Dermatophytes Are Fungi That Can Infect the __________ Because They Can Grow on/in __________.

Learning Objectives

• Explain why the study of fungi such as yeast and molds is within the discipline of microbiology
• Describe the unique characteristics of fungi
• Describe examples of asexual and sexual reproduction of fungi
• Compare the major groups of fungi in this chapter, and requite examples of each
• Identify examples of the master causes of infections due to yeasts and molds
• Place examples of toxin-producing fungi
• Classify fungal organisms according to major groups

The fungi contain a diverse group of organisms that are heterotrophic and typically saprozoic. In improver 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 important to humans in a diversity of ways. Both microscopic and macroscopic fungi have medical relevance, with some pathogenic species that can cause mycoses (illnesses caused by fungi). Some pathogenic fungi are opportunistic, pregnant that they mainly cause infections when the host's allowed defenses are compromised and do not normally cause affliction in healthy individuals. Fungi are important in other ways. They act as decomposers in the surround, and they are critical for the production of sure foods such as cheeses. Fungi are also major sources of antibiotics, such as penicillin from the fungus Penicillium .

Characteristics of Fungi

Fungi have well-divers characteristics that set them apart from other organisms. Most multicellular fungal bodies, commonly called molds, are fabricated up of filaments called hyphae. Hyphae can class a tangled network called a mycelium and grade the thallus (torso) 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 called nonseptate or coenocytic hyphae) (Figure i).

Figure ane. Multicellular fungi (molds) form hyphae, which may be septate or nonseptate. Unicellular fungi (yeasts) cells form pseudohyphae from individual yeast cells.

In dissimilarity 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 every bit a brusk chain or pseudohypha (Figure 1). Candida albicans is a common yeast that forms pseudohyphae; information technology is associated with various infections in humans, including vaginal yeast infections, oral thrush, and candidiasis of the skin.

Some fungi are dimorphic, having more than one appearance during their life bicycle. These dimorphic fungi may exist able to appear as yeasts or molds, which can exist important for infectivity. They are capable of irresolute their advent 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.half dozen °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 fungus (Effigy 2).

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

In that location are notable unique features in fungal cell walls and membranes. Fungal cell walls contain chitin, as opposed to the cellulose found in the cell walls of plants and many protists. Additionally, whereas animals have cholesterol in their cell membranes, fungal cell membranes accept 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 cross- or self-fertilization. Haploid fungi form hyphae that have gametes at the tips. Two different mating types (represented as "+ blazon" and "– blazon") are involved. The cytoplasms of the + and – type gametes fuse (in an event called plasmogamy), producing a cell with 2 distinct nuclei (a dikaryotic cell). Later, the nuclei fuse (in an outcome called 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 (Figure three). Depending on the taxonomic group, these sexually produced spores are known every bit zygospores (in Zygomycota), ascospores (in Ascomycota), or basidiospores (in Basidiomycota) (Figure iv).

Fungi may also exhibit asexual reproduction past mitosis, mitosis with budding, fragmentation of hyphae, and germination of asexual spores by mitosis. These spores are specialized cells that, depending on the organism, may take unique characteristics for survival, reproduction, and dispersal. Fungi exhibit several types of asexual spores and these can exist important in classification.

Effigy 3. Click for a larger image. Zygomycetes take sexual and asexual life cycles. In the sexual life cycle, + and – mating types cohabit to form a zygosporangium.

Think well-nigh It

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

Fungal Diversity

The fungi are very diverse, comprising vii major groups. Not all of the seven groups comprise pathogens. Some of these groups are mostly associated with plants and include plant pathogens. For instance, Urediniomycetes and Ustilagomycetes include the constitute rusts and smuts, respectively. These class reddish or dark masses, respectively, on plants every bit rusts (ruby-red) or smuts (nighttime). Some species accept substantial economical bear upon because of their ability to reduce ingather yields. Glomeromycota includes the mycorrhizal fungi, of import symbionts with plant roots that can promote found growth by acting like an extended root organization. The Glomeromycota are obligate symbionts, meaning that they tin only survive when associated with plant roots; the fungi receive carbohydrates from the establish and the establish benefits from the increased ability to take up nutrients and minerals from the soil. The Chytridiomycetes (chytrids) are small-scale fungi, only are extremely ecologically important. Chytrids are by and large aquatic and have flagellated, motile gametes; specific types are implicated in amphibian declines effectually the globe. Because of their medical importance, we volition 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 utilize sporangiospores for asexual reproduction. The group name comes from the zygospores that they use for sexual reproduction (Figure three), which have difficult walls formed from the fusion of reproductive cells from two individuals. Zygomycetes are of import for food scientific discipline and every bit crop pathogens. One case is Rhizopus stolonifer (Figure 4), an important staff of life mold that as well causes rice seedling blight. Mucor is a genus of fungi that can potentially cause necrotizing infections in humans, although most species are intolerant of temperatures institute in mammalian bodies (Figure 4).

Figure 4. These images show asexually produced spores. (a) This brightfield micrograph shows the release of spores from a sporangium at the stop of a hypha called a sporangiophore. The organism is a Mucor sp. fungus, a mold often found indoors. (b) Sporangia grow at the ends of stalks, which appear equally 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 by Centers for Illness Control and Prevention; credit b right: modification of work by "Andrew"/Flickr)

The Ascomycota include fungi that are used equally food (edible mushrooms, morels, and truffles), others that are common causes of food spoilage (bread molds and plant pathogens), and still others that are human pathogens. Ascomycota may have septate hyphae and cup-shaped fruiting bodies chosen ascocarps. Some genera of Ascomycota apply sexually produced ascospores every bit well as asexual spores called conidia, but sexual phases have not been discovered or described for others. Some produce an ascus containing ascospores within an ascocarp (Figure 5).

Effigy five. (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 blazon of toxic fungus found 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 work by Centers for Illness Control and Prevention)

Figure half dozen. These ascospores, lined upwards within an ascus, are produced sexually. (credit: Peter Thousand. Werner)

Examples of the Ascomycota include several bread molds and pocket-sized pathogens, as well as species capable of causing more serious mycoses. Species in the genus Aspergillus are of import causes of allergy and infection, and are useful in research and in the product of sure fermented alcoholic beverages such every bit Japanese sake. The fungus Aspergillus flavus , a contaminant of basics and stored grains, produces an aflatoxin that is both a toxin and the nigh strong known natural carcinogen. Neurospora crassa is of particular use in genetics inquiry considering the spores produced past meiosis are kept within the ascus in a row that reflects the jail cell divisions that produced them, giving a direct view of segregation and array of genes (Figure 6). Penicillium produces the antibiotic penicillin (Figure 5).

Many species of ascomycetes are medically of import. A large number of species in the genera Trichophyton , Microsporum , and Epidermophyton are dermatophytes, pathogenic fungi capable of causing skin infections such as athlete's foot, jock crawling, and ringworm. Blastomyces dermatitidis is a dimorphic fungus that can cause blastomycosis, a respiratory infection that, if left untreated, tin can become disseminated to other torso sites, sometimes leading to expiry. Another important respiratory pathogen is the dimorphic fungus Histoplasma capsulatum (Figure two), 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 most common cause of vaginal and other yeast infections, is likewise an ascomycete mucus; it is a part of the normal microbiota of the skin, intestine, genital tract, and ear (Figure 5). Ascomycetes also cause found diseases, including ergot infections, Dutch elm affliction, and powdery mildews.

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

Figure 7. Click for a larger image. The life cycle of an ascomycete is characterized by the product of asci during the sexual phase. The haploid phase is the predominant stage of the life cycle.

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

Figure 8. Click for a larger image. The life bike of a basidiomycete alternates a haploid generation with a prolonged stage in which 2 nuclei (dikaryon) are nowadays in the hyphae.

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

Table one. Select Groups of Fungi[ane]
Group	Characteristics	Examples	Medically Important Species	Prototype
Ascomycota	Septate hyphae Ascus with ascospores in ascocarp Conidiospores	Loving 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	Guild 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 exist associated with the greatest number of human diseases?

Eukaryotic Pathogens in Eukaryotic Hosts

When we think virtually antimicrobial medications, antibiotics such as penicillin often come to mind. Penicillin and related antibiotics interfere with the synthesis of peptidoglycan prison cell walls, which finer targets bacterial cells. These antibiotics are useful because humans (like all eukaryotes) do not have peptidoglycan jail cell walls.

Developing medications that are effective against eukaryotic cells but not harmful to man cells is more difficult. Despite huge morphological differences, the cells of humans, fungi, and protists are like in terms of their ribosomes, cytoskeletons, and cell membranes. As a outcome, information technology 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 unlike 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 still tend to have more toxic side effects than antibiotics for leaner.

Clinical Focus: Anthony, Function 3

This example continues Anthony'southward story that started in Unicellular Eukaryotic Parasites and Parasitic Helminths.

Anthony is relieved the ringworm is not an actual worm, but wants to know what information technology really is. The physician explains that ringworm is a mucus. He tells Anthony that he will not meet mushrooms popping out of his skin, because this fungus is more like the invisible part of a mushroom that hides in the soil. They physician reassures Anthony that they are going to get the mucus out of him too.

The physician cleans and then carefully scrapes the lesion to identify a specimen on a slide. Past looking at it under a microscope, the md is able to confirm that a fungal infection is responsible for Anthony's lesion. In Effigy ix, it is possible to see macro- and microconidia in Trichophyton rubrum . Cell walls are also visible. Fifty-fifty if the pathogen resembled a helminth under the microscope, the presence of cell walls would rule out the possibility considering beast cells lack cell walls.

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

The physician prescribes an antifungal cream for Anthony's mother to apply to the ringworm. Anthony's female parent asks, "What should we practise if information technology doesn't become away?"

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

Nosotros'll return to Anthony'southward example in after pages.

Fundamental Concepts and Summary

• The fungi include various saprotrophic eukaryotic organisms with chitin cell walls
• Fungi tin can exist unicellular or multicellular; some (like yeast) and fungal spores are microscopic, whereas some are large and conspicuous
• Reproductive types are important in distinguishing fungal groups
• Medically of import 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 between homo and fungal cells brand it difficult to find targets for medications and these medications ofttimes have toxic agin effects

Multiple Choice

Mushrooms are a type of which of the following?

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

Bear witness Answer

Answer d. Mushrooms are a basidiocarp.

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

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

Show Answer

Reply a.Candida albicans is the well-nigh common cause of human yeast infections.

Which of the post-obit is an ascomycete mucus associated with bat debris that tin can crusade a respiratory infection if inhaled?

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

Show Answer

Respond b.Histoplasma capsulatum is an ascomycete mucus associated with bat debris.

Make full in the Blank

Nonseptate hyphae are also chosen _________.

Show Reply

Nonseptate hyphae are also chosen coenocytic.

Unicellular fungi are called _________.

Show Answer

Unicellular fungi are called yeasts.

Some fungi accept proven medically useful considering they can be used to produce _________.

Show Answer

Some fungi take proven medically useful because they can be used to produce antibiotics.

Think about Information technology

1. Which genera of fungi are common dermatophytes (fungi that cause pare 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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