Introduction to Mycology: Fungi Characteristics

• Mycology is the study of fungi, derived from the Greek words "mykos/mykes" (mushroom) and "logos" (study of).
• Fungi are classified as thallophytes, possess true nuclei, and are heterotrophic, lacking stems, roots, and chlorophyll.
• Fungi are larger and more complex than bacteria.

Characteristics of Fungi

• Saphrophytic: They obtain nutrients from dead or decaying organic matter.
• Eukaryotic: Their cells contain a nucleus and other complex organelles.
• Achlorophyllous: They lack chlorophyll and cannot perform photosynthesis.
• Spore-bearing: They reproduce through spores.
• Aerobic: They require oxygen for growth.
• Possess a well-defined cell wall, which may contain chitin.

Key Characteristics of Fungi

• The presence of chitin in the cell wall.
• The presence of ergosterol in the cell membrane.
• Reproduction by means of spores, produced either asexually or sexually.
• The lack of chlorophyll.
• Lack of susceptibility to antibacterial antibiotics.
• Their heterotrophic nature, deriving nutrition from organic materials.

Traditional Classification of Fungi

• Ascomycota
• Basidiomycota
• Deuteromycota
• Zygomycota

Ascomycota

• Reproduce asexually via conidia (asexual spores) and sexually via ascospores.
• All produce true septate hyphae.
  • Examples include Ajellomyces (H. capsulatum, B. dermatitidis), Piedraia hortae, Pseudallescheria boydii, some Candida species, and Saccharomyces.

Basidiomycota

• Reproduce sexually by forming basidiospores on basidia.
• Generally plant pathogens or environmental organisms, rarely causing human disease.
  • Examples include smuts, rusts, mushrooms, and Cryptococcus neoformans.

Deuteromycota

• Lack a sexual reproductive cycle.
• Characterized by asexual reproductive structures, primarily conidia.
  • Examples include Aspergillus, Coccidioides, Microsporum, Paraccocidiodes, and Sporothrix.

Zygomycota

• Produce aseptate hyphae.
• Exhibit asexual reproduction via sporangiospores and sexual reproduction via zygospores.
  • Examples include Rhizopus, Mucor, Rhizomucor, Absidia, Cunninghamella, and Saksenaea.

Fungal Phases

• Capable of existing in two phases:
  • Multicellular mold phase: Consists of a cottony, mycelial mass.
  • Unicellular yeast phase: Creamy, resembling a bacterial colony.

Dimorphic Fungi

• Exhibit mold phase at 25°C (room temperature), which is the infective stage for humans, then convert to yeast phase.
• Yeast phase occurs at 37°C and in tissues, representing the invasive form.
• Monomorphic fungi exist either as mold at both 25°C and 37°C (e.g., Sepedonium).

Mycelium

• An intertwining structure of tubular filaments called hyphae.
  • Aseptate hyphae/Coenocytic hyphae: Lack cross walls or divisions (e.g., Zygomycetes like Rhizopus, Mucor, Absidia).
  • Septate hyphae: Possess cross walls or divisions (all fungi except Zygomycetes).
• Consists of a vegetative portion (thallus) that absorbs nutrients and a reproductive, aerial part that produces conidia or spores.

Spores Involved in Sexual Reproduction

• Ascospores: Enclosed in an ascus, containing 2-8 spores (perfect fungi).
• Zygospores: Result from the fusion of two identical cells from the same hypha.
• Oospores: Result from the fusion of two cells from different hyphae.
• Basidiospores: Enclosed in a club-shaped basidium.

Spores Involved in Asexual Reproduction

• Conidia: Arise from the side of hypha (imperfect fungi).
  • Microconidia: Small and unicellular.
  • Macroconidia: Large and multicellular.
  • Catenate: Conidia in chains.
  • Echinulate: Rough and spiny.
• Chlamydoconidia: Thick-walled spores formed during unfavorable conditions, germinate when environment improves.
  • Types: Terminal (tip of hypha), Intercalary (within hyphal strand), Sessile (side of hyphal strand).
• Arthroconidia: Rectangular or barrel-shaped, from fragmentation of mycelium (e.g., Geotrichum, Coccidioides, Trichosporon).
• Sporangium: Sac-like structure where asexual sporangiospores develop.
• Sporangiospores: Asexual spores produced within a sporangium.
• Blastoconidia: Develop from simple budding.

Virulence Factors

• Adhesion: Ability to stick to host cells and tissues.
• Invasion: Ability to invade host cells.
• Nutrient competition: Competing with the host for nutrients.
• Immune evasion: Ability to evade the host's immune defenses.
• Capsule: Such as in Cryptococcus, is a major virulence factor.
• Survival capability: Ability to survive in hostile environments.

Specimen of Choice for Laboratory Diagnosis

• Many fungal infections clinically resemble mycobacterial infections.
• Respiratory tract secretions are often included due to the lungs being a primary focus of infection.
• Proper specimen collection and rapid transport are crucial.
  • Respiratory Tract Secretions
  • Cerebrospinal Fluid
  • Blood
  • Tissue, Bone Marrow, and Sterile Body Fluids
  • Hair, Skin, and Nail Scrapings
  • Urine

Respiratory Tract Secretions

• Sputum, induced sputum, bronchial washings, bronchoalveolar lavage, and tracheal aspirations are commonly submitted.
• Cycloheximide should be included in at least one culture medium to prevent overgrowth by rapidly growing molds.

Cerebrospinal Fluid

• CSF should be filtered through a 0.45 μm membrane filter.
• Cultures should be examined daily, with the filter moved every other day.
• Media should not contain antibacterial or antifungal agents.
• Process CSF specimens promptly or keep them at room temperature or in a 30°C incubator.

Blood

• Only a few manual fungal blood culture systems are available, and most are not used routinely.

Tissue, Bone Marrow, and Sterile Body Fluids

• Tissues should be minced, ground, or processed in a Stomacher before culturing.
• Spread at least 1.0 mL of specimen onto appropriate culture media.
• Incubate at 30°C for 21 days, potentially longer if high clinical suspicion exists.
• Culture specimens as soon as they are received.

Hair, Skin, and Nail Scrapings

• Collect samples by scraping skin or nails with a scalpel blade or slide, and pluck infected hairs with forceps.
• Place specimens in a sterile container and do not refrigerate.

Urine

• Process urine samples as soon as possible post-collection.
• Twenty-four-hour urine samples are unacceptable.
• Centrifuge all urine samples and culture the sediment.
• Use media with antibacterial agents due to potential contamination with gram-negative bacteria.

Diagnostic Techniques

• Fungal Media
• Direct Examination Methods
• Serologic Identification

Culture Media and Incubation Requirements

• Use media with and without cycloheximide.
• Use media with and without an antibacterial agent for specimens likely to contain bacteria, but not for sterile sites.
• Incubate cultures at room temperature, or preferably at 30°C, for 21 to 30 days before reporting as negative.

Direct Examination Methods

• Saline Wet Mount
• Lactophenol Cotton Blue Wet Mount
• KOH
• Gram Stain
• Giemsa/ Wright Stain
• Histopathologic Stains
• Immunohistochemical Stains
• India Ink
• Calcoflour White Stain

Saline Wet Mount

• Used to view fungal elements like hyphae, conidia, and budding yeast cells.
• Most applicable for diagnosing vaginitis via vaginal secretions.

Lactophenol Cotton Blue Wet Mount

• Used to stain and preserve fungal elements and hair samples.

KOH (Potassium Hydroxide)

• Used to stain and preserve fungal elements in culture isolates.

Gram Stain

• Can be used to view yeast.

Giemsa/ Wright Stain

• Used when histoplasmosis is suspected to demonstrate yeast cells within macrophages.

Immunohistochemical Stains

• Molecular Identification-For tissue samples.

India Ink

• Used for encapsulated fungi like Rhodotorula spp., Exophiala spinifera, or other cryptococcal spp.
• CSF is the specimen of choice.

Calcoflour White Stain

• A fluorochrome that stains chitin in fungal cell walls.
• Viewed under ultraviolet light, fungi appear white to blue to green.

Serologic Identification

• Used for detecting antigens and antibodies to specific fungal pathogens.
• Tests to detect fungal antigens or metabolic byproducts in serum or other body fluids are useful for diagnosis and management of fungal diseases.

Molecular Identification

• Aims to define genetic loci within the genome for use as molecular targets; ribosomal DNA (rDNA) gene complex.
• Sequence homology within the rDNA genes and differences within the spacer used as the genetic basis for organizing fungi into taxonomic groups.

Learn about mycology, the study of fungi. Fungi are thallophytes with true nuclei, lacking stems, roots, and chlorophyll. Key characteristics include chitin in the cell wall, ergosterol in the cell membrane and reproduction through spores.