Fungi & Parasites Flashcards
Basic characteristics of fungi
Eukaryotic Unicellular Aerobic Heterotrophic Contain rigid cell wall - chitin + cellulose Cell membrane - contains ergosterol
4 Subclassifications of fungi
Saprobes - live upon dead/decaying organic matter
Symbionts - Live upon another organism, to the mutual advantage of both
Commensals - Live upon another organism with no detriment to the host
Parasites - Live upon another organism, to the detriment of the host
Dermatophytes
Superficial fungi that utilize keratin as a substrate for growth and energy production; grows on skin, hair, and nails
Often geophilic (from soil) or zoophilic (from animals)
Yeast - Basic Characteristics
Unicellular growth form
Reproduce asexually by blastoconidia (budding) or fission
Ex: Crytptococcus neoformans, Candida albicans
Pseudohyphae
Elongated yeast linked together like sausages; they do not have cytoplasmic connections between the compartments
Ex: Candida albicans often forms pseudohyphae
Mold - Basic Characteristics
Filamentous growth form in which fungus reproduces via formation of spores
Ex: Dermatophytes, Aspergillosis
Hyphae
Filamentous elements of mold; hyphae grow by extension and may form a mat that grows as one multicellular colony, called a mycelium
Hyphae may be septate or non-septate and may grow root-like structures (rhizoids)
Dimorphic
Fungi that do not have a fixed morphology but may exist in either yeast or hyphal form; transition is usually triggered by environmental changes, most often temperature
Sporothrix Coccidioides Histoplasmosa Blastomyces Paracoccidioides
(Some Can Have Both Phases)
Conidia
Asexual spores usually borne off of specialized aerial hyphae projections called conidophores
A conidum may be large and multi-nucleated (macroconidia) or small and unicellular (microconidia)
Sporangia
Asexual spores similar to macroconidia except that the endospores are enclosed in a membranous sac supported by a sporangiophore
Chlamydospore
Asexual spores that are round, thick-walled, and resistant to adverse environmental conditoins; may be terminal (formed at the end of a hyphae) or intercalary (formed along and within the hyphae)
Arthrospores
Asexual spores that develop along the hyphae (like chlamydospores) but are more “barrel shaped” - may be consecutive or alternative
Spherules
Large, asexual spores; similar to sporangia but without the connection to the hyphae via the sporangiophore
Blastoconidia
Asexual spores that form from the asymmetrical budding of yeast
Candida albicans
Non-dermatophyte, superficial yeast that utilizes the glucose of interstitial fluids for growth
Pityrosporum
Non-dermatophyte, superficial yeast that utilizes breakdown products of sebum for growth
Direct microscopic examination of fungi
Done by taking a scraping of affected skin or mucosa; KOH or DMSO is added to denature human material and leave the chitinous walls of fungi more visible
Sabouraud’s Agar
Sabouraud’s agar - non-selective, will grow dermatophytes and non-dermatophytes
Periodic Acid Schiff / Diastase (PAS/D)
Highlights the chitinous cell wall magenta/purple; most human tissue doesn’t stain
Mechanisms that parasites use to avoid immune detection
Encapsulation
Intraluminal location
Immunosuppression
Aquisition of host antigens
Definitive vs. Intermediate Host
Definitive Host is the species in which the parasite undergoes sexual replication; intermediate host is any additional species in which asexual replication occurs
Epidemiology of Schistosomiasis
Transmitted to humans by exposure to contaminated fresh water, infecting intestinal or urinary system; causes 200,000 deaths/year, second only to malaria; 85% of affected live in sub-Saharan Africa
Schistosomiasis - Life Cycle
Eggs are shed in the feces or urine (S. haematobium) of infected humans, hash in fresh water, and release miracidia which infects snails
Miracidia develop into sporocysts within the snail host; free swimming cercaria are released and cercaria penetrate the skin of humans to initiate infection
Cercaria lose their tails and develop into schistosomulae in tissues of humans; they gain access to circulation and migrate to the portal blood where they mature into adult worms
Adult worms migrate to mesenteric veins or venous plexus of the bladder, where sexual mating occurs
Eggs that are produced circulate in the venous blood to the liver, and are released into the intestine or urinary bladder; from there, they are released into the environment
Schistosomiasis - Dermatitis
Caused by cercarial invasion into the skin; occurs within 2-3 days of infection
Katayama fever
Serum sickness-like illness occurring acutely (4-8 weeks) after infection with schistosomiasis; presents with lymphadenopathy and hepatosplenomegaly
Co-incident with worm maturation and onset of oviposition
Schistosomiasis - Chronic Stage
Granulomatous and fibrotic changes in the liver / bladder, leading to the formation of inflammatory infiltrate (primarily eosinophils)
Diarrhea, abdominal pain, and ascites in intestinal disease; hematuria and bladder cancer in urinary disease
Malaria - Life Cycle
Sporogony (gamete fusion to form zygotes) occurs in the mosquito and leads to the production of sporozoites in the salivary gland
Infected mosquitos bite humans and inject sporozoites into blood; sporozoites infect liver cells and replicate
After primary replication in the liver, merozoites are released into the blood; they infect erythrocyte and undergo additional asexual replication and release, causing RBC lysis
Which malarial species can establish dormant hepatic phase?
P. vivax
P. ovale
May cause recurrent disease
Malaria - effects on RBCs
Malarial replication cycle within RBCs becomes synchronized; periodic fever is every 3 or 4 days
Anemia results from lysis of RBCs and RBC phagocytosis in the stimulated RE system; hemolysis may be severe, causing hemoglobinuria
Malaria - Physical Exam
Jaundice, hypotension (septic shock), tachycardia, fever, hepatosplenomegaly
Malarial species & preferred cell types
P. falciparum invades erythrocytes of all ages - achieves highest parasitemia and mortality
P. vivax and P. ovale infect young erythrocytes
P. malariae infects old erythrocytes
Genetic diseases that confer resistance to Malaria
Sickle cell disease
Ovalocytosis
Glucose-6-phosphate deficiency
Thalassemia
Praziquantel
Anti-trematode drug
Mycocel / Mycobiotic Agar
Sabouraud’s agar + antibiotic to select for fungi
Corn meal agar
Allows Candida to grow out Chlamydophores
Dermatophyte Test Medium (DTM)
Mycosel + pH indicator that turns the agar from yellow to red in the presence of a dermatophyte
GMS Stain
Highlights fungus jet black against a silver background
Mucicarmine
Highlights the mucoid capsule of Cryptococcus red
Chlorazol E Black Stain
Stains the chitinous fungal cell walls grey/green
India Ink Stain
Used in fungal culture; staine everything except the mucoid capsule surrounding a fungus (i.e. Cryptococcus), highlighting the organism
