Fungi: Candida albicans Dimorphism

Fungi: Cell Structure and Classification

• Fungi are eukaryotic, and there are approximately 150,000 species, with only about 300 species causing human diseases.
• They grow in nature as saprophytes on dead organic material and are important catalysts in the carbon cycle.
• Fungi produce drugs, such as antibiotics (penicillin) and immunosuppressants (cyclosporin), and are involved in food microbiology and industrial microbiology.

Pathogenic Species

• Plant pathogens
• Animal pathogens
• Human pathogens

Human Pathogens

• To be a human pathogen, a fungus must: • Grow at a high temperature (at or above 37°C) • Reach the tissues it will parasitize by penetrating host tissue barriers or circumventing them through small airborne cells • Digest and absorb components of human tissues • Withstand the human immune system

Fungal Reproduction

• Fungi reproduce sexually and/or asexually
• Ascospores are usually arranged in a linear order in an ascus (sac) and can be 4-8 in number
• Basidiospores are the sexual spores of Basidiomycota, formed in a club-shaped structure called a basidium
• Zygospores are typical of Zygomycota, thick-walled spores formed when two sexually compatible hyphae or gametangia fuse together
• Oospores are sexually produced spores of Oomycota, formed within a special female structure called an oogonium

Fungi Classification: Based on Sexual Reproduction

• Ascomycota
• Basidiomycota
• Zygomycota
• Deuteromycota (Fungi Imperfecti) - no longer formally accepted as a taxon, many of these fungi have yet to find a place in modern fungal classification

Opportunistic Pathogens

• Microorganisms that do not cause disease in a healthy host
• Take advantage of a host with a weakened immune system
• Examples include some bacteria, viruses, fungi, and protozoa

Dimorphism of Candida albicans

• DYC (daughter yeast cell)
• GT (germ tube)
• H (hyphae)
• Ph (pseudohyphae)
• YMC (yeast mother cell)

Fungi: Molds and Yeasts

• Molds (filamentous fungi)
• Yeasts • Unicellular, spherical, oval, 3-15 μm • Reproduce by budding • Pseudohyphae: a chain of elongated budding cells that have failed to detach (not true hyphae) • Form a bacteria-like colony (creamy)

Molds (Filamentous Fungi)

• Multicellular, forming a network of filaments called hyphae
• Septate-coenocytic (aseptate hyphae) - identification
• Woolly (fuzzy) appearance in culture
• The hyphae form together to produce a mat-like structure called a mycelium

Types of Hyphae

• Vegetative hyphae: penetrate the media and absorb food
• Aerial hyphae: directed above the surface of the media
• Reproductive hyphae: aerial hyphae that carry different spores
• Mycelium: a collection of hyphae

Hyphal Growth from Spore

Capsule

• Cryptococcus neoformans
• Complex polysaccharides
• Main virulence factor
• In the environment, the capsule plays a role in protecting the organism against some stress conditions
• Disease caused by Cryptococcus species = Cryptococcosis

Growth Conditions

• Optimum growth temperature: 20-35°C
• Most fungi require a humidity level over 75% to grow
• pH 5-7 (generally like acidic pH levels)
• Aerobic or facultative anaerobic
• Chemoheterotrophs
• Growth rate: 1-2 days for yeasts, 4-21 days for molds

Nutrition and Physiology

• Feed by secreting extracellular enzymes that digest polymeric materials (e.g., polysaccharides or proteins) into monomers that are assimilated as sources of carbon and energy
• Decomposers (digest dead animal and plant materials)
• Inorganic substances and salts
• Fungi tend to be more resistant to environmental stress conditions than bacteria

Fungi Groups: On the Basis of Morphology

• Yeasts: unicellular
• Molds: multicellular (filamentous fungi)
• Dimorphic fungi: ability to exist in two forms, e.g., yeast phase at 35-37°C and mold phase at 25°C

General Characteristics

• Fungi lack chlorophyll and do not engage in photosynthesis (different from plants)
• They produce spore structures (sexual-asexual spores)
• Fungal spores are microscopic biological particles that allow fungi to be reproduced
• Spore morphology is one of the most important features used for visual identification of fungi

Fungal Cell Wall

• Complex and rigid cell wall composed of mannoproteins, chitins, α- and β-linked glucans (90%), lipids, protein (% 10)

Fungal Cell Membrane

• Bilayered with inclusion of sterol (ergosterol, zymosterol), in contrast to the cholesterol typically found in higher eukaryotic membranes
• ERGOSTEROL is a specific target for antifungal agents