DD 02-28-14 09-10am Introduction to Fungal Pathogens - High Flashcards
Basic Tenets of Medical Mycology - Linnaen system, Differences between Fungi & others
Divides living world into 5 kingdoms:
- Plantae
- Animalia
- Fungi (Mycota
- Protista (protozoa)
- Monera (bacteria)
Fungi are wholly unrelated to bacteria or protozoa
- these differences can be exploited therapeutically
- unaffected by antibacterial antibiotics
- instead use antifungal antibiotics
Fungi - basics of life / reproduction
- eukaryotic
- aerobic
- unicellular or filamentous
- heterotrophic
- encased in a rigid cell wall
- May reproduce sexually or asexually (nature of reproduction is used in classification)
Fungi as Eukaryotes
As Eukaryotes, have…
- contain membrane bound organelles (nuclei, mitochondria, Golgi, ER lysosomes)
Fungi as Heterotrophs
As Heterotrophs…
- lack chlorophyll
- NOT photosynthetic (autotrophic) like plants/algae
- obtain necessary organic substrates from surroundings
Fungi - Cell wall
Fungi have rigid cell wall
- like plants
- unlike animals
- contains chitin (as in exoskeleton of insects) and cellulose (as in plant matter)
Fungi - Cell membrane
- inside rigid cell wall
- contains ergosterol
Fungi - Motility
- Only a few specialized fungi (Chytridiomycota) are mobile
- NO medically-relevant species are motile
Subclassifications of Fungal Species
- saprobes
- symbionts
- commensals
- parasites
- This same system of classification applies to bacteria and protozoa.
Saprobes (subclassification of fungal species)
- live upon dead / decaying organic matter
Symbionts (subclassification of fungal species)
- live upon other organisms to the mutual advantage of both
Commensals (subclassification of fungal species)
- live upon another orangism with no detriment to the host
Parasites (subclassification of fungal species)
- live upon another organism w/clear detriment to the host
How fungi are sub-classified
Poses problem when fungus formerly thought to be “imperfect” (w/out a sexual state) is later discovered to be capable of sexual reproduction
–> thus, phyla recognized within mycology are in great flux
Cryptococcus classification & reproduction
Cryptococcus = fungal infection common in HIV/AIDS patients, caused by Cryptococcus neoformans
- 1st thought it was “imperfect” (w/out sexual state)
- Later discovery of sexual state prompted “re-naming” to Filobasidiella neoformans
- Clinicians have not embraced the change and, in the medical realm, the original name persists
Kingdom Fungi/Mycota - 6 Phyla
Chytridiomycota Zygomycota* Ascomycotina * Glomeromycota Basidomycotina* Deuteromyoctina*
*medially important
Zygomycota - reference species
Mucor, Rhizomucor, and Rhizopus
Ascomycotina - reference species
Dermatophytes
Basidomycotina - reference species
Cryptococcus
Deuteromyoctina - reference species
Asexual / imperfect fungi
2 Fungi Growth Forms
Yeasts - unicellular, round & oval
Molds - filamentous
Yeast growth form
- unicellular growth form
Fungus reproduces via…
- budding to form blastoconidia
- dividing in half through fission
Colonies of yeast are usually moist or mucoid in appearance
May form pseudohyphae (different than mold hyphae)
Yeast of medical relevance
Cryptococcus neoformans
Candida albicans
Mold growth form
- filamentous growth form
- Fungus reproduces via formation of spores or conidia
Filamentous elements = hyphae
Molds of medical relevance
common dermatophytes
Aspergillosis
Hyphae of Mold forms
- en mass = mycelium
- often branched
- grow by apical extension
- may be septate (w/internal divisions, like a cattail stalkca) or non-septate
Septa of Hyphae
Septa divide hyphae into compartment but do NOT strictly divide the fungus into “cells”
-cytoplasm or even organells may flow between compartments via pores w/in the septa
Pseudohyphae (vs. hyphae)
- formed by some yeasts (not the true hyphae of molds)
- simply elongated yest linked together like sausages
- typically demonstrate some degree of rounding
- DO NOT have cytoplasmic connections between compartments
- Candida albicans is an organism that often forms pseudohyphae
Dimorphic fungi
- Fungi that do not have fixed morphology but may exist in YEAST OR HYPHAL form
- Typically transition triggered by an environmental change (atmosphere, temperature, food supply)
*Thermal dimorphism refers to dimorphism that is dictated by temperature (most common context of “dimorphism”)
Histoplasmosis & Dimorphism
- disease caused by Histoplasmosa capsulatum
= thermally dimorphic fungi often found in bird/bat feces - at ambient temp in fecal material, exists mold and saprophyte
- if inhaled & at body temp, can transform into parasitic yeast of macrophages
Medically relevant thermally dimorphic organisms (w/pneumonic)
Some Can Have Both Phases S- Sporothrix schenckii C- Coccidioides immitis H- Histoplasmosa capsulatum B- Blastomyces dermatitidis P- Paracoccidioides brasiliensis
Hyphal Characteristics - Rhizoids
= specialized form of hyphal elements that grow like roots from larger hyphae
Medically-relevant fungi that form Rhizoids
- seen only in a few medically-relevant fungi (e.g. Rhizopus)
Hyphal Characteristics - Septa
- Septate hyphae are those that demonstrate complete cell walls that subdivide the hyphae into compartments
- These subdivisions are NOT equivalent to “cells,” as microscopic pores in septae allow for free exchange of cytoplasm & nuclei between compartments.
Medically-relevant SEPTATE fungi
Aspergillus fumigatus
Dermatophytes
Hyphal Characteristics - Non-septate
- Non-septate hyphae have no cell walls compartmentalizing the hyphae
- In truth, sometime very sparse or incomplete septae may be present, but fungus is still considered to be aseptate
Medically-relevant NON-SEPTATE fungi
Rhizopus
Spores & Fungal Reproduction
- Sexual & asexual fungi may reproduce via spore formation
- Yet at an introductory level, the spores useful to identify & classify medically-relevant fungi that are asexual spores
Examples of important types of asexual spores
Conidia Sporangia Chlamydospores Arthrospores Spherules Blastoconidia Sclerotic bodies
Conidia
- type of asexual spore of medical relevance
- usually borne off of specialized aerial hyphae (upward-projecting hyphae) called conidophores
- may macroconidia or microconidia
Microconidia vs. Macroconidia
Conidia may be…
- large & multinucleated (macroconidia)
- small & unicellular (microconidia)
(see pics in notes)
- Some fungal species may produce both macroconidia & microconidia
- Generally, macroconidia of certain shapes or with certain features are more useful in speciation than microconidia
Sporangia
– type of asexual spore of medical relevance
Similar to macroconidia
- BUT asexual spores (endospores) are enclosed in membranous sac
- sac that breaks & entire structure is borne by sporangiphore
(see pic in notes)
- Shapes / colors of sporangium may be useful in speciation
Chlamydospores
- type of asexual spore of medical relevance
- thick-walled, round spores
- highly resistant to adverse environmental conditions
Further classified based upon where they form along hyphae
- Terminal = form at ends of hyphae
- Intercalary = form along & w/in hyphae
(see pic in notes)
Arthrospores
– type of asexual spore of medical relevance
Like chlamydospores, develop along hyphae
- BUT generally are more numerous & elongated
- often “barrel-shaped”
Classical example of barrel-shaped arthrospores:
- Mycelial phase of Coccidioides immitis
(see pic in notes)
It is inhalation of these arthospores from the environment that yields the disease.
Spherules
- type of asexual spore of medical relevance
- large, asexual spores that develop during the yeast phase of some organisms growth
EX: Yeast form of dimorphic Coccidioides immitis forms spherules in tissue that are filled w/ endospores
(see pic in notes)
Blastoconidia
– type of asexual spore of medical relevance
- yeasts that bud asymmetrically form these
(see pic in notes)
Sclerotic bodies
- type of asexual spore of medical relevance
- aka Medlar bodies
- thick-walled, environmentally protective forms of yeast
- produced by some medically-relevant fungi that reproduce by fission
Thallus
another name for a fungal colony growing on a culture dish
Obverse vs. Converse
Obverse = top side of growing thallus Converse = flip-side (seen by looking through the media, and also known as the reverse)
Color of obverse/converse, or other characteristics (“mucoid”, “tangled”, “cerebriform”) of growing thallus may be used in speciation
Geophilic
- subclassifcation of Dermatophytes
= a species of fungus found principally in soil - can be transmitted to humans (but often less clinically relevant)
Anthrophilic
- subclassifcation of Dermatophytes
= a species of fungus found principally in humans - typically less inflammatory
Zoophilic
- subclassifcation of Dermatophytes
= a species of fungus found principally in animals - can be transmitted to humans
- typically more inflammatory
Dematiaceous
- subclassifcation of Dermatophytes
= a fungus which produces its own pigment, usually melanin
Number of fungi implicated in human disease
250,000 know forms of fungus
100-150 implicated in disease of humans
Only a dozen cause regularly-seen fungal infections
Superficial fungal infections - Examples
- Tinea (dermatophytes, dermatophytosis – “ringworm”)
- Candida (candidiasis, thrush, “[vaginal] yeast infections”)
- Pityrosporum (pityrosporum versicolor/tinea versicolor)
Deep fungal infections/Systemic mycoses - Examples
Sporotrichosis Cryptococcosis Coccidioidomycosis North American Blastomycosis Histoplasmosis Paracoccidiodomycosis Lobomycosis Mucormycosis
3 most common superficial fungal infections
dermatophytes
Candida
Pityrosporum
*usually cutaneously limited diseases of the skin & mucosa (except in immunocompromised & critically-ill)
Dermatophytes
- = a group of hyphal fungi that utilizes keratin as a substrate for growth
- a rather unique property of this group of fungi
- limits areas of body where organism will be found (skin, hair, nails)
- predicts its behavior
Candida
= a non-dermatophyte yeast that prefers the glucose of interstitial fluids for growth
- Candida also causes superficial fungal infections, but its need for glucose explains subtle differences in its body distribution & behavior
Pityrosporum
= another non-dermatophyte yeast that prefers breakdown products of sebum (“skin oil”) for growth
- this substrate dependence explains differences in its body distribution & behavior
Characteristic of Deep mycoses
= able to cause systemic infection
- Many begin in the lungs (through inhalation of arthrospores and the like)
- Later, disseminate widely to variety of tissues
- Often the extent of disease depends upon immunological status of the host
4 means to Dx Fungal infections
- empirical
- direct examination
- culture
- tissue
Other advanced techiques to Dx Fungal infections
- serological
- PCR
- used for certain infections or in specific circumstances
Autofluorescence under black-light
- “Wood’s Lamp,” 365 nm
- some fungal species autofluoresce under black-light
- Before, an easy way to highlight some fungal species that cause tinea capitis (dermatophyte infection of the scalp)
- Nowadays 95% of all tinea capitis is caused by an organism that DOES NOT fluoresce, limiting the utility of this technique today
Direct Microscopic Examination of Fungal Infection
- Often it is most appropriate to take scraping of epithelium from affected skin or mucosa
- Add drop of KOH (5-20%) or surfactant DMSO, to denatures human material & leaves chitinous walls of fungi more visible
- Allows for direct observation for presence of yeast & hyphae indicative of superficial fungal infection
- Even in some deeper fungal infections, elimination of yeast forms through skin allows Dx to be confirmed using KOH technique
Variation of KOH/DMSO techniques for Direct Microscopic Examination of Fungal Infection
- May add chlorazol E black stain to KOH or DMSO
- Stains chitinous fungal cell walls a grey-green color
- Makes direct microscopic examination easier for many inexperienced practitioners
India Ink for fungal infections
- Cryptococcosis is a systemic infection prevalent in HIV/AIDS patients.
- This yeast is surrounded by a thick mucoid capsule
- If cryptococcosis is suspected, CSF from lumbar puncture may be IDed by mixing it w/India ink
- -> stains everything EXCEPT mucoid capsule, thereby highlighting the organism
Gram stain for Fungal Infections
A simple Gram stain not only highlights bacteria on clinical material, but also highlights the yeast, Candida albicans
Culturing fungal infections
- Just as for bacterial cultures
- Clinical material (skin, hair, nails, soft tissue, etc.) may be cultured for growth of fungal organisms
- Chief advantage of culture is that it allows for direct speciation of infecting organism, although in many clinical situations the sensitivity of culture may be less than direct examination or histological examination (via biopsy)
Media for Fungal Cultures
Cultures may be performed using a variety of different media including:
- Sabouraud’s agar
- Mycosel®/Mycobiotic® agar
- Dermatophyte Test Medium®
Special agars may be used for specific purposes or to grow fastidious organisms
- EX: cornmeal agar induces Candida albicans to produce chlamydospores
Sabouraud’s agar for Fungal Culture
= the most sensitive culture media, as it will allow for growth of both dermatophytes & non-dermatophytes
- Unfortunately, unrelated contaminants also grow well, and may obscure the primary pathogen
Mycosel/Mycobiotic agar for Fungal Culture
= fungal media impregnated w/ chloramphenicol & chlorheximide to inhibit growth of bacteria & saprobes
Dermatophyte Test Medium (DTM) for Fungal Culture
= essentially mycosel-like media w/ a pH indicator that turns agar from red in presence of a dermatophyte
Pros & Cons of Culturing Fungus to Dx Infection
PROS:
- Cheap
- Allows for speciation
- Does not require much professional time to collect a sample
CONS:
- Results are often delayed (2-4 weeks)
- Less sensitive than KOH in many settings
Lactophenol cotton blue stain
- often used on fungal specimens taken from colonies grown via culture
- not a stain used in any clinical setting outside of the examination of cultured fungi
Histology to Dx Fungal Infection
= biopsy of skin, hair, and nails
= a sensitive & specific diagnostic technique for fungal infection
Pros & Cons of Histology of a Biopsy to Dx Fungal Infection
PROS:
- rapidity of diagnosis (2-3 days, more rapid than culture but less than direct examination)
- high degree of sensitivity, esp. when special stains are performed
CONS:
- higher cost
- invasive nature of the procedure
- procurement of representative sample is essential to the sensitivity of the test
- some organisms are not usually well demonstrated in histological sections & culture is there preferred
H&E vs. Special Stains for Fungus
- Many fungi are apparent w/ H&E staining alone
- Several special stains may make fungus more apparent during histologic examination
Special Stains for fungus - examples
- PAS/D (Periodic acid Schiff followed by Diastase)
- GMS – Gomori methenamine silver
- Mucucarmine (mucin stain)
- Calcoflour white stain
- Fluorescent antibody stains
PAS/D – Periodic acid Schiff followed by diastase
- special fungal stain
- highlights chitinous cell wall of fungi, yielding a magenta/purple color
- Most human tissue does not stain w/ PAS-D
- Glycogen rich material will stain with PAS, but this staining is lost w/ diastase digestion, while fungal chitin is not affected by diastase
GMS – Gomori methenamine silver
- special fungal stain
- utilizes silver to highlight fungus a jet black color
Mucucarmine (mucin stain)
- special fungal stain
- useful to highlight mucoid capsule of Cryptococcus a red color
Calcoflour white stain
- special fungal stain
- may be used for direct examination of most specimens using fluorescent microscopy
- Cell walls of fungi bind stain & fluoresce blue-white or apple-green depending upon fluorescent light source used on the fluorescent microscope
Skin Testing for fungal infections
- works using principles similar to that of TB screening
- Killed antigenic material, such as histoplasmin derived from Histoplasmosa capsulatum, is placed under the skin
- A delayed-type hypersensitivity response is checked for in 48-72 hours
- Just like a TB screen, a positive reaction is only indicative of prior exposure & not necessarily active disease
Serology for fungal infections
For some types of fungal infections, blood tests may be useful.
EX: Pts w/ disseminated cryptococcosis
- serological test of cryptococcal antigen is positive in 75% of cases
PCR for fungal infections
- becoming increasingly useful for speciation of some types of fungal infections
- has not yet reached wide commercial availability
Selective toxicity in fungal infections
- Just as antibacterial agents seek to exploit physiological differences btwn human cells & bacteria, so to do antifungal agents seek to exploit differences in fungal cells
- Ergosterol is a component of fungal cell membranes, serving the same function that cholesterol serves in animal cells
- Presence of ergosterol in fungal cell membranes & its absence in animal cell membranes, makes it a useful target for antifungal drugs
Polyenes - action
= a circular molecule consisting of a hydrophobic & hydrophilic region, creating an amphoteric molecule
- bind w/ ergosterol in fungal cell membrane
- react w/ animal sterols to much lesser extent
- considered FUNGICIDAL
Polyenes - examples
Amphotericin B
Nystatin
Amphotericin B
- binds to ergosterol, creating pore in fungal membrane
- -> causes ions & other molecules to leak out
- Many side-effects including fever, seizures, & kidney damage
- Newer liposomal formulations have made it better tolerated, but w/ greater financial cost
- Still, it remains a common medication for life-threatening fungal infections
Nystatin
= another polyene used topically for Candida infections
- not absorbed when given PO & too toxic for IV
Imidazole & Triazole antifungals - actions
Inhibit the enzyme, 14α-demethylase
- this enzyme converts lanosterol to ergosterol
- it is required in fungal cell membrane synthesis
These drugs block steroid synthesis in humans too, but to a much lesser extent.
Generally considered to be FUNGISTATIC
Drug-Drug Interactions of Imidazole & Triazole antifungals
- often interfere w/ CYP450 enzymes
- -> leads to potentially fatal drug interactions w/ certain meds
- Topical imidazoles are not absorbed to any significant extent & are considered free of worry WRT drug-drug interactions (why they are available OTC)
Triazoles vs. Imidazoles
Triazoles are similar to imidazoles but simply newer & for the most part better tolerated
Examples of Topical Imidazoles
Clotrimazole
Miconazole
Econazole
Sertaconazole
Examples of Oral Imidazoles
Ketoconazole
Examples of Oral Trianzoles
Itraconazole
Fluconazole
Voriconazole
Posaconazole
Allylamines & benzylamines antifungals - action
Inhibit enzyme squalene epoxidase
- another enzyme required for ergosterol synthesis
–> leads to accumulation of squalene w/in fungal cell = directly toxic
= FUNGICIDAL
Terbinafine
= most common allylamine
- available in topical & oral forms
Reports of unmasking of lupus-like conditions w/ oral terbinafine
= poor choice for those w/ pre-existing Hx of connective tissue disease
Butenafine
= a topical benzylamine, related to terbinafine
Echinocandins - action
= the newest class of antifungal agents
- inhibit synthesis of glucan in cell walls of some fungi
- probably via inhibition of enzyme 1,3-β glucan synthase
Pros & Cons of Echinocandins
PROS:
- low toxicity
- rapid fungicidal activity against most isolates of Candida
- favorable kinetics that allow for once daily dosing
- also fungicidal against Aspergillus
CONS:
- not useful against many other forms of fungus
Griseofulvin
- inhibits fungal cell mitosis by disrupting mitotic spindle formation (critical step in cellular division)
- well tolerated & has been around for decades
- still used in children with tinea capitis b/c of its long record of safety
- in many other indications, replaced by newer triazole medications
-FUNGISTATIC
Flucytosine
- small molecule transported into fungal cells by specific enzyme, cytosine permease
- then converted in cytoplasm by cytosine deaminase to 5-fluorouracil (5-FU)
- 5-FU is a pyrimidine anti-metabolite that interrupts DNA synthesis, inhibiting fungal growth
- FUNGISTATIC
Ciclopirox olamine
- does NOT affect ergosterol metabolism
- thought to chelate polyvalent metal cations, such as Fe3+ & Al3+
- -> leads to inhibition of many different fungal enzymes, including cytochromes
- -> inhibits important cellular activities such as mitochondrial e- transport & energy production
- FUNGISTATIC and/or FUNGICIDAL activity in vitro against a broad spectrum of fungal organisms
