriate biosafety cabinets (class II).

Types of Molecular Assays

DNA probes provide rapid and earlier diagnosis of fungal infections than other techniques.

Types of Immunoassays

Antibody detection or titer measurement can assist in diagnosis of immunocompromised individuals.
Tests like complement fixation and latex agglutination are often used.

Molecular and Immunoassays

These testings provide details about detection and confirmation of proper diagnoses for fungal infections

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Description

Explore the complex world of pathogenic fungi, focusing on their general properties, virulence, and classification. Learn about their unique structures, metabolism, reproduction processes, and how their cell composition influences antifungal treatments.