Fungi & Parasites Lessons | Microbiology

🍄 Fungi & Parasites

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🍄 Fungi & Parasites

Candida: normal flora that overgrows when defenses drop — "thrush, diaper rash, or systemic if immunocompromised"

Candida albicans — opportunistic yeast; normal oral/GI/vaginal flora that overgrows with antibiotics, immunosuppression, or diabetes

The most common opportunistic fungal pathogen — already living in you

Pseudohyphae on microscopy. Oral thrush (white plaques). Vulvovaginal: cottage cheese discharge, pruritus. Invasive candidemia: ICU patients with central lines — risk with broad-spectrum antibiotics, TPN, immunosuppression. Treat: fluconazole (mild-moderate), echinocandin (invasive/azole-resistant).

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🃏 🍄 Fungi & Parasites

Candida — when does it cause disease?

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Candida: normal flora that overgrows when defenses drop — "thrush, diaper rash, or systemic if immunocompromised"

Candida albicans — opportunistic yeast; normal oral/GI/vaginal flora that overgrows with antibiotics, immunosuppression, or diabetes

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🍄 Fungi · Molds

Aspergillus: "Septate hyphae at 45°." Invasive in neutropenic. Aspergilloma = fungus ball in old TB cavities.

Aspergillus fumigatus — ubiquitous mold; invasive disease in neutropenic/immunocompromised patients; classic halo sign on CT chest

The mold that kills immunocompromised patients — three clinical presentations

ABPA (allergic bronchopulmonary aspergillosis): asthma/CF patients — IgE-mediated reaction. Aspergilloma: fungus ball colonizes pre-existing cavity (TB cavities, sarcoidosis). Invasive aspergillosis: neutropenic patients (AML, transplant) → angioinvasion → hemorrhagic infarcts — halo sign on CT. Treat invasive: voriconazole (first-line), amphotericin B.

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🃏 🍄 Fungi · Molds

Aspergillus — key features?

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🃏 Answer

Aspergillus: "Septate hyphae at 45°." Invasive in neutropenic. Aspergilloma = fungus ball in old TB cavities.

Aspergillus fumigatus — ubiquitous mold; invasive disease in neutropenic/immunocompromised patients; classic halo sign on CT chest

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🍄 Fungi · AIDS

Cryptococcus: "Soap bubbles in brain." India ink stain shows capsule. Meningitis in AIDS (CD4 <100).

Cryptococcus neoformans — encapsulated yeast from pigeon droppings; causes meningitis in AIDS patients with CD4 <100

The encapsulated yeast causing the most common AIDS-related meningitis

Found in pigeon droppings and soil. Large polysaccharide capsule = antiphagocytic. India ink: clear halo around yeast. Latex agglutination: detects capsular antigen in CSF (most sensitive). CSF: India ink+, very high opening pressure ("soap bubbles" on MRI). Treat: amphotericin B + flucytosine (induction 2 weeks), then fluconazole maintenance (lifelong in AIDS).

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🃏 🍄 Fungi · AIDS

Cryptococcus — the classic brain finding?

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🃏 Answer

Cryptococcus: "Soap bubbles in brain." India ink stain shows capsule. Meningitis in AIDS (CD4 <100).

Cryptococcus neoformans — encapsulated yeast from pigeon droppings; causes meningitis in AIDS patients with CD4 <100

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🍄 Fungi · AIDS

PCP: AIDS-defining illness at CD4 <200. "Bat-wing infiltrates." TMP-SMX treats AND prevents.

Pneumocystis jirovecii Pneumonia — now classified as fungus; AIDS-defining illness when CD4 <200; bilateral interstitial infiltrates

The AIDS-defining lung infection — bilateral infiltrates + high LDH

Cannot be cultured — diagnose by BAL + GMS (Gomori methenamine silver) stain showing cysts. Presentation: progressive dyspnea, dry cough, fever, hypoxia. CXR: bilateral interstitial infiltrates (bat wings). High LDH. Treat: TMP-SMX high dose × 21 days. Add steroids if PaO₂ <70 mmHg. Prophylaxis with TMP-SMX when CD4 <200.

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🃏 🍄 Fungi · AIDS

PCP — CD4 threshold, X-ray, treatment?

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PCP: AIDS-defining illness at CD4 <200. "Bat-wing infiltrates." TMP-SMX treats AND prevents.

Pneumocystis jirovecii Pneumonia — now classified as fungus; AIDS-defining illness when CD4 <200; bilateral interstitial infiltrates

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🍄 Parasites · Protozoa

Malaria: mosquito → sporozoites → liver → merozoites → RBCs → rupture → fever spikes

Plasmodium Life Cycle — Anopheles mosquito injects sporozoites → exoerythrocytic liver phase → erythrocytic RBC phase → symptoms on RBC rupture

The two-host life cycle that explains malaria symptoms and drug targets

P. falciparum: most severe — cytoadherence, cerebral malaria, no hypnozoites. P. vivax/ovale: tertian fever (48 hr), hypnozoites in liver → relapse — add primaquine. P. malariae: quartan (72 hr). Blood stage treatment: chloroquine (sensitive), artemisinin (resistant). Cerebral malaria: IV artesunate.

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🃏 🍄 Parasites · Protozoa

Malaria — the life cycle in humans?

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Malaria: mosquito → sporozoites → liver → merozoites → RBCs → rupture → fever spikes

Plasmodium Life Cycle — Anopheles mosquito injects sporozoites → exoerythrocytic liver phase → erythrocytic RBC phase → symptoms on RBC rupture

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🍄 Parasites · Helminths

NTC: Nematodes (roundworms) · Trematodes (flukes) · Cestodes (tapeworms)

Helminth Classification — N=Nematodes (roundworms) · T=Trematodes (flatworms/flukes) · C=Cestodes (tapeworms)

Three major worm categories — each with distinctive biology and treatment

Nematodes: Ascaris (Loeffler syndrome), hookworm (iron-deficiency anemia), Strongyloides (autoinfection in immunocompromised), pinworm (scotch tape test), Trichinella (undercooked pork). Trematodes: Schistosoma (freshwater snail, hematuria or portal hypertension), liver flukes. Cestodes: T. solium (neurocysticercosis), Echinococcus (hydatid cysts). Treat: albendazole/mebendazole (most); praziquantel (schistosomiasis, tapeworms).

N — Nematodes

Roundworms — Ascaris, hookworm, Strongyloides, pinworm, Trichinella; treat with albendazole

T — Trematodes

Flukes — Schistosoma (water), liver flukes (undercooked fish); treat with praziquantel

C — Cestodes

Tapeworms — T. solium (neurocysticercosis), Echinococcus (hydatid cysts); praziquantel or surgery

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🃏 🍄 Parasites · Helminths

NTC

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NTC: Nematodes (roundworms) · Trematodes (flukes) · Cestodes (tapeworms)

N — NematodesRoundworms — Ascaris, hookworm, Strongyloides, pinworm, Trichinella; treat with albendazole

T — TrematodesFlukes — Schistosoma (water), liver flukes (undercooked fish); treat with praziquantel

C — CestodesTapeworms — T. solium (neurocysticercosis), Echinococcus (hydatid cysts); praziquantel or surgery

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🍄 Fungi · Skin

Tinea = dermatophyte ringworm. Named by body location: capitis (scalp) · pedis (foot) · cruris (groin) · unguium (nails)

Dermatophyte Infections — three genera (Trichophyton, Microsporum, Epidermophyton) cause superficial fungal infections of keratinized tissue

Same organisms, different location names — know the treatment by site

KOH prep: hyphae visible on scraping. Tinea pedis (athlete's foot): most common — interdigital. Tinea unguium (onychomycosis): yellow thickened nails — treat with oral terbinafine. Tinea capitis in children: treat systemically (griseofulvin or terbinafine — topicals don't penetrate hair follicle). Tinea corporis/cruris: topical clotrimazole/miconazole sufficient.

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🃏 🍄 Fungi · Skin

Tinea — how are the types named?

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Tinea = dermatophyte ringworm. Named by body location: capitis (scalp) · pedis (foot) · cruris (groin) · unguium (nails)

Dermatophyte Infections — three genera (Trichophyton, Microsporum, Epidermophyton) cause superficial fungal infections of keratinized tissue

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🍄 Parasites · Protozoa

Toxoplasma: "cats + pregnant women + AIDS." Ring-enhancing brain lesions. Reactivation at CD4 <100.

Toxoplasma gondii — definitive host is cats (oocysts in feces); causes congenital disease and CNS reactivation in AIDS

Harmless in healthy adults — devastating in pregnancy and AIDS

Transmission: cat feces (oocysts) or undercooked meat (tissue cysts). Congenital: chorioretinitis, hydrocephalus, intracranial calcifications ("C-H-I-C"). AIDS reactivation (CD4 <100): encephalitis, ring-enhancing lesions on MRI. Treat: pyrimethamine + sulfadiazine + leucovorin. Prophylaxis: TMP-SMX when CD4 <100.

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🃏 🍄 Parasites · Protozoa

Toxoplasma — the source, and who is at risk?

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Toxoplasma: "cats + pregnant women + AIDS." Ring-enhancing brain lesions. Reactivation at CD4 <100.

Toxoplasma gondii — definitive host is cats (oocysts in feces); causes congenital disease and CNS reactivation in AIDS

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🍄 Parasites · Protozoa

Giardia: "backpacker's diarrhea." Trophozoite = two nuclei = "owl eyes." Foul fatty stool = malabsorption.

Giardia lamblia — fecal-oral via contaminated water; trophozoites attach to small intestine mucosa → malabsorption; treat with metronidazole

Most common intestinal parasitic infection in the US — foul-smelling fatty diarrhea

Cysts in contaminated water (streams, lakes — resistant to chlorine). Trophozoites attach to small intestine via ventral disc → malabsorption. Symptoms: foul-smelling fatty diarrhea (steatorrhea), bloating, no blood/mucus. Diagnosis: stool O&P, antigen test. Treat: metronidazole or tinidazole.

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🃏 🍄 Parasites · Protozoa

Giardia — the classic nickname and source?

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Giardia: "backpacker's diarrhea." Trophozoite = two nuclei = "owl eyes." Foul fatty stool = malabsorption.

Giardia lamblia — fecal-oral via contaminated water; trophozoites attach to small intestine mucosa → malabsorption; treat with metronidazole

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🍄 Fungi · Dimorphic

Dimorphic fungi: "Mold in cold, yeast in heat." Temperature-dependent — mold at room temp, yeast at body temp.

Dimorphic Fungi — grow as mold at room temperature (25°C) and switch to yeast at body temperature (37°C)

Fungi that change form to evade immune defenses at body temperature

Histoplasma capsulatum: Ohio/Mississippi river valleys, bird/bat droppings. Blastomyces: North America, Great Lakes, skin lesions + lung. Coccidioides: Southwest US (California, Arizona), spherules. Paracoccidioides: South America. All: inhale spores → convert to yeast at 37°C → pneumonia. Treat: itraconazole (mild-moderate), amphotericin B (severe/CNS).

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🃏 🍄 Fungi · Dimorphic

Dimorphic fungi — which form at which temperature?

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Dimorphic fungi: "Mold in cold, yeast in heat." Temperature-dependent — mold at room temp, yeast at body temp.

Dimorphic Fungi — grow as mold at room temperature (25°C) and switch to yeast at body temperature (37°C)

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🍄 Parasites · Protozoa

Entamoeba histolytica: "flask-shaped ulcers" + liver abscess. Bloody diarrhea + RUQ pain. Anchovy paste pus.

Entamoeba histolytica — fecal-oral; invades colonic mucosa causing bloody diarrhea; can disseminate to liver (amoebic abscess)

The amoeba that causes bloody diarrhea and liver abscesses

Fecal-oral, contaminated water, endemic in developing countries. Trophozoites invade colonic mucosa → flask-shaped ulcers → bloody diarrhea (dysentery). Amoebic liver abscess: RUQ pain, fever, no jaundice, "anchovy paste" pus (sterile). Diagnosis: serology + stool O&P. Treat: metronidazole + luminal agent (paromomycin or iodoquinol).

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🃏 🍄 Parasites · Protozoa

Entamoeba histolytica — key findings?

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Entamoeba histolytica: "flask-shaped ulcers" + liver abscess. Bloody diarrhea + RUQ pain. Anchovy paste pus.

Entamoeba histolytica — fecal-oral; invades colonic mucosa causing bloody diarrhea; can disseminate to liver (amoebic abscess)

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🍄 Parasites · Ectoparasites

Scabies: "burrows" between fingers. Sarcoptes scabiei mite. Intense nocturnal pruritus. Treat entire household.

Scabies — Sarcoptes scabiei mite burrows in stratum corneum; Type IV hypersensitivity reaction causes intense itching; treat with permethrin

The mite that causes intense itching — and why you must treat the whole household

Sarcoptes scabiei burrows in stratum corneum — especially between fingers, wrists, genitals. Intense pruritus (worse at night) — delayed-type hypersensitivity to mite proteins. Diagnosis: scraping + mineral oil → mites/eggs under microscope. Treat: topical permethrin (entire body from neck down); treat all household contacts simultaneously; wash all bedding.

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🃏 🍄 Parasites · Ectoparasites

Scabies — signs and treatment?

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Scabies: "burrows" between fingers. Sarcoptes scabiei mite. Intense nocturnal pruritus. Treat entire household.

Scabies — Sarcoptes scabiei mite burrows in stratum corneum; Type IV hypersensitivity reaction causes intense itching; treat with permethrin

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