11.2 Fungi and Microsporidians

SECTION OBJECTIVES

Describe the structure of the fungal mycelium.
Explain how fungi contribute to ecosystems.
Describe how different types of fungi and microsporidians cause disease.

How do fungi contribute to our ecosystems? Fungi are heterotrophs that catabolize (break down and oxidize) complex molecules from other organisms. They break down wood and leaves, including substances such as lignin that animals cannot digest. Many fungi are saprophytes (heterotrophs that live on dead organisms). They decompose plant detritus, recycling nutrients for other plants. Underground, fungal filaments called mycorrhizae (singular, mycorrhiza) extend the root systems of trees, forming a nutritional “internet” that interconnects plant communities. Mycorrhizae could have inspired the fictional underground tree network shown in the 2009 film Avatar.

At the same time, some fungi are pathogens that threaten the lives of humans, animals, and plants. The yeast Candida albicans (Figure 11.4) is normally carried by most people, but it can kill a susceptible patient. The related yeast Candida auris is a deadly pathogen spreading in hospitals. C. auris is resistant to nearly all antifungal agents, and infected patients have a mortality rate of 35%.

A scanning electron micrograph of the structure of Candida albicans, a unicellular fungus.

Yeasts Are Single-Celled Fungi

Single-celled fungi are known as yeasts. Yeasts have the advantage of rapid growth and dispersal in aqueous environments. Many different single-celled yeasts evolved independently in different fungal taxa. Baker’s yeast, Saccharomyces cerevisiae, is used to leaven bread and to brew wine and beer. Other yeasts, such as Candida albicans, are important members of human vaginal biota. But C. albicans is also an opportunistic pathogen, causing vaginal infections as well as infections such as thrush in the mouth of patients with AIDS. Other yeasts that are opportunistic pathogens, commonly infecting AIDS patients, include Pneumocystis jirovecii, a yeast-form ascomycete; and Cryptococcus neoformans, a yeast-form basidiomycete.

Some yeasts are asexual, whereas others, such as S. cerevisiae, undergo alternation of generations between diploid (2n) and haploid (n) forms (Figure 11.5). For S. cerevisiae, both haploid and diploid forms are single cells. The haploid (n) cells undergo several generations of mitosis and budding. This asexual reproduction is referred to as vegetative growth.

A diagram of the lifecycle of Saccharomyces cerevisiae, with an inset micrograph and photo. — The life cycle of *Saccharomyces cerevisiae*. Haploid (n) cells reproduce by budding (asexually). For sexual reproduction, a pair of opposite mating types fuse, forming a diploid (2n) cell. The 2n cell reproduces by budding (asexual) until it undergoes meiosis (sexual). Meiosis generates an ascus full of ascospores, which germinate as haploid cells.

At some point, however, various stress conditions, such as nutrient limitation, may lead the haploid cells to develop into gametes. The gametes are of two mating types. Cells of the two mating types may fertilize each other by fusing together and combining their nuclei to make a diploid (2n) zygote. The diploid cell—like the haploid cell—may undergo vegetative budding and reproduction. Under stress, however, a 2n cell may undergo meiosis, regenerating haploid cells. The haploid progeny possess chromosomes reassorted and recombined from those of their “parental” gametes.

Some fungi, such as C. albicans, can grow either as yeast or as filamentous fungi, depending on the environment; these are known as dimorphic fungi. Another dimorphic fungal pathogen is Blastomyces dermatitidis, the cause of blastomycosis, a type of pneumonia. B. dermatitidis grows as a yeast within the infected lung, but in culture and in soil environments it forms multicellular filaments. Filamentous fungi are described next.

Filamentous Fungi

CASE HISTORY 11.1

Fungus Growing in the Lung

A scanning electron micrograph of Aspergillus.

Frank, a 45-year-old man, visited the hospital with respiratory distress and a high fever. Frank had diabetes mellitus and had smoked for 25 years. His white blood cell (WBC) count was 24,000/μl, and he had a C-reactive protein concentration of 24 mg/ml—both signs that his immune system was fighting an infection. His chest X-ray showed an abnormal shadow, and his chest computed tomography (CT) scan showed thickened bronchial walls. He was coughing up black sputum. The physician prescribed an antibiotic, cefotiam, but Frank’s condition worsened. By the tenth day he was producing 100 ml of sputum a day, and the CT scan of his lungs showed multiple lesions. Compare the normal lung scan (Figure 11.6A) with the scan of lungs infiltrated by an infectious agent (Figure 11.6B). Since antibacterial antibiotics had failed, Frank was tested for fungi. A culture of his sputum and PCR amplification of fungal DNA revealed the fungus Aspergillus fumigatus (Figure 11.6C). Frank was treated with the antifungal agents itraconazole, meropenem, and amphotericin B, but he died on the twenty-fifth day.

A photo of a chest C T scan of healthy lungs. — A. Chest CT, healthy lungs.

A photo of a chest C T scan of lungs infiltrated by Aspergillus mycelium. — A. Chest CT, healthy lungs.

Aspergillus is a filamentous fungus of the ascomycete group (see Table 11.2). Filaments of Aspergillus typically grow on soil or indoors on air conditioner coils or as basement mold. It forms spores that can reach significant concentrations in the air, where they are taken into the lungs and may cause allergic reactions. In patients with impaired immune function, Aspergillus spores may germinate and grow “fungus balls” that require surgical removal. The fungus is a common source of hospital-acquired (nosocomial) infection. In Frank’s case, however, the infection was community acquired. Frank’s predisposing conditions (depressed immune function due to diabetes and smoking) had allowed the fungal infection to take hold.

Table 11.2 Fungi
Phylum and Genus	Habitat or Disease
Ascomycetes	Form asci, pods of eight spores
Aspergillus	Breaks down plant detritus; infects lungs of immunocompromised patients
Candida	Yeast infections; thrush; opportunistic pathogen
Penicillium	Grows on bread and fruit; original source of penicillin
Saccharomyces	Ferments beer and bread dough
Stachybotrys	Black mold; contaminates dwellings
Trichophyton	Ringworm and athlete’s foot skin infections
Basidiomycetes	Mushrooms and mycorrhizae
Amanita	Extremely poisonous; produces alpha-amanitin toxin
Lycoperdon	Edible mushrooms
Chytridiomycetes	Motile zoospores with single flagellum
Batrachochytrium	Global disease of frogs
Zygomycetes	Saprophytes, insect pathogens, and mycorrhizae
Rhizopus	Bread mold; food production (tempeh)

Many fungi grow as multicellular branched filaments called hyphae (singular, hypha; Figure 11.7A). The hyphae undergo a life cycle that produces spores for dissemination. Inhalation of fungal spores can lead to aspergillosis and histoplasmosis, conditions particularly dangerous for immunocompromised patients. Other fungi cause skin infections such as ringworm and athlete’s foot (medical term, tinea pedis). Examples of fungi commonly found in humans or in our environment are presented in Table 11.2.

A scanning electron micrograph of fungal mycelium. — A. A mycelium: *Penicillium* fungal hyphae form the skin of Brie cheese.

An illustration of the chemical structure of Chitin. — A. A mycelium: *Penicillium* fungal hyphae form the skin of Brie cheese.

Hyphae have tough cell walls that are composed of chitin, an acetylated amino-polysaccharide stronger than steel (Figure 11.7B). Its strength derives from multiple hydrogen bonds between fibers. Hyphae grow by extending multinucleated cells (Figure 11.7C). As a fungal hypha extends, its nuclei divide mitotically without cell division. The hyphae form branching tufts called a mycelium (plural, mycelia). The mycelium may grow large enough to be seen as a fuzzy colony or puffball. As the hypha expands, its chitinous cell wall enables it to penetrate softer materials, such as plant or animal cells.

The biosynthesis of chitin provides a target for antifungal agents such as nikkomycin Z. Another drug target is the membrane lipid ergosterol, an analog of cholesterol not found in animals or plants. Ergosterol biosynthesis is inhibited by antifungal agents called triazoles.

The hyphal growth process means that most fungal forms are nonmotile. Fungi cannot ingest particulate food, as do protists. This is because their cell walls cannot part and re-form like the flexible pellicle of amebas and ciliates does (discussed next, in Section 11.3). Instead, fungi secrete digestive enzymes and then absorb the broken-down molecules from their environment. Fungi such as Aspergillus produce toxins (called mycotoxins) that contaminate food and harm human health. But fungi are so effective at secreting small molecules that they are engineered for biotechnology to make medical products; engineered Aspergillus is a billion-dollar industry.

Filamentous fungi can expand at great length by asexual division (mitosis). Eventually, however, they run out of nutrients, and the cells undergo meiosis to form gametes. The gametes develop into spores for dispersal. An example of a spore-forming life cycle is that of ascomycete fungi such as Aspergillus or Morchella (Figure 11.8). In an ascomycete, the tips of the diploid mycelium undergo meiosis and develop into haploid ascospores within pods called asci (singular, ascus). The ascospores disperse until they germinate in a region of fresh resources. Each ascus undergoes cell divisions to form a haploid mycelium, which is male or female. The male and female structures fuse, followed by migration of all the male nuclei into the female structure. The paired nuclei then undergo several rounds of mitotic division while migrating into the growing mycelium. In the mycelial tips, the paired nuclei finally fuse (becoming 2n) and re-form a diploid mycelium. Overall, the diploid-haploid life cycle enables a fungus to respond genetically to environmental change by reassorting its genes through meiosis and fertilization. Gene reassortment provides new genotypes, some of which may increase survival in the changed environment.

A diagram of the lifecycle of an ascomycete with an inset photo of a morel mushroom and a micrograph of asci.

Some fungi pack their asci (or other spore-producing forms) into a larger structure called a fruiting body. Ascomycetes such as Aspergillus and Penicillium mold form relatively small fruiting bodies, called conidiophores, that disperse airborne spores. Conidiospore-forming ascomycetes are the major type of mold associated with dampness in human dwellings; they caused massive damage to homes flooded in the wake of Hurricane Katrina (see IMPACT).

impact

Mold after Hurricane Katrina

In the wake of Hurricane Katrina in 2005, much of the city of New Orleans was flooded. Entire neighborhoods were submerged for days or weeks. The flooding of homes full of drywall and organic materials made ideal conditions for the growth of “mold,” which generally consists of airborne ascomycete fungi. Mold grew not only on the materials submerged but also on the surfaces above, exposed to water-saturated air. Figure IMPACT 11.1 shows how mold grew up to 3 feet above the flood line (the highest level submerged) within the kitchen of a flooded home. Unfortunately, most homeowner insurance policies covered only damage “up to the flood line.”

A photo of a student pointing to mold on a wall. — An undergraduate student volunteer indicates mold colonies growing above the flood line on a kitchen wall in a New Orleans home, 6 months after flooding.

Mold growing on damp surfaces releases spores into the air. Inhaled mold spores cause respiratory problems, even at relatively low levels (such as 2,000 spores per cubic meter). In New Orleans after the waters receded, the mold counts in the air inside homes reached as high as 650,000 spores per cubic meter. Such spore levels cause asthma attacks and hypersensitivity pneumonitis, a condition in which fever rises and the lungs fail. All remediation workers were advised to wear an N95 respirator mask. The Natural Resources Defense Council (NRDC) found that even after all contaminated items—including furniture, carpets, and drywall down to the studs—had been removed from flooded homes, airborne spore counts still reached 70,000.

The most common types of mold found by the NRDC researchers were ascomycetes such as Cladosporium (Figure IMPACT 11.2). Cladosporium species form black or dark green colonies of conidia that readily break off and become airborne. Other ascomycetes found at high levels included Aspergillus and Penicillium species. These organisms occur at low levels even in clean air. Their spores cause allergy attacks and infect immunocompromised individuals. Some ascomycete molds produce mycotoxins, fungal toxins that cause long-term health problems. In addition, some flooded homes showed significant levels of Stachybotrys species, known as “black mold,” a possible cause of neurological problems and immunosuppression. Once these molds establish growth, their eradication is a major challenge.

A light micrograph of Cladosporium cladosporioides. — A. Conidiophores (branched hyphae) producing conidia (asexual spores) of *Cladosporium cladosporioides*.

A photo of Cladosporium mold growing on a petri plate. — A. Conidiophores (branched hyphae) producing conidia (asexual spores) of *Cladosporium cladosporioides*.

DISCUSSION QUESTION

What can homeowners and communities do to decrease the incidence and prevalence of harmful airborne fungi?

The fruiting bodies of other ascomycete fungi can be impressively large and edible as prized foods. Morels (Morchella hortensis; see Figure 11.8) and truffles (Tuber aestivum) form large, mushroom-like fruiting bodies whose ascospores are dispersed by animals attracted by their delicious flavor. The famous truffles grow underground, where human collectors traditionally use muzzled pigs to detect and unearth them.

SECTION SUMMARY

Single-celled fungi are called yeasts. The yeast Saccharomyces cerevisiae is used to make fermented drinks and to leaven bread. Candida albicans is a part of normal human biota, but certain strains can cause opportunistic infections. Candida auris is an emerging drug-resistant pathogen.
Filamentous fungi grow as multinucleated hyphae. Hyphae are exceptionally strong filaments that can penetrate many materials. A branching mass of hyphae is called a mycelium.
Fungi have cell walls of chitin. Chitin biosynthesis is a target for antifungal agents.
Fungi are heterotrophs that absorb nutrients. Most fungi in nature absorb nutrients from decaying organisms, although some are pathogens that infect animal or plant hosts.
Ascomycete fungi form asci that disperse spores. Spores of fungi such as Aspergillus can spread disease or cause allergies.

Thought Question 11.1 For a fungus, what are the advantages of growing a mycelium? What are the advantages of growing as a single-celled yeast?

SHOW ANSWER HIDE ANSWER

Mycelial growth enables coordinated expansion into a food resource, such as a host tissue. The large size of the mycelium enables the fungus to avoid being engulfed by amebas or host macrophages. Single-celled yeasts, however, can disperse readily to new habitats, finding new resources to exploit.

mycorrhiza ( pl. mycorrhizae): A fungus involved in an intimate mutualism with plant roots, in which nutrients are exchanged.
yeast: A unicellular fungus.
dimorphic fungi: Species of fungi that can grow as a mycelium or as a yeast form.
hypha ( pl. hyphae): The threadlike filament that forms the mycelium of a fungus.
chitin: A polymer of N-acetylglucosamine that is a major structural component of fungal cell walls.
mycelium (pl. mycelia): A mass of branched fungal hyphae that projects into the air (aerial mycelium) or into the growth substrate (substrate mycelium).
ascospore: The spore produced by an ascomycete fungus.
ascus (pl. asci): The case that contains the spores of an ascomycete fungus.
fruiting body: A multicellular fungal or bacterial reproductive structure.
mycorrhizae: A fungus involved in an intimate mutualism with plant roots, in which nutrients are exchanged.
hyphae: The threadlike filament that forms the mycelium of a fungus.
mycelia: A mass of branched fungal hyphae that projects into the air (aerial mycelium) or into the growth substrate (substrate mycelium).
asci: The case that contains the spores of an ascomycete fungus.

11.2 Fungi and Microsporidians

Yeasts Are Single-Celled Fungi

Filamentous Fungi

DISCUSSION QUESTION

Glossary