Microbiology
High-yield MBBS Microbiology question bank — bacteriology, immunology, virology, mycology and parasitology, with laboratory diagnosis, at Ananthanarayan & Paniker / Jawetz depth.
12chapters144questions82High-Yield
Definition
Fungi are eukaryotic organisms with a chitinous cell wall and ergosterol-containing membrane; the diseases they cause are called mycoses.
General Features
- Eukaryotic; cell wall of chitin, membrane contains ergosterol
- Yeasts — unicellular, reproduce by budding
- Moulds — filamentous hyphae forming mycelium
- Dimorphic — mould in environment, yeast in tissue
- Culture: Sabouraud’s dextrose agar at 25°C and 37°C
Classification of Mycoses
- Superficial — outermost skin/hair (pityriasis versicolor)
- Cutaneous — skin, hair, nails (dermatophytes)
- Subcutaneous — mycetoma, sporotrichosis
- Systemic (deep) — histoplasmosis, blastomycosis
- Opportunistic — candidiasis, aspergillosis, mucormycosis
Mycoses are classified by the depth of tissue they invade. Type Example Cutaneous Tinea Subcutaneous Mycetoma Opportunistic Candidiasis Applied
- Diagnosis: KOH mount, culture, histopathology
- Opportunistic mycoses are rising with HIV, diabetes and steroid use
🔑KEY POINTS TO REMEMBER- Fungi are eukaryotes with chitin walls and ergosterol membranes.
- Forms: yeasts, moulds, dimorphic fungi; grown on Sabouraud’s agar.
- Mycoses: superficial, cutaneous, subcutaneous, systemic, opportunistic.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Dermatophytoses are cutaneous mycoses caused by keratinophilic fungi infecting skin, hair and nails.
Causative Genera
- Trichophyton — skin, hair, nails
- Microsporum — skin and hair
- Epidermophyton — skin and nails (not hair)
- Digest keratin; do not invade living tissue
Clinical Types (by site)
- Tinea capitis — scalp; tinea corporis — body (ring-shaped, central clearing)
- Tinea cruris — groin; tinea pedis — athlete’s foot
- Tinea unguium (onychomycosis) — nails
- Itchy, scaly, annular lesions with an active advancing edge
Outward growth with central healing gives the classic ring appearance. Test Finding KOH mount Branching septate hyphae Wood’s lamp Fluorescence (Microsporum) Culture Sabouraud’s agar Applied
- Topical azoles/terbinafine; oral therapy for scalp and nail infection
- Keep areas dry; treat family members and pets
🔑KEY POINTS TO REMEMBER- Dermatophytes: Trichophyton, Microsporum, Epidermophyton — keratinophilic.
- Annular itchy lesions with central clearing; named by body site.
- Diagnosed by KOH mount; nail and scalp need oral antifungals.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Candidiasis is an opportunistic mycosis caused mainly by Candida albicans, a normal commensal yeast.
Organism & Identification
- Oval budding yeast with pseudohyphae
- Normal flora of mouth, gut and vagina
- Germ tube test positive (diagnostic for C. albicans)
- Forms chlamydospores on cornmeal agar; grows on Sabouraud’s agar
Predisposing Factors & Diseases
- Diabetes, antibiotics, steroids, pregnancy, HIV, indwelling catheters
- Oral thrush — white removable plaques
- Vulvovaginitis (curdy white discharge), intertrigo, napkin dermatitis
- Systemic — candidaemia, endocarditis, oesophagitis (AIDS-defining)
Loss of normal defences converts a commensal into a pathogen. Test Result Germ tube Positive (C. albicans) Cornmeal agar Chlamydospores Applied
- Treatment: topical/oral fluconazole; echinocandins for systemic disease
- Oesophageal candidiasis suggests HIV — check CD4
🔑KEY POINTS TO REMEMBER- C. albicans is a commensal yeast with pseudohyphae; germ tube positive.
- Predisposed by diabetes, antibiotics, steroids and HIV.
- Causes thrush, vaginitis and systemic candidiasis; treated with fluconazole.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Aspergillosis is disease caused by Aspergillus species, ubiquitous moulds acquired by inhalation of spores.
Organism
- Septate hyphae branching at acute (45°) angles
- A. fumigatus commonest; also A. flavus (aflatoxin), A. niger
- Widespread in soil, decaying vegetation, hospital dust
- Grows rapidly on Sabouraud’s agar
Clinical Forms
- Allergic bronchopulmonary aspergillosis (ABPA) — asthma, eosinophilia, ↑ IgE
- Aspergilloma (fungus ball) — in an old tuberculous cavity; haemoptysis
- Invasive aspergillosis — neutropenic patients; angioinvasion, high mortality
- Otomycosis; aflatoxin → hepatocellular carcinoma
Host immunity decides whether infection is allergic, saprophytic or invasive. Form Host ABPA Asthmatic Aspergilloma Old TB cavity Invasive Neutropenic Applied
- Treatment: voriconazole (invasive); steroids for ABPA; surgery for aspergilloma
- Halo sign on CT suggests invasive disease
🔑KEY POINTS TO REMEMBER- Aspergillus: septate hyphae branching at 45°, inhaled from the environment.
- Three forms: ABPA (allergic), aspergilloma (cavity), invasive (neutropenic).
- Voriconazole is the drug of choice for invasive disease.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Cryptococcosis is an opportunistic mycosis caused by the capsulated yeast Cryptococcus neoformans, chiefly causing meningitis in AIDS.
Organism & Transmission
- Round budding yeast with a thick polysaccharide capsule
- Source: pigeon droppings, soil; acquired by inhalation
- Capsule is the main virulence factor (antiphagocytic)
- Urease-positive; produces melanin on birdseed agar
Disease & Diagnosis
- Subacute meningitis in AIDS (CD4 <100/μL) — headache, fever, altered sensorium
- Pulmonary and disseminated forms
- India ink negative staining of CSF — clear halo around yeast
- Cryptococcal antigen (CrAg) in CSF/serum — most sensitive; mucicarmine stain
Inhaled yeast spreads from lung to meninges when immunity fails. Test Finding India ink Capsular halo CrAg Most sensitive Culture Urease positive Applied
- Treatment: amphotericin B + flucytosine, then fluconazole
- Repeated lumbar punctures relieve raised intracranial pressure
🔑KEY POINTS TO REMEMBER- C. neoformans is a capsulated yeast from pigeon droppings.
- Causes subacute meningitis in AIDS (CD4 <100/μL).
- India ink shows a capsular halo; CrAg is most sensitive.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Dimorphic fungi exist as moulds at 25°C in the environment and as yeasts at 37°C in tissue.
Members
- Histoplasma capsulatum — histoplasmosis (bat/bird droppings, caves)
- Blastomyces dermatitidis — broad-based budding yeast
- Coccidioides immitis — spherules with endospores
- Penicillium (Talaromyces) marneffei — in AIDS, South-East Asia
- Sporothrix schenckii — subcutaneous
Significance
- Acquired by inhalation of spores
- Often subclinical; disseminate in immunosuppression
- Histoplasma seen as intracellular yeasts within macrophages
- Culture at both 25°C and 37°C confirms dimorphism
Temperature-dependent conversion is the hallmark of these fungi. Fungus Tissue form Histoplasma Intracellular yeast Coccidioides Spherule Blastomyces Broad-based bud Applied
- Treatment: itraconazole; amphotericin B for severe disease
- Consider in fever with hepatosplenomegaly in HIV
🔑KEY POINTS TO REMEMBER- Dimorphic fungi: mould at 25°C, yeast at 37°C.
- Include Histoplasma, Blastomyces, Coccidioides, Sporothrix, T. marneffei.
- Acquired by inhalation; disseminate in immunosuppression.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Pneumocystis pneumonia (PCP) is an opportunistic lung infection caused by Pneumocystis jirovecii, a fungus of low virulence.
Organism & Host
- Classified as a fungus (formerly thought a protozoan)
- Cannot be cultured in vitro
- Occurs when CD4 <200/μL — an AIDS-defining illness
- Also in malnutrition, malignancy, steroid and transplant patients
Features & Diagnosis
- Dry cough, progressive breathlessness, fever; few chest signs
- Marked hypoxia out of proportion to findings
- Chest X-ray: bilateral perihilar interstitial (bat-wing) shadows
- Diagnosis: induced sputum or bronchoalveolar lavage — silver (GMS) or immunofluorescence stain; raised LDH
Alveolar filling causes hypoxia despite unremarkable chest signs. Feature Detail Risk CD4 <200/μL X-ray Bat-wing shadowing Stain Silver (GMS) Applied
- Treatment and prophylaxis: co-trimoxazole
- Add steroids if severe hypoxia
🔑KEY POINTS TO REMEMBER- P. jirovecii is a fungus causing pneumonia when CD4 <200/μL.
- Dry cough with hypoxia out of proportion to signs; bat-wing shadows.
- Co-trimoxazole for both treatment and prophylaxis.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Mucormycosis (zygomycosis) is an aggressive, angioinvasive fungal infection caused by Mucor, Rhizopus and related moulds.
Organism & Risk Factors
- Broad, aseptate (coenocytic) hyphae branching at wide (90°) angles
- Ubiquitous in soil and decaying matter; inhaled
- Risk: uncontrolled diabetes with ketoacidosis, neutropenia, steroids, iron overload (deferoxamine)
- Rose sharply during COVID-19 (steroid use)
Clinical Forms
- Rhino-orbito-cerebral — commonest; nasal blockage, black necrotic eschar on palate/turbinate, orbital swelling, cranial nerve palsy
- Pulmonary, cutaneous, gastrointestinal, disseminated
- Angioinvasion → thrombosis and tissue necrosis; rapidly fatal
Vascular invasion causes the black necrotic tissue that defines the disease. Feature Mucor Aspergillus Hyphae Aseptate Septate Branching 90° (wide) 45° (acute) Applied
- Treatment: urgent surgical debridement + liposomal amphotericin B
- Control diabetes and stop steroids
🔑KEY POINTS TO REMEMBER- Mucormycosis: broad aseptate hyphae branching at 90°.
- Occurs in diabetic ketoacidosis and immunosuppression; angioinvasive.
- Rhino-orbito-cerebral form with black eschar; needs surgery + amphotericin B.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Mycetoma is a chronic subcutaneous infection marked by the triad of swelling, sinuses and discharging grains, usually of the foot.
Types
- Eumycetoma — true fungi (Madurella mycetomatis); usually black grains
- Actinomycetoma — bacteria (Nocardia, Actinomadura); usually red/white grains
- Acquired by traumatic implantation (thorn prick, barefoot walking)
- Commonest in agricultural workers
Features & Management
- Painless slow-growing swelling with multiple discharging sinuses
- Grains (fungal or bacterial colonies) in the discharge
- Spreads to fascia and bone; disability but rarely systemic
- Diagnosis: grain examination, culture, imaging
- Treatment: eumycetoma — itraconazole + surgery; actinomycetoma — co-trimoxazole + dapsone (medical cure possible)
Implanted organisms form granulomas that discharge characteristic grains. Type Cause Treatment Eumycetoma Fungus Itraconazole + surgery Actinomycetoma Bacteria Co-trimoxazole Applied
- Distinguishing the two types is essential — treatments differ completely
- Footwear prevents the disease
🔑KEY POINTS TO REMEMBER- Mycetoma triad: swelling, sinuses, discharging grains.
- Eumycetoma (fungal, black grains) vs actinomycetoma (bacterial).
- Actinomycetoma responds to co-trimoxazole; eumycetoma often needs surgery.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Pityriasis (tinea) versicolor is a superficial mycosis caused by Malassezia furfur, producing hypo- or hyperpigmented scaly macules.
Organism & Features
- Malassezia furfur — lipophilic yeast, normal skin flora
- Favoured by heat, humidity, sweating, oily skin
- Well-defined hypo- or hyperpigmented macules with fine scaling
- Distribution: upper trunk, neck, arms; usually asymptomatic
Diagnosis & Treatment
- KOH mount — ‘spaghetti and meatballs’ appearance (short hyphae + round yeasts)
- Wood’s lamp — golden-yellow fluorescence
- Culture needs lipid-supplemented media (olive oil overlay)
- Treatment: topical selenium sulphide, ketoconazole shampoo; oral fluconazole if extensive
- Pigment change takes months to normalise; recurrence common
The yeast disturbs melanin production, changing skin colour. Test Finding KOH Spaghetti and meatballs Wood’s lamp Golden-yellow Applied
- Also causes Malassezia folliculitis and seborrhoeic dermatitis
- Reassure that repigmentation is slow
🔑KEY POINTS TO REMEMBER- Pityriasis versicolor is caused by the lipophilic yeast Malassezia furfur.
- KOH shows spaghetti-and-meatballs; Wood’s lamp golden-yellow.
- Treated with selenium sulphide or topical azoles; recurrence common.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Sporotrichosis is a subcutaneous mycosis caused by the dimorphic fungus Sporothrix schenckii, classically the ‘rose gardener’s disease’.
Organism & Transmission
- Dimorphic — mould at 25°C, cigar-shaped yeast at 37°C
- Present on thorns, soil, moss, timber
- Acquired by traumatic implantation (thorn prick)
- Occupational: gardeners, farmers, florists
Clinical Features & Treatment
- Lymphocutaneous form (commonest) — nodule at the site, then a chain of nodules along lymphatics, which may ulcerate
- Fixed cutaneous, osteoarticular and pulmonary forms occur
- Diagnosis: culture (most reliable); tissue yeasts scanty
- Treatment: itraconazole; saturated potassium iodide (traditional); amphotericin B for disseminated disease
Lymphatic spread produces the characteristic chain of nodules. Feature Detail Fungus Dimorphic Classic form Lymphocutaneous Treatment Itraconazole Applied
- Suspect in a gardener with a linear chain of nodules
- Gloves prevent inoculation injuries
🔑KEY POINTS TO REMEMBER- Sporothrix schenckii is dimorphic, implanted by thorn pricks.
- Lymphocutaneous form gives a chain of nodules along lymphatics.
- Treated with itraconazole (or potassium iodide).
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Antifungal agents exploit differences between fungal and human cells, chiefly the presence of ergosterol in the fungal membrane.
Classes & Mechanism
- Polyenes — amphotericin B, nystatin: bind ergosterol → membrane pores
- Azoles — fluconazole, itraconazole, voriconazole: inhibit ergosterol synthesis
- Echinocandins — caspofungin: inhibit β-glucan cell-wall synthesis
- Allylamines — terbinafine (squalene epoxidase)
- Others — griseofulvin (microtubules), flucytosine (nucleic acid synthesis)
Clinical Use
- Superficial — topical azoles, terbinafine
- Candidiasis — fluconazole; echinocandins if resistant/severe
- Aspergillosis — voriconazole; mucormycosis — amphotericin B
- Cryptococcal meningitis — amphotericin B + flucytosine
Ergosterol and β-glucan are absent in humans, giving selective toxicity. Class Target Example Polyene Binds ergosterol Amphotericin B Azole Ergosterol synthesis Fluconazole Echinocandin β-glucan Caspofungin Applied
- Amphotericin B is nephrotoxic — liposomal form is safer
- Azoles inhibit cytochrome P450 → drug interactions
🔑KEY POINTS TO REMEMBER- Antifungals target ergosterol (polyenes, azoles) or β-glucan (echinocandins).
- Voriconazole for aspergillosis; amphotericin B for mucormycosis and cryptococcosis.
- Amphotericin B is nephrotoxic; azoles cause P450 interactions.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).