Fungi

FUNGI

Fungi are generally saprophytic, meaning they thrive on decay. They are included in the discussion of microbiology because there are many that are microscopic in nature. They lead to mycoses or fungal diseases in humans. Some are strictly opportunistic because they infect only the immunosuppressed human host.

Remember that a major feature is a chitin-containing cell wall. There are molds that are multicellular and made from hyphae, which are filaments. Multiple entangled hyphae are called a mycelium. There are those that have walls separating them, called septate hyphae and those that do not have septa, called coenocytic. Pseudohyphae are short chains of daughter cells that have budded off of a budding yeast. Figure 24 depicts the different types of hyphae:

Figure 24.

Dimorphic fungi have more than one morphological appearance, depending on their life cycle. They can be either yeasts or molds and change morphology based on the environment. Histoplasma capsulatum and Candida albicans are two fungal organisms considered to be dimorphic. Histoplasma causes the lung disease called histoplasmosis, while Candida species result in yeast infections. Histoplasma makes hyphae in the environment but are unicellular yeasts in the human body.

Fungi reproduce sexually or asexually, through self-fertilization or cross-fertilization. Most of their life cycle is haploid instead of diploid. Spores can be formed that can germinate to begin the haploid mycelial stage. Some groups form zygospores; others will produce basidiospores; and others will form ascospores, depending on their taxonomy.

There are seven major groupings of fungi but not all of these are pathogenic. Some will only be pathogenic in plants, causing smuts and plant spores. These can affect the food supply. Others are symbiotic with plant roots, including those that can only survive this way. The Chytrids are small types of fungi that live in water and have motile gametes. They infect amphibians and have threatened their survival. There are four groups that are important in human illnesses, including Ascomycota, Zygomycota, Microsporidia, and Basidiomycota.

Zygomycota will make sporangiospores for asexual reproduction and zygospores for sexual reproduction. There is one genus called Rhizopus, which leads to bread mold and Mucor, which is a genus that can cause severe necrotizing human infections.

Ascomycota include the morel mushrooms, truffles, and other edible mushrooms. Some will spoil food. They make cup-shaped fruiting bodies called ascocarps. There are sexual spores called ascospores and asexual spores called conidia. Besides bread molds, some will cause human disease, particularly Aspergillus, which makes an aflatoxin when it contaminates grains or nuts. Aflatoxin can cause cancer. Penicillium is what makes penicillin.

The ascomycetes an also cause the various skin infections, such as ringworm, jock itch, and athlete’s foot. Blastomyces can cause blastomycosis, which is a respiratory infection. Histoplasma is in this classification. Coccidioides leads to Valley fever and