lec.1 Introduction to Medical Mycology

Introduction to Medical Mycology

Medical Mycology Medical mycology is the study of mycoses of man and their eitiologic agents .It is an integral part of general mycology as well as an important specialty in medicine. The mycoses are disease caused by fungi. fungal invasion of human tissue was recognized in the early 1800s before the science of bacteriology was developed.

What is a Fungus ?

Eukaryotic – a true nucleus Do not contain chlorophyll Have cell wallsProduce filamentous structuresProduce spores

Kingdom Fungi Eukaryocytes

Ascomycota Basidiomycota Zygomycota Mitosporic Fungi (Fungi Imperfecti)

MYCOTIC DISEASES(Four Types)

Hypersensitivity Allergy Mycotoxicosis Production of toxin Mycetismus (mushroom poisoning) Pre-formed toxin Infection

PATHOGENIC FUNGI

NORMAL HOST Systemic pathogens - 25 species Cutaneous pathogens - 33 species Subcutaneous pathogens - 10 species IMMUNOCOMPROMISED HOST Opportunistic fungi - 300 species

PATHOGENICITY OF FUNGI

Thermotolerance Ability to survive in tissue environment Ability to withstand host defenses

MORPHOLGY

Yeasts Hyphae (filamentous fungi, mycelium) Septate Coenocytic (non-septate) Dimorphic Yeast Mycelium

Budding Yeast/Germ Tubes

Hyphae

Dimorphic Fungi

Yeast Form Parasitic form Tissue form Cultured at 37 C Mycelial Form Saprophytic form Cultured at 25 C

Dimorphic nature of true fungal pathogens

Figure 22.2

SPORES

ASEXUAL Arthrospore Blastospore Chamydospore Conidia Microconidia Macroconidia


Chlamydospore - thick-walled asexual spore formed by direct differentiation of the mycelium (concentration of protoplasm and nutrients). Sporangiospore - an asexual spore contained in a sporangium at the end of a sporangoiphore.
Asexual Spores

Sexual spores

- exhibit fusion of nuclei. Ascospore - spore formed in a sac-like cell known as an ascus. Often eight (8) spores formed. (Ascomycetes) Basidiospore - sexual spore produced on a specialized club-shaped structure, called a basidium. (Basidiomycetes) Zygospore - a thick-walled spore formed during sexual reproduction (Phycomycetes)

Sexual spores

Conidiophore - a specialized branch of hypha on which conidia are developed. Dematiaceous - pigmented, dark in color, usually gray to black Sterigmata - a specialized structure that arises from a basidium and supports basidiospores .
Miscellanous terms:

Miscellanous terms: (continued)

Pseudohyphae - a chain of elongated budding cells that have failed to detach (not true hyphae). Rhizoids - root-like structures. Sporangiophore - a special aerial hypha or stalk bearing a sporangium. Sporangium - a sac or cell containing spores produced asexually.

Immunology of Mycosis

Antibody mediated immunity (B-cell, humoral) Antibodies are often produced in response to a fungal infection, but do not confer immunity. Serological tests for identification of fungal diseases detect these antibodies. Cellular mediated immunity (T-cell) T-cell immunity is effective in resistance to fungal infections.

Classifications of Fungi

Geographic grouping - where they exist. Epidemiologic grouping - how organism is transmitted. Taxonomy grouping - according to morphologic and cultural characteristics. Topographic Grouping - type of mycosis produced.

EPIDEMIOLOGY

MOST MYCOTIC AGENTS ARE SOIL SAPRPHYTES Mycotic Diseases Are NOTContagious

ESTABLISHMENT OF INFECTION WITH A MYCOTIC AGENT DEPENDS ON

Inoculum size Resistance of the host , rather than virulence of fungus


most often used Superficial - Confined to the outer most layers of the skin and hair. No host cellular or inflammatory response due to organisms being remote from living tissue. Essentially no pathology; the disease is recognized purely on cosmetic basis. Cutaneous - in the keratin of the skin, nails, and hair. These organisms prefer non-living cornified layers. The disease is called a dermatophytosis or dermatomycosis. Host response is patchy scaling or eczema eruptions. They are classified according to the area of the body that is involved.
Topographic Grouping of Fungi


Subcutaneous - Involve the deeper layers of skin and often muscle tissue. Man is an accidental host following inoculation of fungal spores via some form of trauma. This type of infection is often identified by the presence of a characteristic tissue reaction or granule. Systemic - Attack the deep tissues and organ systems; often creating symptoms that resemble other diseases.
Topographic Grouping of Fungi: (continued)


Categories of systemic disease:
Those caused by truly pathogenic fungi with the ability to cause disease in the normal human host when the inoculum is of sufficient size (Histoplasma capsulatum, Blastomyces dermatitidis, Coccidioides immitis, Paracoccidioides braziliensis). Those caused by opportunistic fungi, low virulence organisms, which require the patient's defenses to be lowered before the infection is established (Aspergillus spp. Candida albicans, Cryptococcus neoformans).


EYE
SKIN
UROGENITAL TRACT
ANUS
MOUTH
RESPIRATORY TRACT
COLONIZATION
Multiplication of an organism at a given site without harm to the host

EYE
SKIN
UROGENITAL TRACT
ANUS
MOUTH
RESPIRATORY TRACT
INFECTION
Invasion and multiplication of organisms in body tissue resulting in local cellular injury.


Diagnosis1. Wet Mount (KOH)2. Skin test (dermal hypersensitivity)3. Serology (Latex agglutination , Complement fixation ….)4. Fluorescent antibody5. Biopsy and histopathology(pyogenic , granulomatous ,or necrotic)6. Culture (Sabouraud dextrose agar)7. DNA probes

KOH Wet Mount

ISOLATION MEDIA(culture)
SABOURAUD DEXTROSE AGAR (pH ~ 5.6) Plain With antibiotics With cycloheximide

INCUBATION TEMPERATURE

37 C - Body temperature 25 C - Room temperature

DNA Probes

Rapid (1-2 Hours) Species specific Expensive

THERAPY

Because they are eukaryotic, fungi are biochemically similar to the human host. Therefore it is difficult to develop chemotherapeutic agents that will destroy the invading fungus without harming the patient.

IN FUNGAL THERAPY

We attempt to induce cell injury by causing the cell membrane of the fungus to become permeable.

PROBLEM

Finding an agent that will selectively injure fungal cell walls without damaging the host cell.

ALL EUKARYOTIC CELLS CONTAIN STEROLS

Mammalian cells – cholesterolFungal cells - ergosterol

PRIMARY ANTI-FUNGAL AGENTS

Polyene derivatives Amphotericin B Nystatin Azoles Ketoconazole Fluconazole Itraconazole Voriconazole Posaconazole

PRIMARY ANTI-FUNGAL AGENTS

3. Griseofulvin 4. 5-fluorocytosine (5-FC) 5. Allylamines -Terbinafine (Lamasil) 6. Echinocandins - Caspofungin

AMPHOTERICIN BMechanism of Action

Amphotericin B binds to sterols Ergosterol is a constituent of the fungal cell wall AMB has a greater avidity for ergosterol than for the cholesterol in the human cell wall Binding to the fungal cell wall alters the permeability and the intracellular contents leak

AMPHOTERICIN BDisadvantages

Intravenous administration Thrombophlebitis Nephrotoxic Fever Chills Anemia Long term administration


Azoles
There are a few rare serious side effects from Itraconazole and Fluconazole

Griseofulvin

A slow acting drug used for skin and nail infections. It accumulates in the stratum corneum and prevent hyphal penetration through these layers

5-fluorocytosine(5-FC)

Interferes With RNA Synthesis

MECHANISMS OF ACTION

Polyenes Azoles Griseofulvin 5 - FC
Ergosterol in cell membrane Interfere with ergosterol synthesis Forms a barrier to fungal growth Inhibits RNA synthesis

End of Introduction