Fungi Flashcards
(42 cards)
1
Q
Fungi Basic Characteristics
A
- Fungi are eukaryotic organisms (e.g., have a nucleus, membrane-bound organelles, linear DNA). Much more similar to animals and plants than to prokaryotic organisms like bacteria and archaea.
- Cell wall made of the following polysaccharides:
- Chitin
- β-glucan
- Mannan
- Cell membrane contains the sterol ergosterol
- Comparable function to cholesterol for mammalian membranes. Helps w/ fluidity and stabilization of the cell membrane.
2
Q
Fungi Uses in Human Society
A
- Edible
- Food production
- Ripening of cheese, leavening of bread
- Alcoholic beverages (e.g., yeast)
- Key to discovery/development of many pharmaceutical products especially antibiotics (e.g., penicillin is produced by the Penicillium species of fungus that kills bacteria). The majority of fungi are not pathogenic to humans, and are more frequently found as plant, insects, and amphibian pathogens.
- They also produce a variety of enzymes important to humans (e.g., Aspergillus produces asparaginase, amylase, and cellulase and Penicillium produces catalase))
3
Q
3 Major Categories of Pathogenic Fungi in Humans
A
- Yeasts
- Candida
- Cryptococcus
2. Molds
- Aspergillus
- Mucor spp
- Fusarium
3. Dimorphics - can switch between yeast/mold
- Histoplasma
- Coccidioides
4
Q
Yeasts
A
- Unicellular organism, 3-10μm (larger than most bacterial pathogens)
- Replicate by asexual budding or binary fission
- Can form pseudohyphae (strings of connecting, budding cells) or true hyphae (elongated cells)
- Examples:
- Candida spp
- Cryptococcal spp
5
Q
Molds
A
- Multicellular
- Comprised of nucleated filaments called hyphae, which together are called mycelium, and spores. To survive in harsh conditions, they upregulate the production of spores and the structures that release the spores are called conidia.
- They are involved in the decomposition of organic matter. When inhaled, they can cause allergies and asthma. They are also opportunistic pathogens in immunocompromised hosts.
- Examples
- Aspergillus spp
- Pseudallescheria spp
6
Q
Fungal Infections
A
- Superficial infections (tend to occur in healthy humans)
- Skin
- Mucous membranes
- Invasive/systemic infections, mainly in immuno-compromised patients. At risk populations include: HIV/AIDS, organ transplant patients, cancer patients undergoing chemotherapy, and extremes of age e.g., very young or very old.
- Pneumonia
- Meningitis
- Blood stream infections
7
Q
Trends in Fungal Infections
A
- The prevalence of invasive fungal infections has been increasing over time but the trends are varied depending on the fungal pathogen.
- Pneumosystis pneumonia and cryptococcosis peaked in the 1990s due to the HIV/AIDS epidemic and has been declining in the US ever since due to better treatments.
- Other fungal pathogens, including Candidemia and Invasive aspergillosis have been increasing.
8
Q
Superficial Fungal Infections
A
- Tinea corporis (Ring worm)
- Tinea pedis (Athlete’s foot)
- Tinea capitis (Cradle cap)
Cause by dermatophytes (e.g., trichophyton and microsporum)
Tend to infect people that are otherwise healthy and usually are easily treated with topical treatments
9
Q
Candida species
A
- Yeast – unicellular form
- 4 to 6 µM
- thin-walled, ovoid cells (blastospores) that reproduce by budding
- Form hyphae and pseudohyphae (important for pathogenesis)
- Culture: forms smooth, creamy white, glistening colonies
- Candida species are the most common cause of opportunistic fungal infections.
10
Q
Candida albicans
A
- A commensal organism
- Found in the GI tract, skin, lungs, and genitourinary tract of most people.
- Frequent cause of mucosal infections
- Oral (thrush)
- Skin (diaper rash)
- Genital (vaginal yeast infection)
- Fungemia (fungus in the blood) in immunocompromised patients
- C. albicans is also the most common species found among Candida
11
Q
Mucosal infections
A
- Caused by endogenous (exists within our own body) Candida that has overgrown
- Risk factors include:
- antibiotic use
- mucosal damage
- immune defect
- As those factors cause imbalance in normal biome of microorganisms.
12
Q
Thrush
A
- Oral candidiasis
- Infection of the mucous membranes of the mouth
- 50% of people are colonized with oral C. albicans
- When host immune system is compromised (e.g., HIV/AIDS) or at extremes of ages (i.e., babies)
- Treated with systemic (oral) or topical anti-fungal drugs
13
Q
Yeast Infections
also known as Candidal vulvovaginitis
A
- Candidal vulvovaginitis (medical term)
- Affects ~75% of women in their lifetimes
- Infection of vaginal mucous membranes
- Caused by overgrowth of the endogenous Candida present on the mucous membranes of the vagina.
- Symptoms: vaginal itching, pain, burning w/ urination, discharge
- Treatment: oral or topical anti-fungals
14
Q
Candida Bloodstream Infections
A
Pathogenesis
- Candidas ability to transition from a yeast cell to a hyphal (i.e. elongated) cell is critical to it becoming an invasive infection because it allows it to penetrate between cells/breach the mucosa and epithelium and into the bloodstream. Once in the bloodstream it can disseminate to other organs and cause infection to:
- Heart valves (endocarditis)
- Eye (endophthalmitis)
- Liver (hepatic abscess)
- Spleen (splenic abscess)
- Biofilms are also critical to Candida’s pathogenicity.
- Biofilms are dense communities of both yeast and hyphal cells. They are sticky and tightly adhere to a surface (usually catheters and any other foreign objects in the body) and are enclosed in an extracellular matrix. They are also metabolically inert.
- Biofilms are highly resistant to antifungals, and catheters must be removed to fully treat.
- Key risk factors:
- Immune defect
- Direct vascular access (indwelling catheter e.g., central venous catheter or hemodialysis catheter). C. albicans is the 3rd leading cause of catheter infections.
- Symptoms: Non-specific symptoms including fever, chills, low BP, fast heart rate. WILL NOT improve with antibiotics. The fact that antibiotics aren’t helping helps clue physicians in that it might be a fungal bloodstream infection.
- 3rd most common hospital-acquired bloodstream infection in US
- Mortality ~50% increased by the fact that physicians first assume a bacterial infection so may go untreated longer.
15
Q
Candida Virulence Mechanisms
A
- Morphogenetic change from yeast to hyphae is critical to its virulence.
- Inability to switch to hyphal cells prevents invasion/infection.
- Hyphal cells produce proteases, which help breakdown host tissue and get through epithelial layer, and also produce adhesin proteins, which help them adhere to surfaces and produce more biofilms.
- Not mentioned in slide, but formation of biofilms is clearly another important virulence factor.
16
Q
Other Candida Species
A
- C. albicans is the most common clinically
- Other species becoming more prevalent and drug resistant, including
- C. glabrata
- C. krusei
- C. auris
17
Q
C. auris
A
- Emerging fungal pathogen in immunocompromised hosts
- First described in Japan in 2009
- Some isolates are pan-resistant (meaning no therapy that is effective against it), and have very high mortality
- Often misidentified b/c it’s so rare
18
Q
Diagnosis of Candida Infections
A
3 methods:
- Direct visualization with staining
- First get the fungal sample either through:
- Tissue biopsy
- Skin/mucosal scraping
- Direct visualization includes 2 components
- Morphology: yeast and/or hyphae form
- Fungal-specific stains:
- Calcofluor white stains chitin in the fungal cell wall, visualized with fluorescence
- First get the fungal sample either through:
- Culture
- Readily cultured in agar and blood cultures
- Antibiotics added to the media to inhibit growth of contaminating bacteria
- Distinguish Candida species by culturing on CHROMagar media, which will show different species as different colors (e.g., C. albicans is green)
- Readily cultured in agar and blood cultures
- Serum biomarkers (most common, at least as initial screen)
- 1,3 B-glucan levels
- Non-specific fungal marker found in cell wall. Doesn’t tell you the type of fungus.
- 1,3 B-glucan levels
19
Q
Candida Summary
A
- Candida is a very common colonizer
- Infections range from superficial to invasive
- Vaginitis, thrush, diaper rashes
- Bloodstream infection (BSI) related to immune status and presence of vascular catheter
- Virulence in BSI depends on hyphal cells
- C. albicans is the most common species, but drug-resistant species (e.g., C. auris) are on the rise.
20
Q
Cryptococcus Summary
A
-
Cryptococcus are ubiquitous environmental yeast (unlike Candida, which tend to be endogenous)
- Decaying trees, soil, pigeon excrement
- Transmitted through inhalation
- Cause pneumonia or spread to CNS through Trojan Horse mechanism
- Highly prevalent in Sub-Saharan Africa
- Risk factors for infection are low CD4 counts, often found in AIDS patients
-
C. gattii newly discovered in the Pacific Northwest but prevalent in numerous sub-tropical areas
- Can affect immunocompetent hosts
21
Q
Cryptococcus Basic Characteristics
A
- Yeast
- Ubiquitous in decaying trees, soil. Environmental yeast. Whereas Candida is endogenous to humans.
- Distinctive cellular capsule
- 2 species known to cause disease in humans
- C. neoformans
- major human and animal pathogen
- C. gattii
- an emerging pathogen
- C. neoformans
22
Q
C. neoformans Transmission
A
- Steps:
- Inhalation of spores. Common sources are soil, decaying trees, and pigeon excrement.
- Once inhaled, C. neoformans are phagocytized by alveolar macrophages. In healthy humans, macrophages usually kill the C. neoformans, but in immunocompromised hosts, some may survive the macrophages and live intracellularly, which is thought to be the source of the latent infection. C. neoformans can also survive extracellularly in the lung alveoli.
- Primary lung infection or establishment of latent infection
- Can spread from lung to CNS, leading to meningitis
- Causes 2 types of infection:
- Pneumonia
- Meningitis
23
Q
C. neoformans Pathogenesis
A
- Symptoms
- All: fever, fatigue
- Pneumonia: dry cough
- Meningitis: headaches, blurred vision, confusion
- C. neoformans alone, or in macrophages can then enter the bloodstream and disseminate to the CNS.
- It is believed that C. neoformans cross the blood-brain barrier via a Trojan horse mechanism, where it crosses within a monocyte/macrophage, and then exits the cell to cause infection.
- It can cause infection in other sites of the body but that doesn’t generally happen unless a large amount of C. neoformans has entered the blood stream and/or the immune system is severely compromised.
24
Q
Epidemiology of C. Neoformans
A
- Main risk factor is a low CD4 count
- Opportunistic infection in HIV/AIDS
- In sub-Saharan Africa 20-80% of AIDS patients have cryptococcal infections
- ~1 million cases of cryptococcal meningitis per year
- High mortality rates in sub-Saharan Africa (50-70%)
25
C. *gattii*
* Diverged from *C. neoformans* 30-40M years ago
* Designated a new species in 2002
* First identified outside a tropical climate on Vancouver Island in 1999
* Now known to be present in many sub-tropical areas
* Outbreaks in the Pacific Northwest of US throughout 2000s
* Similar routes of transmission and clinical presentations to C. neoformans
* Spread to humans from trees. Reservoir thought to be the Douglas fir
* **Unlike C. neoformans, can infect both immuno-compromised an immuno-competent patients.**
* Very rare (~500 human cases) and lower mortality (20%) compared to C. neoformans infection, possibly because also infects immuno-competent patients
26
Cyptococcus Virulence
* Polysaccharide capsule
* Very large/dense
* Protects the cell form phagocytosis by macrophages
* Blocks the ability of antibodies to attach to it and the opsonic effect of complement, which is an immune process for tagging foreign pathogens for elimination by phagocytes. Also called **complement-mediated opsonization**.
27
Cryptococcus Diagnosis
* 3 methods:
* Direct visualization of cerebrospinal fluid (CSF) in the case of meningitis or a sputum sample in the case of pneumonia
* India ink stain
* When India ink is applied, you can see a clear halo develop around the cells, which is due to the polysaccharide capsule excluding the dye - a unique feature of cryptococcus
* Serum/CSF biomarkers (Most common)
* Latex agglutination/ELISAs to detect capsular antigen (see next slide)
* Cultures (rarely used)
* Rarely used b/c because doesn't grow well outside of host
28
Cryptococcal Antigen (CrAg)
* ELISA
* Antibodies recognize and attach to the cryptococcal capsular antigens
* Then a secondary antibody that has a colored/fluorescent label attaches to that antibody-antigen complex , allowing for detection.
* Latex agglutination
* Capsular antigen-specific antibodies attached to latex beads
* Beads clump when mixed with Cryptococcus
29
Aspergillus Epidemiology
* A mold found worldwide in soil, decaying vegetation, air, and water. Similar to Cryptococcus, it tends not to be something that is endogenous to human body.
* Unknown how it infects us. Presumably through inhalation of particles. Exposure is common, every day in air on food.
* Requires immunosuppression to cause illness
* Reduced pulmonary defenses
* Neutropenia - low levels of neutrophils
* Steroids
* Organ and bone marrow transplants
30
*Aspergillus Fumigatus*
* Most common pathologic species
* Most pathogenic (make you sicker)
* Found in soil, decaying vegetation, air, water
* Colonies: Smoky gray or green with a wooly texture
* Microscopy: roughened conidia 2-3.5 µm
* Hyphae are hylanie (lightly pigmented), have septa (have walls), and are usually branched at acute (typically 45 degrees) angles
31
*Aspergillus flavus*
* Causes sinusitis and skin infections
* Found in soil and decaying vegetation
* **Produces aflatoxin ("A.-flavus-toxin"), which can lead to liver damage and liver cancer**
* Colony: Olive to lime green
* Micro: conidia 3-6 µm
32
Aspergillus Clinical Manifestations
* Invasive Pulmonary Aspergillosis
* Most common form of aspergillus infections
* Most commonly seen in patients with neutropenia (low neutrophil counts), usually patients receiving chemotherapy. Neutrophils are non-specific immune cells that help combat infections
* Symptoms: progressive dry cough, dyspnea (difficulty breathing or feeling like you can't catch your breath), pleuritic chest pain (pain when you take a deep breath), chest pain, fever despite broad spectrum antibiotics, and presence of pulmonary infiltrates
* Presents as a “halo” sign on CT due to the pulmonary infiltrates
* Allergic Bronchopulmonary Aspergillosis (ABPA)
* An allergic response to Aspergillus
* Seen in Cystic Fibrosis patients
* Clinical critieria
1. asthma
2. central bronchiectasis
3. immediate skin test reactivity to *Aspergillus*
4. elevated serum IgE
5. elevated serum IgE and/or IgG antibody to *A. fumigatus,*
6. fleeting infiltrates on chest radiography,
7. serum precipitating antibodies to *A. fumigatus,* and
8. peripheral blood eosinophilia
* Fungal Ball, also known as Aspergilloma
* A solid mass of hyphae growing in a previously existing pulmonary cavity
* Patients that present with this tend to have an underlying chronic disease that has led to the creation of a pulmonary cavity
* Relatively rare, and are either asymptomatic or if they have invaded the vasculature within the lungs they can cause fatal hemoptysis
* Treatment: Surgical excision
* Other invasive diseases:
* Sinusitis
* Cerebral Aspergillosis
* Bone Aspergillosis
* Skin Aspergillosis
33
Aspergillus Diagnosis
* Easily cultured from clinical samples (e.g. from sterile site or sputum sample)
* Biopsy
* Elevated fungal markers
* **Galactomannan - specific for Aspergillus\***
* 1,3 β-D-glucan - nonspecific fungal marker
34
Challenge of Treating Fungi
* Evolutionary similarity between fungi and mammals means many antifungal protein targets have structural similarity to human proteins, increasing risk of side effects
* So it's difficult to find drugs that kill fungi without hurting humans.
35
Antifungal Drugs
* Effective antifungals exploit differences between fungi and humans
* Cell membrane targets (fungi have ergosterol; humans have cholesterol): Polyenes and -azoles
* Cell wall targets (fungi have cell walls; humans don't have cell walls): Echinocandins
* KNOW which antifungals target the wall vs the membrane
36
Antifungal Drugs - the -azoles
* Inhibit ergosterol biosynthesis
* leads to membrane stress
* Most widely used antifungals
* Tend to have more mild side effects and are generally available in oral forms. Side effects related to inhibiting the CYP protein system in human cells, which is used to metabolize numerous compounds in the body. Can lead to hormonal imbalances, liver damage, and a lot of drug interactions with other medications
* Examples (more broad spectrum as you go down the list):
* Clotrim**azole** (topical)
* Flucon**azole**
* Voricon**azole**
* Itracon**azole**
* Isavucon**azole**
* Posacon**azole**
37
Antifungal Drugs - the Polyenes
* Bind to fungal ergosterol, causing pores to form in the fungal membrane
* Results in loss of integrity of fungal membrane
* Can cause very severe side effects, including liver and kidney damage, primarily due to the fact that it can sometimes bind to human cholesterol
* They are used as last ditch anti-fungals
* Examples: **Amphotericin B** known as “ampho-terrible” due to bad side effects, **Nystatin** its topical so doesn't lead to the systemic side effects
38
Antifungal Drugs - the Echinocandins
* Only class of anti-fungal to be introduced in past 2 decades.
* **Non-competitive inhibition of 1,3** **β-D-glucan synthase**
* Target cell wall biosynthesis by blocking beta-glucan synthesis. Cell wall doesn't form properly and collapses in on itself.
* Examples (All end in -fungin):
* Anidulafungin
* **Micafungin**
* **Caspofungin**
39
Treatment of Superficial Fungal Infections
* Topical or lower potency oral antifungals
* Usually -azoles or nystatin
40
Treatment of Invasive Fungal Disease
* Often begin with IV and then switch to orals when patient can tolerate it
* Sometimes combination therapy due to risk of resistance to a single class
* Choice of therapy depends on clinical suspicion for specific pathogens
* Can be narrowed based on knowing what the sensitivities are of the specific pathogen species
* Cryptococcal meningitis treatment occurs in phases (induction, maintenance) and can last many months and require multiple agents
41
Antifungal Drug Resistance
**Azoles**
* Resistance common among some species of *Candida* and on the rise due to high use
* Fluconazole-resistant *Candida* (including *albicans*), a serious threat in the US, even more prevalent worldwide. In some places, upwards of 50% of Candida species are resistant to the -azoles
**Polyenes**
* Some fungal species with intrinsic resitance
* *Aspergillus terreus, Fusarium, C. parapsilosis*
* Acquired resistance is rare
**Echinocandins**
* _Intrinsic resistance in_ *_Cryptococcus, including C. neoformans_*
* Resistance can develop quickly in *Candida,* and resistance to Echinocandin is becoming more common in several *Candida* species
42
Summary of Fungi
* Fungal pathogens are diverse, including yeasts and molds
* Common fungal infectious agents are *Candida, Cryptococcus,* and *Aspergillus* species
* Infection range from superficial (skin/mucosa) to invasive/systemic
* Invasive fungal infections predominantly affect immun-compromised hosts
* There are only a few classes of drugs to target fungal pathogens
* Antifungals target cell membrane (polyene and -azoles) and cell wall (echinocandins)
* Many fungi are intrinsically resistant to antifungals, and acquire resistance is an increasing problem
* *C. neoformans* is intrinsically resistant to the echinocandins
