Eukaryotes 2 Flashcards
explain how fungal diseases in humans can manifest in various ways, including through the production of mycotoxins, allergic reactions, or infections known as mycoses
It’s important to note that fungi are a diverse group of organisms, and while there are approximately 1.5 million known fungal species, only a relatively small number of them cause human disease. Many of these fungi are considered opportunistic pathogens, meaning they primarily affect individuals with compromised immune systems. Fungal diseases in humans can manifest in various ways, including through the production of mycotoxins, allergic reactions, or infections known as mycoses.
Types of Fungal Infections:
Superficial Infections: These infections are limited to the outermost layers of the skin, hair, and nails. Examples include ringworm (caused by dermatophytes), athlete’s foot, and fungal nail infections.
Subcutaneous Infections: Subcutaneous fungal infections involve the skin and tissues just below the skin’s surface. These infections are often the result of traumatic injuries or inoculation of the fungus into the skin. Examples include sporotrichosis and chromoblastomycosis.
Systemic Infections (Deep Infections): Systemic fungal infections are more severe and can affect internal organs, spreading throughout the body. They are typically associated with immunocompromised individuals and can be life-threatening. The four main genera commonly associated with systemic fungal infections are:
Candida: Candida species are known for causing candidiasis, which can affect various mucous membranes and internal organs, leading to conditions like oral thrush, esophageal candidiasis, and invasive candidiasis.
Aspergillus: Aspergillus species are responsible for a range of infections, including aspergillosis, which can be invasive and affect the lungs, sinuses, and other organs, particularly in individuals with weakened immune systems.
Cryptococcus: Cryptococcus neoformans and Cryptococcus gattii are the primary species responsible for cryptococcosis, which primarily affects the lungs and central nervous system, often in people with compromised immune function.
Histoplasma: Histoplasma capsulatum is the cause of histoplasmosis, a fungal infection that primarily affects the lungs and can become systemic in individuals with weakened immune systems.
Mycotoxins, as you mentioned, are toxic compounds produced by some fungi, and they can contaminate food supplies, leading to a range of health issues in humans and animals.
three main taxonomic groups of fungi
The three main taxonomic groups of fungi you mentioned are:
Zygomycetes: Zygomycetes are a diverse group of fungi known for their sexual reproduction involving the formation of durable, resistant structures called zygospores. They include molds like Rhizopus, which is commonly found in soil and can be involved in food spoilage. Zygomycetes are characterized by their coenocytic (multinucleate) hyphae.
Ascomycetes: Ascomycetes, also known as sac fungi, are one of the largest and most diverse groups of fungi. They are characterized by their sexual reproductive structures called asci, which contain ascospores. Ascomycetes include a wide range of fungi, from yeasts like Saccharomyces cerevisiae used in baking and brewing to filamentous fungi like Penicillium, which is involved in cheese production and the production of the antibiotic penicillin.
Basidiomycetes: Basidiomycetes, also known as club fungi, are characterized by their club-shaped sexual reproductive structures called basidia. These basidia produce basidiospores. Basidiomycetes include some of the most familiar and ecologically important fungi, such as mushrooms (Agaricus), bracket fungi (Ganoderma), and rusts (Puccinia) that can affect plants.
These three groups represent some of the major fungal taxa, but there are additional groups as well, such as Chytridiomycetes (including chytrids) and Glomeromycetes (including arbuscular mycorrhizal fungi), among others.
explain some key features of yeast cell structure and reproduction
- Eukaryotic Cell Structure: Yeast cells, like all fungi, are eukaryotic. This means they have a well-defined nucleus enclosed within a membrane, as well as various membrane-bound organelles, including mitochondria, endoplasmic reticulum, and Golgi apparatus. Yeast cells also have a prominent cell wall, which gives them structural support and protection.
- Thick Cell Wall: The cell wall of yeast is a distinguishing feature. It is made of a complex sugar called chitin, which is different from the cellulose found in the cell walls of plant cells. The cell wall helps maintain the cell’s shape and provides protection.
- Reproduction by Budding: Yeast cells typically reproduce asexually through a process called budding. In budding, a small daughter cell (the bud) starts to grow on the surface of the mother cell. Over time, the bud enlarges and eventually detaches from the mother cell, becoming an independent yeast cell. This is a rapid and efficient means of reproduction.
- Sexual Reproduction: While yeast primarily reproduce asexually through budding, they can also undergo sexual reproduction when conditions are less favorable. Yeast can have a sexual cycle involving the fusion of specialized mating types. The mating types are often referred to as “a” and “α.” During sexual reproduction, yeast cells can fuse together, and their nuclei combine to form a diploid cell. This diploid cell can later undergo meiosis to produce haploid spores, which can then germinate into new yeast cells.
- “Kissing” Schmoos: The term “kissing schmoos” is a playful way to describe the process of sexual reproduction in yeast. When yeast cells of opposite mating types (a and α) come into close proximity, they extend projections known as “schmoo tips” towards each other. These projections “kiss,” facilitating cell fusion and the initiation of sexual reproduction.
outline the sexual reproduction cycle of yeast cells, particularly Saccharomyces cerevisiae
Compatible Haploid Schmoos Meet: When two yeast cells of compatible mating types (a and α) come into close proximity, they extend projections known as “schmoo tips” toward each other. These schmoo tips facilitate the recognition and interaction between the two mating types.
Outgrowths Meet and Dividing Walls Break Down: The schmoo tips of the two mating cells come into contact, and their cell walls at the point of contact break down. This allows the two cells to fuse together.
Schmoos Fuse to Form a Diploid Zygotic Nucleus: After the fusion, the contents of the two haploid cells combine to form a single diploid cell with a zygotic nucleus. This diploid cell contains genetic material from both parent cells.
Diploid Nucleus Enters a New Budding Cell: The diploid zygotic nucleus can enter a new budding cell that forms on the surface of the diploid cell. This budding process can continue in a manner similar to asexual reproduction, forming a chain of cells.
New Diploid Cell Buds Off: Eventually, the new diploid cell formed through budding can also bud off to create more diploid cells. This process can continue, leading to the production of multiple diploid cells.
Development into a Flask or Ascus: Under specific conditions, some diploid cells may undergo further development. In the case of Saccharomyces cerevisiae, they may develop into specialized structures called asci. Within the ascus, meiosis occurs, resulting in the production of four haploid spores.
The cycle described represents the sexual reproduction of yeast, which is a mechanism for creating genetic diversity. This is different from the more common asexual reproduction through budding, which results in genetically identical daughter cells.
explain the reproductive structures and processes associated with ascomycetes
Flask-Like Structure Containing Ascospores: Ascomycetes reproduce sexually by forming specialized structures called asci (singular: ascus). The ascus is a sac-like or flask-like structure that contains sexually produced spores called ascospores. These ascospores are the result of the sexual fusion of nuclei within the asci.
Asci Lining the Hymenium in Cup-Fungi: Cup-fungi are a specific group of ascomycetes that have a cup-shaped or saucer-like fruiting body. The asci in these fungi line the hymenium, which is the fertile, spore-bearing layer located inside the cup. When the asci mature, they release their ascospores into the environment.
Mitosis Following Meiosis in Ascomycetes: In ascomycetes, the reproductive process typically involves meiosis, which reduces the chromosome number, followed by a round of mitosis. As a result, each ascus contains multiple ascospores. This process results in the production of typically eight ascospores per ascus, although the exact number can vary in some species.
The life cycle of ascomycetes is characterized by this sexual reproductive process, which leads to the formation of asci and ascospores. Ascomycetes are a diverse group of fungi with a wide range of ecological roles and forms, including cup-fungi, morel mushrooms, and many plant pathogens.
explain the various forms and structures found in fungi
Your descriptions are accurate and pertain to various forms and structures found in fungi, including yeasts, hyphae, pseudohyphae, and terms like pleiomorphic, dimorphic, and polymorphic, which describe the ability of some fungi to exhibit different growth forms. Here’s an elaboration on these concepts:
Yeasts: Yeasts are a type of fungi that consist of single-celled organisms. They are typically oval or spherical in shape and reproduce asexually through a process known as budding, as previously mentioned. Yeasts are known for their ability to ferment sugars and produce carbon dioxide and alcohol, which has applications in baking and brewing.
Hyphae: Hyphae are the thread-like, filamentous structures that make up the body (thallus) of many fungi. These hyphae can be multinucleate, meaning they contain multiple nuclei within the same cell. They are involved in nutrient absorption and the growth of the fungus. Some hyphae have cross-walls or septa at intervals, dividing the hyphal structure into individual cells.
Pseudohyphae: Pseudohyphae are structures formed by elongated yeast cells that remain attached end to end. While they may resemble true hyphae in appearance, pseudohyphae are different in structure and function. True hyphae have septa that separate individual cells, whereas pseudohyphae do not.
Pleiomorphic / Dimorphic / Polymorphic: These terms describe the ability of some fungi to exhibit different growth forms or morphologies, often in response to changes in environmental conditions.
Pleiomorphic: Fungi that are pleiomorphic can exist in various forms. They may alternate between yeast-like cells and filamentous hyphal forms depending on the environmental conditions and nutrient availability.
Dimorphic: Dimorphic fungi have two distinct growth forms, typically transitioning between a yeast phase and a filamentous phase. This dimorphic behavior is often associated with temperature sensitivity, with one form favored at cooler temperatures and the other at warmer temperatures.
Polymorphic: Polymorphic fungi can exhibit multiple forms, which can include yeast-like cells, pseudohyphae, and true hyphae. These transitions can be part of their life cycle or responses to changing environmental conditions.
explain long, thin filaments
Long, thin filaments, called hyphae, make up the fungal mycelium. The mycelium is the vegetative part of a fungus and consists of a network of hyphae. These hyphae are responsible for nutrient absorption, growth, and exploration of the fungus’s environment.
The hyphae are composed of individual, thread-like cells that can be septate (divided by cross-walls or septa) or coenocytic (multinucleate with no septa). They secrete enzymes that break down organic matter, allowing the fungus to absorb nutrients from its surroundings. As the mycelium grows, it can form extensive networks, which enable the fungus to extract nutrients from a larger area, making it an efficient decomposer and symbiotic partner in various ecosystems.
what is the advantage of having a body composed of such thin filaments?
Increased Surface Area: The filamentous structure of hyphae provides a high surface-to-volume ratio. This allows fungi to maximize their contact with the environment, making them efficient at absorbing nutrients, particularly from organic matter, such as decaying plant material and organic debris.
Nutrient Absorption: Fungal hyphae are well-suited for breaking down complex organic molecules, such as cellulose and lignin. The increased surface area and enzymatic activity of the hyphae enable efficient decomposition of these materials, making fungi essential decomposers in ecosystems.
Exploration and Resource Capture: The mycelium’s thin filaments can extend and explore their surroundings, seeking out new sources of nutrients. This exploratory ability allows fungi to efficiently colonize and extract nutrients from their environment.
Symbiotic Relationships: Fungi often form symbiotic relationships with other organisms, such as mycorrhizal associations with plants. In these interactions, fungal hyphae can extend into the plant root systems, increasing the plant’s ability to absorb water and nutrients. This mutualistic partnership is beneficial for both the fungi and the host plant.
Efficient Spore Production: The filamentous structure also provides a suitable environment for the formation and release of spores, which are the reproductive units of fungi. Spores can be produced at the tips of hyphae, allowing for efficient dispersal and reproduction.
Adaptability to Various Environments: Fungal mycelium can grow in a wide range of environments and substrates, thanks to its exploratory and absorptive capabilities. This adaptability makes fungi highly successful in diverse ecosystems, from soil and decaying wood to aquatic environments and even extreme conditions.
Effective Competition: Fungi can compete effectively with other microorganisms for resources by rapidly colonizing and decomposing organic matter. Their thin filamentous structure aids in resource capture and the outcompeting of other organisms.
explain hyphae
Hyphae:
Long Branching Fungal Filament: Hyphae are the long, thread-like, filamentous structures that make up the body of a fungus. They are responsible for nutrient absorption and growth.
Separated into ‘Cells’ by Septae: Some hyphae are divided into individual cells by septa (singular: septum), which are cross-walls. These septa can have pores, allowing for the movement of organelles and cytoplasm between adjacent cells. However, some fungi have coenocytic hyphae, where there are no septa, and the entire hyphal structure is one continuous cell with multiple nuclei.
Grow from the Tip of the Hypha: Hyphae extend and grow predominantly from their tips. This growth is crucial for exploring the environment and obtaining nutrients.
Mycelia Often Used to Describe Interwoven Hyphae: Mycelia is the plural form of mycelium, which is the mass or network of interwoven hyphae. It is the visible part of the fungus that you might see on the surface of decaying matter or in the soil.
Pseudohyphae:
Form Long Chains of Elongated Cells: Pseudohyphae are structures that resemble hyphae in that they are chains of elongated cells. However, unlike true hyphae, pseudohyphae do not have true septa, and their structure is different.
Not True Hyphae: Pseudohyphae are a distinctive growth form associated with certain yeasts, like Saccharomyces cerevisiae. They are characterized by the elongated, connected yeast cells but differ from true hyphae in structure and function. Pseudohyphae are important in the budding process of these yeasts.
explain the major components of the fungal cell wall
Polysaccharides (80%):
β(1,3) Glucan: β(1,3) glucan is a primary component of the fungal cell wall. It forms a structural network, providing strength to the cell wall.
β(1,6) Glucan: β(1,6) glucan is another type of glucan that branches off from the β(1,3) glucan network. It plays a role in cell wall flexibility and as a virulence factor in some pathogenic fungi.
Chitin: Chitin is a long-chain polymer of N-acetylglucosamine (GlcNAc) and is structurally similar to the chitin found in arthropods and insects. It provides rigidity to the cell wall and is a target for antifungal agents.
Mannose: Mannose is a type of sugar and is a component of the mannoproteins, which are glycoproteins found in the cell wall.
Proteins (20%):
Mannoproteins: Mannoproteins are a class of glycoproteins that are highly glycosylated with mannose sugars. They are an important part of the fungal cell wall and have various functions, including adherence to host cells, recognition by the host immune system, and structural support.
Non-Mannoproteins: These are proteins that are not highly glycosylated with mannose sugars. They may be attached to the cell wall via glycosylphosphatidylinositol (GPI) anchors or associated with the glucan and chitin structure of the cell wall.
Hydrophobins: Hydrophobins are a class of cell wall proteins that have hydrophobic properties. They play a role in the attachment of fungal cells to surfaces and in the formation of structures such as spore coatings.
The cell wall’s anionic surface charge, primarily due to the presence of negatively charged mannoproteins, can help reduce cell wall permeability and protect against the entry of harmful substances.
explain the candida species
You’ve provided information about different Candida species, specifically C. albicans, C. glabrata, and C. dubliniensis, highlighting their dimorphism and pathogenicity, especially in the context of HIV patients. Let’s delve into these Candida species in more detail:
Candida albicans:
Dimorphic, opportunistic pathogen: C. albicans is one of the most well-known Candida species and is dimorphic, meaning it can exist in both yeast and filamentous forms. This adaptability allows it to cause a range of infections, especially in individuals with weakened immune systems. C. albicans is responsible for the majority of candidal infections, including oral thrush, vaginal yeast infections, and systemic infections.
Candida glabrata:
Increasingly found in HIV patients: C. glabrata is another species of Candida. It’s notable for its increasing prevalence in individuals with HIV and other immunocompromised conditions. Unlike C. albicans, C. glabrata is considered non-dimorphic, meaning it primarily exists as a yeast form. This species is of concern due to its resistance to some antifungal drugs, making it more challenging to treat.
Candida dubliniensis:
2-3% of Candida infections in HIV patients: C. dubliniensis is related to C. albicans and shares many similarities. It has been found in a small percentage of Candida infections, often in individuals with HIV. However, it is generally considered to be less pathogenic than C. albicans.
Candida species, including C. albicans, C. glabrata, and C. dubliniensis, can cause a range of infections, from superficial to systemic, in individuals with compromised immune systems.
explain candida albicans
Opportunistic Fungal Pathogen: Candida albicans is an opportunistic pathogen, meaning it typically causes infections in individuals with compromised immune systems, such as those with HIV/AIDS, cancer patients, or individuals on immunosuppressive medications.
~60% of Population Carriers: Candida albicans is a common member of the human microbiota, and approximately 60% of the population carries it as part of their natural flora. It resides in various parts of the body, such as the mouth, gastrointestinal tract, and genital areas, without causing illness in healthy individuals.
4th Most Common Bloodstream Infection: Candida albicans is one of the leading causes of bloodstream infections (BSI), ranking as the fourth most common bloodstream infection. These infections can be life-threatening, especially in immunocompromised individuals.
Mortality Rate of BSI = 40%: Bloodstream infections caused by Candida species, including C. albicans, can be associated with a high mortality rate, with approximately 40% of affected individuals succumbing to the infection.
Pleiomorphic, Obligate Diploid: Candida albicans is pleiomorphic, which means it can exhibit various morphologies, including yeast-like, pseudohyphal, and hyphal forms. It is an obligate diploid organism, which means it predominantly exists in a diploid state, but it can undergo a parasexual cycle to generate genetic diversity.
Parasexual Cycle: The parasexual cycle is a genetic recombination mechanism that allows Candida albicans to generate genetic diversity without undergoing traditional sexual reproduction. It contributes to the adaptability and genetic variability of the species.
Superficial or Systemic Infections: Candida albicans can cause a wide range of infections, from superficial infections like oral thrush and vaginal yeast infections to more severe systemic infections, particularly in immunocompromised individuals. Superficial infections are often easily treatable, while systemic infections can be life-threatening.
why might this be a major clinical concern?
Opportunistic Pathogen: Candida albicans is an opportunistic pathogen, meaning it can cause infections in individuals with compromised or weakened immune systems. This includes patients with conditions such as HIV/AIDS, cancer, organ transplantation recipients, and those on immunosuppressive medications. In these vulnerable populations, Candida infections can become life-threatening.
High Prevalence: As you mentioned, around 60% of the population carries Candida albicans as part of their natural flora. This prevalence makes it more likely for the pathogen to cause infections, particularly in hospital settings where patients may have weakened immune systems.
Bloodstream Infections (BSI): Candida species, including C. albicans, are one of the leading causes of bloodstream infections (BSI). These infections are associated with significant morbidity and mortality rates. The presence of Candida in the bloodstream can lead to systemic infections that can involve vital organs and result in severe illness or death.
Resistance to Antifungal Drugs: Some Candida strains have developed resistance to commonly used antifungal drugs, which makes treatment more challenging. Multidrug-resistant Candida strains have emerged, necessitating alternative treatment strategies.
Mortality Rate: Bloodstream infections caused by Candida species have a high mortality rate, with estimates of around 40%. This makes them a serious clinical concern, particularly in critical care settings.
Superficial and Invasive Infections: Candida infections can range from superficial infections like oral thrush or vaginal yeast infections to invasive, life-threatening infections affecting internal organs. The spectrum of infections poses a challenge to healthcare providers in terms of diagnosis and management.
Complications: Candida infections can lead to various complications, including endocarditis, pneumonia, peritonitis, and systemic candidiasis, which can be difficult to treat and manage.
Cost of Treatment: Candida infections require significant healthcare resources for diagnosis, treatment, and prevention. The economic burden of managing Candida infections is substantial.
outline the general steps involved in the pathogenesis of invasive fungal infections, particularly those caused by certain fungal species, including Candida and Aspergillus
Adhesion and Colonization: In the first stage, fungal cells adhere to the host tissue, typically mucosal surfaces or damaged areas. Fungal adhesion is mediated by specific adhesins on the fungal cell surface. Once adhered, the fungal cells can start to colonize the host tissue.
Hyphal Penetration and Invasion: Some fungal species, like Candida, can transition from yeast to filamentous hyphal forms when exposed to specific environmental cues. Hyphae can penetrate host tissues more effectively than yeast cells. They can extend into and invade the underlying tissues, creating a network of filaments.
Vascular Dissemination: The invasive hyphae can extend deeper into the host, potentially reaching the bloodstream or lymphatic system. This stage marks a critical point in the infection’s progression, as it allows the fungus to disseminate throughout the host’s body.
Endothelial Colonization and Penetration: Once in the bloodstream, the invasive fungal hyphae may colonize the endothelial lining of blood vessels. This can damage the endothelial cells, which are important for maintaining vascular integrity. The fungi may then penetrate the blood vessel walls, leading to vascular invasion.
Infections caused by fungi like Candida and Aspergillus can lead to serious medical conditions, including candidemia (Candida infection of the bloodstream) and invasive aspergillosis (Aspergillus infection that invades lung tissues and can spread systemically). The ability of these fungi to adhere, penetrate, and invade host tissues, as well as their potential to disseminate through the vascular system, contributes to the severity of these infections, particularly in immunocompromised individuals.
Managing these invasive fungal infections often requires antifungal medications, such as azoles or echinocandins, as well as addressing the underlying immunosuppressive conditions to improve the host’s ability to fight the infection.
explain how candida albicans is capable of colonizing a wide range of tissues and causing systemic infections due to its adaptability and virulence factors.
Candida albicans is capable of colonizing a wide range of tissues and causing systemic infections due to its adaptability and virulence factors. Here’s how it achieves this:
Metabolic Versatility: Candida albicans has a broad range of metabolic capabilities. It can utilize many different sugars and all amino acids as carbon and energy sources. This metabolic versatility allows it to grow and persist in diverse host environments.
Secreted Proteases: Candida albicans produces enzymes like secreted aspartic proteases (Saps) that can degrade host tissue. These proteases break down host proteins, including those in the extracellular matrix, liberating amino acids and other nutrients for the fungus to use as a food source.
Altered Cell Wall Composition: Candida species can alter their cell wall composition in response to environmental cues. Changes in the cell wall can affect the fungus’s tolerance to various stress conditions and can influence its immunogenicity, enabling it to evade or modulate the host immune response.
Hyphal Formation and Invasion Inducers:
Serum: Exposure to serum components, particularly fetal bovine serum, is a common inducer of hyphal formation in Candida albicans. Serum can mimic the host environment and promote the transition from yeast to hyphal forms.
Temperature: Candida albicans is temperature-responsive. The shift from lower temperatures (e.g., 30°C) to higher temperatures (e.g., 37°C, which is closer to human body temperature) can trigger the yeast-to-hyphal transition.
Contact and Physical Factors: Physical factors such as contact with host tissues, surface roughness, or exposure to certain host molecules can also induce hyphal formation and invasion. The presence of certain signaling molecules or gradients can influence the fungus’s behavior.
Systemic Dissemination: Systemic dissemination can occur in several ways:
Hematogenous Spread: The fungus can enter the bloodstream, either from a local infection site or through the gastrointestinal tract, and then disseminate to distant organs. This is a common route of systemic dissemination.
Lymphatic Spread: Candida may enter the lymphatic system and spread to lymph nodes and other tissues.
