Which Term Most Accurately Describes A Fungus Growing On Live Human Skin?

Learning Objectives

• Explain why the written report of fungi such equally yeast and molds is within the discipline of microbiology
• Depict the unique characteristics of fungi
• Depict examples of asexual and sexual reproduction of fungi
• Compare the major groups of fungi in this chapter, and give examples of each
• Identify examples of the principal causes of infections due to yeasts and molds
• Identify examples of toxin-producing fungi
• Classify fungal organisms according to major groups

The fungi comprise a diverse group of organisms that are heterotrophic and typically saprozoic. In addition to the well-known macroscopic fungi (such as mushrooms and molds), many unicellular yeasts and spores of macroscopic fungi are microscopic. For this reason, fungi are included within the field of microbiology.

Fungi are of import to humans in a diversity of means. Both microscopic and macroscopic fungi take medical relevance, with some pathogenic species that can crusade mycoses (illnesses acquired by fungi). Some pathogenic fungi are opportunistic, meaning that they mainly crusade infections when the host'due south immune defenses are compromised and do not normally crusade illness in healthy individuals. Fungi are of import in other ways. They act equally decomposers in the environment, and they are disquisitional for the production of sure foods such as cheeses. Fungi are likewise major sources of antibiotics, such equally penicillin from the fungus Penicillium .

Characteristics of Fungi

Fungi take well-divers characteristics that set them apart from other organisms. Well-nigh multicellular fungal bodies, commonly called molds, are made up of filaments called hyphae. Hyphae can class a tangled network called a mycelium and form the thallus (body) of fleshy fungi. Hyphae that have walls between the cells are called septate hyphae; hyphae that lack walls and cell membranes between the cells are chosen nonseptate or coenocytic hyphae) (Figure 1).

Effigy one. Multicellular fungi (molds) form hyphae, which may be septate or nonseptate. Unicellular fungi (yeasts) cells grade pseudohyphae from private yeast cells.

In contrast to molds, yeasts are unicellular fungi. The budding yeasts reproduce asexually by budding off a smaller daughter cell; the resulting cells may sometimes stick together as a brusk chain or pseudohypha (Figure 1). Candida albicans is a common yeast that forms pseudohyphae; it is associated with various infections in humans, including vaginal yeast infections, oral thrush, and candidiasis of the peel.

Some fungi are dimorphic, having more than than 1 appearance during their life cycle. These dimorphic fungi may be able to appear equally yeasts or molds, which can be of import for infectivity. They are capable of irresolute their appearance in response to environmental changes such as nutrient availability or fluctuations in temperature, growing as a mold, for example, at 25 °C (77 °F), and as yeast cells at 37 °C (98.6 °F). This ability helps dimorphic fungi to survive in various environments. Histoplasma capsulatum , the pathogen that causes histoplasmosis, a lung infection, is an example of a dimorphic mucus (Effigy 2).

Effigy two. Histoplasma capsulatum is a dimorphic mucus that grows in soil exposed to bird carrion or bat carrion (guano) (top left). It can alter forms to survive at different temperatures. In the outdoors, it typically grows every bit a mycelium (every bit shown in the micrograph, bottom left), but when the spores are inhaled (right), it responds to the high internal temperature of the body (37 °C [98.6 °F]) by turning into a yeast that can multiply in the lungs, causing the chronic lung disease histoplasmosis. (credit: modification of work by Centers for Disease Control and Prevention)

There are notable unique features in fungal cell walls and membranes. Fungal prison cell walls contain chitin, as opposed to the cellulose found in the cell walls of plants and many protists. Additionally, whereas animals accept cholesterol in their prison cell membranes, fungal cell membranes take different sterols called ergosterols. Ergosterols are often exploited as targets for antifungal drugs.

Fungal life cycles are unique and circuitous. Fungi reproduce sexually either through cantankerous- or cocky-fertilization. Haploid fungi form hyphae that accept gametes at the tips. 2 different mating types (represented as "+ type" and "– type") are involved. The cytoplasms of the + and – blazon gametes fuse (in an outcome called plasmogamy), producing a prison cell with ii distinct nuclei (a dikaryotic cell). Later, the nuclei fuse (in an event chosen karyogamy) to create a diploid zygote. The zygote undergoes meiosis to form spores that germinate to start the haploid stage, which eventually creates more haploid mycelia (Effigy 3). Depending on the taxonomic group, these sexually produced spores are known as zygospores (in Zygomycota), ascospores (in Ascomycota), or basidiospores (in Basidiomycota) (Figure 4).

Fungi may besides showroom asexual reproduction past mitosis, mitosis with budding, fragmentation of hyphae, and formation of asexual spores past mitosis. These spores are specialized cells that, depending on the organism, may have unique characteristics for survival, reproduction, and dispersal. Fungi exhibit several types of asexual spores and these can be of import in classification.

Figure 3. Click for a larger prototype. Zygomycetes have sexual and asexual life cycles. In the sexual life cycle, + and – mating types conjugate to grade a zygosporangium.

Remember about It

• Is a dimorphic fungus a yeast or a mold? Explain.

Fungal Diverseness

The fungi are very various, comprising vii major groups. Not all of the 7 groups contain pathogens. Some of these groups are mostly associated with plants and include plant pathogens. For example, Urediniomycetes and Ustilagomycetes include the found rusts and smuts, respectively. These form carmine or dark masses, respectively, on plants every bit rusts (red) or smuts (nighttime). Some species accept substantial economic bear upon because of their ability to reduce ingather yields. Glomeromycota includes the mycorrhizal fungi, important symbionts with plant roots that tin can promote constitute growth by acting similar an extended root organization. The Glomeromycota are obligate symbionts, meaning that they tin only survive when associated with found roots; the fungi receive carbohydrates from the plant and the plant benefits from the increased ability to take upwardly nutrients and minerals from the soil. The Chytridiomycetes (chytrids) are pocket-size fungi, but are extremely ecologically of import. Chytrids are mostly aquatic and have flagellated, motile gametes; specific types are implicated in amphibian declines effectually the world. Because of their medical importance, we will focus on Zygomycota, Ascomycota, Basidiomycota, and Microsporidia. Figure 9 summarizes the characteristics of these medically important groups of fungi.

The Zygomycota (zygomycetes) are mainly saprophytes with coenocytic hyphae and haploid nuclei. They use sporangiospores for asexual reproduction. The group name comes from the zygospores that they utilise for sexual reproduction (Effigy 3), which have difficult walls formed from the fusion of reproductive cells from ii individuals. Zygomycetes are important for food science and equally crop pathogens. One example is Rhizopus stolonifer (Figure 4), an important breadstuff mold that likewise causes rice seedling bane. Mucor is a genus of fungi that can potentially cause necrotizing infections in humans, although near species are intolerant of temperatures found in mammalian bodies (Figure 4).

Effigy 4. These images show asexually produced spores. (a) This brightfield micrograph shows the release of spores from a sporangium at the end of a hypha chosen a sporangiophore. The organism is a Mucor sp. mucus, a mold often found indoors. (b) Sporangia grow at the ends of stalks, which announced as the white fuzz seen on this bread mold, Rhizopus stolonifer. The tips of staff of life mold are the dark, spore-containing sporangia. (credit a: modification of work past Centers for Illness Control and Prevention; credit b right: modification of work past "Andrew"/Flickr)

The Ascomycota include fungi that are used as food (edible mushrooms, morels, and truffles), others that are mutual causes of nutrient spoilage (bread molds and constitute pathogens), and even so others that are human being pathogens. Ascomycota may accept septate hyphae and cup-shaped fruiting bodies called ascocarps. Some genera of Ascomycota utilize sexually produced ascospores too as asexual spores chosen conidia, but sexual phases have not been discovered or described for others. Some produce an ascus containing ascospores within an ascocarp (Figure v).

Figure 5. (a) This brightfield micrograph shows ascospores being released from asci in the fungus Talaromyces flavus var. flavus. (b) This electron micrograph shows the conidia (spores) borne on the conidiophore of Aspergillus, a type of toxic fungus plant mostly in soil and plants. (c) This brightfield micrograph shows the yeast Candida albicans, the causative agent of candidiasis and thrush. (credit a, b, c: modification of piece of work by Centers for Disease Control and Prevention)

Effigy vi. These ascospores, lined up within an ascus, are produced sexually. (credit: Peter 1000. Werner)

Examples of the Ascomycota include several bread molds and small-scale pathogens, too every bit species capable of causing more than serious mycoses. Species in the genus Aspergillus are important causes of allergy and infection, and are useful in enquiry and in the production of certain fermented alcoholic beverages such equally Japanese sake. The fungus Aspergillus flavus , a contaminant of nuts and stored grains, produces an aflatoxin that is both a toxin and the about potent known natural carcinogen. Neurospora crassa is of particular use in genetics research considering the spores produced by meiosis are kept inside the ascus in a row that reflects the cell divisions that produced them, giving a direct view of segregation and assortment of genes (Figure 6). Penicillium produces the antibody penicillin (Figure five).

Many species of ascomycetes are medically important. A large number of species in the genera Trichophyton , Microsporum , and Epidermophyton are dermatophytes, pathogenic fungi capable of causing pare infections such as athlete's human foot, jock itch, and ringworm. Blastomyces dermatitidis is a dimorphic fungus that can cause blastomycosis, a respiratory infection that, if left untreated, can become disseminated to other body sites, sometimes leading to death. Some other important respiratory pathogen is the dimorphic fungus Histoplasma capsulatum (Figure 2), which is associated with birds and bats in the Ohio and Mississippi river valleys. Coccidioides immitis causes the serious lung disease Valley fever. Candida albicans , the nigh common cause of vaginal and other yeast infections, is too an ascomycete fungus; it is a part of the normal microbiota of the skin, intestine, genital tract, and ear (Effigy five). Ascomycetes too cause plant diseases, including ergot infections, Dutch elm illness, and powdery mildews.

Saccharomyces yeasts, including the bakery'southward yeast S. cerevisiae, are unicellular ascomycetes with haploid and diploid stages (Figure 7). This and other Saccharomyces species are used for brewing beer.

Effigy 7. Click for a larger prototype. The life wheel of an ascomycete is characterized by the production of asci during the sexual stage. The haploid stage is the predominant phase of the life bicycle.

The Basidiomycota (basidiomycetes) are fungi that have basidia (club-shaped structures) that produce basidiospores (spores produced through budding) inside fruiting bodies called basidiocarps (Figure viii). They are important as decomposers and as food. This group includes rusts, stinkhorns, puffballs, and mushrooms. Several species are of particular importance. Cryptococcus neoformans , a fungus usually found as a yeast in the environment, tin can crusade serious lung infections when inhaled by individuals with weakened immune systems. The edible meadow mushroom, Agricus campestris, is a basidiomycete, every bit is the poisonous mushroom Amanita phalloides, known as the death cap. The deadly toxins produced by A. phalloides have been used to study transcription.

Figure viii. Click for a larger image. The life bike of a basidiomycete alternates a haploid generation with a prolonged stage in which two nuclei (dikaryon) are nowadays in the hyphae.

Finally, the Microsporidia are unicellular fungi that are obligate intracellular parasites. They lack mitochondria, peroxisomes, and centrioles, just their spores release a unique polar tubule that pierces the host prison cell membrane to let the fungus to gain entry into the cell. A number of microsporidia are human pathogens, and infections with microsporidia are called microsporidiosis. One pathogenic species is Enterocystozoan bieneusi , which tin cause symptoms such every bit diarrhea, cholecystitis (inflammation of the gall bladder), and in rare cases, respiratory affliction.

Table 1. Select Groups of Fungi[1]
Grouping	Characteristics	Examples	Medically Important Species	Image
Ascomycota	Septate hyphae Ascus with ascospores in ascocarp Conidiospores	Cup fungi Edible mushrooms Morels Truffles Neurospora Penicillim	Aspergillus spp. Trichophyton spp. Microsporum spp. Epidemophyton spp. Blastomyces demititidis Histoplasma capsulatum	Aspergillus niger
Basidiomycota	Basidia Produce basidiospores in basidiocarp	Club fungi Rusts Stinkhors Puffballs Mushrooms Cryptococcus neoformans Amanita phalloides	Cryptococcus neoformans	Amanita phalloides
Microsporidia	Lack mitochondria, peroxisomes, and centrioles Spores produce a polar tube	Enterocystozoan bieneusi	Enterocystozoan bieneusi	Microsporidia (unidentified)
Zygomycota	Mainly saprophytes Coenocytic hyphae Haploid nuclei Zygospores	Rhizopus stolonifera	Mucor spp.	Rhizopus sp.

Think about It

• Which group of fungi appears to be associated with the greatest number of human diseases?

Eukaryotic Pathogens in Eukaryotic Hosts

When we think most antimicrobial medications, antibiotics such every bit penicillin often come up to heed. Penicillin and related antibiotics interfere with the synthesis of peptidoglycan cell walls, which effectively targets bacterial cells. These antibiotics are useful because humans (like all eukaryotes) do not have peptidoglycan cell walls.

Developing medications that are constructive against eukaryotic cells just non harmful to human cells is more difficult. Despite huge morphological differences, the cells of humans, fungi, and protists are similar in terms of their ribosomes, cytoskeletons, and cell membranes. Every bit a result, it is more challenging to develop medications that target protozoans and fungi in the aforementioned way that antibiotics target prokaryotes.

Fungicides have relatively limited modes of action. Considering fungi have ergosterols (instead of cholesterol) in their cell membranes, the different enzymes involved in sterol production can be a target of some medications. The azole and morpholine fungicides interfere with the synthesis of membrane sterols. These are used widely in agriculture (fenpropimorph) and clinically (e.g., miconazole). Some antifungal medications target the chitin cell walls of fungi. Despite the success of these compounds in targeting fungi, antifungal medications for systemic infections notwithstanding tend to have more toxic side furnishings than antibiotics for bacteria.

Clinical Focus: Anthony, Part 3

This case continues Anthony's story that started in Unicellular Eukaryotic Parasites and Parasitic Helminths.

Anthony is relieved the ringworm is non an actual worm, but wants to know what it really is. The doctor explains that ringworm is a mucus. He tells Anthony that he volition not encounter mushrooms popping out of his skin, because this fungus is more like the invisible part of a mushroom that hides in the soil. They dr. reassures Anthony that they are going to get the fungus out of him likewise.

The doctor cleans and then carefully scrapes the lesion to place a specimen on a slide. By looking at it under a microscope, the physician is able to confirm that a fungal infection is responsible for Anthony's lesion. In Effigy 9, information technology is possible to see macro- and microconidia in Trichophyton rubrum . Prison cell walls are as well visible. Even if the pathogen resembled a helminth nether the microscope, the presence of cell walls would rule out the possibility because brute cells lack cell walls.

Figure 9. This micrograph shows hyphae (macroconidium) and microconidia of Trichophyton rubrum, a dermatophyte responsible for fungal infections of the skin. (credit: modification of work by Centers for Disease Control and Prevention)

The doctor prescribes an antifungal foam for Anthony'south mother to employ to the ringworm. Anthony's mother asks, "What should we do if it doesn't go abroad?"

• Can all forms of ringworm be treated with the aforementioned antifungal medication?

We'll return to Anthony's example in later pages.

Key Concepts and Summary

• The fungi include diverse saprotrophic eukaryotic organisms with chitin jail cell walls
• Fungi tin can exist unicellular or multicellular; some (like yeast) and fungal spores are microscopic, whereas some are big and conspicuous
• Reproductive types are of import in distinguishing fungal groups
• Medically important species exist in the four fungal groups Zygomycota, Ascomycota, Basidiomycota, and Microsporidia
• Members of Zygomycota, Ascomycota, and Basidiomycota produce deadly toxins
• Important differences in fungal cells, such as ergosterols in fungal membranes, can be targets for antifungal medications, but similarities betwixt human and fungal cells make information technology difficult to find targets for medications and these medications often take toxic adverse furnishings

Multiple Choice

Mushrooms are a blazon of which of the following?

1. conidia
2. ascus
3. polar tubule
4. basidiocarp

Prove Answer

Answer d. Mushrooms are a basidiocarp.

Which of the following is the most common crusade of human yeast infections?

1. Candida albicans
2. Blastomyces dermatitidis
3. Cryptococcus neoformans
4. Aspergillus fumigatus

Show Answer

Respond a.Candida albicans is the most common crusade of human yeast infections.

Which of the following is an ascomycete mucus associated with bat droppings that can cause a respiratory infection if inhaled?

1. Candida albicans
2. Histoplasma capsulatum
3. Rhizopus stolonifera
4. Trichophyton rubrum

Show Reply

Answer b.Histoplasma capsulatum is an ascomycete fungus associated with bat droppings.

Fill in the Blank

Nonseptate hyphae are also chosen _________.

Show Reply

Nonseptate hyphae are also called coenocytic.

Unicellular fungi are chosen _________.

Bear witness Answer

Unicellular fungi are called yeasts.

Some fungi have proven medically useful because they can be used to produce _________.

Show Answer

Some fungi have proven medically useful considering they tin can be used to produce antibiotics.

Think nigh It

1. Which genera of fungi are mutual dermatophytes (fungi that cause skin infections)?
2. What is a dikaryotic cell?
3. Explain the benefit of research into the pathways involved in the synthesis of chitin in fungi.
4. Which of the drawings shows septate hyphae?

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Source: https://courses.lumenlearning.com/microbiology/chapter/fungi/

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