Microbiology
High-yield MBBS Microbiology question bank — bacteriology, immunology, virology, mycology and parasitology, with laboratory diagnosis, at Ananthanarayan & Paniker / Jawetz depth.
12chapters144questions82High-Yield
Definition
Viruses are obligate intracellular infectious agents containing a single type of nucleic acid (DNA or RNA) enclosed in a protein coat.
Structure
- Nucleic acid core — DNA or RNA (never both)
- Capsid — protein coat of capsomeres
- Nucleocapsid = nucleic acid + capsid
- Envelope (some) — lipid bilayer with glycoprotein spikes
- Size 20–300 nm (need electron microscopy)
Classification Criteria
- Nucleic acid — DNA or RNA; single- or double-stranded
- Symmetry — icosahedral, helical, complex
- Envelope — enveloped or naked
- Site of replication, host range
The presence of an envelope determines resistance and mode of spread. Type Example DNA enveloped Herpesvirus DNA naked Papillomavirus RNA enveloped Influenza, HIV RNA naked Poliovirus Applied
- Naked viruses resist drying and disinfectants → faeco-oral spread
- Enveloped viruses are killed by soap and alcohol
🔑KEY POINTS TO REMEMBER- Virus = nucleic acid (DNA or RNA) + capsid ± envelope.
- Classified by nucleic acid, symmetry and envelope.
- Naked viruses are hardier; enveloped viruses are destroyed by soap.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Viral replication is the process by which a virus produces new virions using the host cell’s synthetic machinery.
Steps of Replication
- Adsorption — attachment to specific host receptors
- Penetration — entry (endocytosis or fusion)
- Uncoating — nucleic acid released
- Biosynthesis — viral nucleic acid and protein made (eclipse phase)
- Assembly (maturation)
- Release — lysis (naked) or budding (enveloped)
Key Concepts
- Eclipse phase — no infectious virus detectable inside the cell
- One-step growth curve describes the cycle
- Some viruses establish latency (herpesviruses)
The virus takes over host machinery to make and release new virions. Step Event Adsorption Receptor binding Eclipse No infectious virus Release Lysis or budding Applied
- Antivirals target specific steps (uncoating, polymerase, release)
- Receptor specificity determines tissue tropism
🔑KEY POINTS TO REMEMBER- Steps: adsorption, penetration, uncoating, biosynthesis, assembly, release.
- Eclipse phase = no detectable infectious virus.
- Naked viruses lyse the cell; enveloped viruses bud.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Herpesviruses are large enveloped DNA viruses characterised by their ability to establish lifelong latency with reactivation.
Members & Diseases
- HSV-1 — oral herpes, encephalitis; HSV-2 — genital herpes
- Varicella-zoster — chickenpox, then shingles on reactivation
- Epstein-Barr virus — infectious mononucleosis, Burkitt lymphoma, nasopharyngeal carcinoma
- Cytomegalovirus — congenital infection, retinitis in AIDS
- HHV-6 (roseola), HHV-8 (Kaposi sarcoma)
Common Features
- Double-stranded DNA, enveloped, icosahedral
- Latency — HSV in sensory ganglia, VZV in dorsal root ganglia, EBV in B cells
- Reactivation with stress, immunosuppression
- Intranuclear Cowdry type A inclusion bodies
After the first illness the virus hides in nerves and can reawaken. Virus Latency site Reactivation HSV Sensory ganglia Cold sores VZV Dorsal root ganglia Shingles EBV B lymphocytes Lymphoma risk Applied
- Aciclovir for HSV and VZV; ganciclovir for CMV
- EBV diagnosed by Paul-Bunnell (monospot) test
🔑KEY POINTS TO REMEMBER- Herpesviruses are enveloped DNA viruses causing lifelong latency.
- HSV in sensory ganglia, VZV in dorsal root ganglia, EBV in B cells.
- Aciclovir for HSV/VZV; ganciclovir for CMV.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Poxviruses are the largest, brick-shaped DNA viruses; other important DNA viruses include adenoviruses, papillomaviruses and parvovirus B19.
Poxviruses
- Largest viruses, brick-shaped, complex symmetry
- Unique — replicate in the cytoplasm (own enzymes)
- Variola (smallpox — eradicated), vaccinia, molluscum contagiosum
- Guarnieri bodies — cytoplasmic inclusions
Other DNA Viruses
- Adenovirus — conjunctivitis, respiratory and gut infection; naked, icosahedral
- Papillomavirus (HPV) — warts, cervical cancer
- Parvovirus B19 — smallest DNA virus (single-stranded); erythema infectiosum, aplastic crisis
- Hepatitis B — hepadnavirus
Poxviruses are unique in replicating outside the nucleus. Virus Feature Poxvirus Largest, cytoplasmic Adenovirus Naked, conjunctivitis Parvovirus B19 Smallest, ssDNA Applied
- Smallpox eradicated in 1980 (vaccinia vaccine)
- Parvovirus B19 causes aplastic crisis in sickle-cell disease
🔑KEY POINTS TO REMEMBER- Poxviruses are the largest and replicate in the cytoplasm (Guarnieri bodies).
- Adenovirus is naked; parvovirus B19 is the smallest DNA virus.
- HPV causes warts and cervical cancer.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Laboratory diagnosis of viral infections uses direct detection of the virus or its components, isolation, or detection of the antibody response.
Direct Detection
- Electron microscopy — morphology
- Antigen detection — ELISA, immunofluorescence, rapid tests
- Molecular — PCR / RT-PCR (most sensitive), viral load
- Cytology — inclusion bodies, multinucleate giant cells
Isolation & Serology
- Cell culture — cytopathic effect (gold standard, slow)
- Embryonated eggs, animal inoculation
- Serology — IgM (recent infection), fourfold IgG rise in paired sera
Three approaches: find the virus, grow it, or detect the immune response. Method Advantage PCR Rapid, most sensitive Culture Gold standard, slow Serology Retrospective Applied
- PCR has largely replaced culture in routine practice
- IgM indicates recent infection; IgG indicates past exposure
🔑KEY POINTS TO REMEMBER- Diagnosis by direct detection (PCR, antigen), isolation (culture) or serology.
- PCR is the most sensitive and rapid method.
- IgM = recent infection; fourfold IgG rise confirms.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Viruses can only be cultivated in living cells, since they are obligate intracellular parasites.
Methods
- Cell (tissue) culture — primary, diploid, continuous cell lines; shows cytopathic effect
- Embryonated eggs — chorioallantoic membrane (pox), amniotic cavity (influenza)
- Animal inoculation — suckling mice (coxsackie), rarely used now
Evidence of Growth
- Cytopathic effect — rounding, giant cells, inclusion bodies
- Haemadsorption — red cells adhere to infected cells
- Interference; plaque formation under agar overlay
Living cells are needed because viruses cannot replicate alone. Method Use Cell culture Most viruses Embryonated egg Influenza vaccine Animal Rarely used Applied
- Egg culture used for influenza vaccine production
- Cell culture is the classical gold standard for isolation
🔑KEY POINTS TO REMEMBER- Viruses need living cells: cell culture, embryonated eggs, animals.
- Growth shown by cytopathic effect, haemadsorption or plaques.
- Egg culture is used for influenza vaccine production.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Inclusion bodies are abnormal structures within infected cells representing sites of viral replication or accumulated viral products.
Types & Location
- Intranuclear — herpesvirus (Cowdry type A), adenovirus, CMV (owl-eye)
- Intracytoplasmic — rabies (Negri bodies), poxvirus (Guarnieri bodies), molluscum bodies
- Both — measles
Accumulated viral material forms visible bodies useful for diagnosis. Inclusion Virus Site Negri body Rabies Cytoplasm Cowdry A Herpes Nucleus Owl-eye CMV Nucleus Guarnieri Poxvirus Cytoplasm Applied
- Negri bodies in hippocampus confirm rabies at autopsy
- Owl-eye cells suggest CMV infection
🔑KEY POINTS TO REMEMBER- Inclusion bodies mark sites of viral replication.
- Negri (rabies) and Guarnieri (pox) are cytoplasmic.
- Cowdry A (herpes) and owl-eye (CMV) are intranuclear.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Oncogenic viruses are viruses capable of causing malignant transformation of host cells.
DNA Oncogenic Viruses
- HPV (16, 18) — cervical, anogenital, oropharyngeal cancer
- EBV — Burkitt lymphoma, nasopharyngeal carcinoma, Hodgkin disease
- Hepatitis B — hepatocellular carcinoma
- HHV-8 — Kaposi sarcoma
RNA Oncogenic Viruses & Mechanisms
- HTLV-1 — adult T-cell leukaemia
- Hepatitis C — hepatocellular carcinoma
- Mechanisms: insertion of viral oncogenes, inactivation of tumour suppressor genes (p53, Rb by HPV E6/E7), chronic inflammation
Persistent infection disrupts growth control, allowing cancer to develop. Virus Cancer HPV 16/18 Cervical cancer EBV Burkitt lymphoma HBV/HCV Liver cancer Applied
- HPV and hepatitis B vaccination prevent these cancers
- About 15–20% of human cancers have a viral cause
🔑KEY POINTS TO REMEMBER- DNA oncoviruses: HPV, EBV, HBV, HHV-8; RNA: HTLV-1, HCV.
- HPV E6/E7 inactivate p53 and Rb.
- HPV and hepatitis B vaccines prevent virus-related cancers.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Human papillomavirus (HPV) is a naked DNA virus; persistent infection with high-risk types is the necessary cause of cervical cancer.
Types & Disease
- Low-risk (6, 11) — genital warts (condyloma acuminata), laryngeal papilloma
- High-risk (16, 18) — cervical, vulval, anal, oropharyngeal cancer
- Sexually transmitted; most infections clear spontaneously
- Persistent infection → dysplasia (CIN) → carcinoma over years
Mechanism, Screening & Prevention
- Viral E6 inactivates p53; E7 inactivates Rb
- Screening: Pap smear (koilocytes), HPV DNA testing, VIA
- HPV vaccine — quadrivalent/nonavalent, given to girls before sexual debut
Only persistent high-risk infection progresses to cancer, over many years. Type Disease 6, 11 Genital warts 16, 18 Cervical cancer Applied
- Koilocytes on Pap smear suggest HPV infection
- Vaccination plus screening can eliminate cervical cancer
🔑KEY POINTS TO REMEMBER- HPV 6/11 cause warts; 16/18 cause cervical cancer.
- E6 inactivates p53 and E7 inactivates Rb.
- Prevented by HPV vaccination and Pap/HPV screening.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Varicella-zoster virus (VZV) is a herpesvirus causing chickenpox as primary infection and shingles on reactivation.
Chickenpox (varicella)
- Primary infection, spread by droplets and vesicle fluid
- Highly contagious; incubation 14–21 days
- Centripetal rash in successive crops — macule, papule, vesicle, crust together (pleomorphic)
- Complications: pneumonia (adults), encephalitis, secondary bacterial infection
Shingles (herpes zoster)
- Reactivation of latent virus in dorsal root ganglia
- Painful vesicular rash in a single dermatome, does not cross midline
- Triggered by age, stress, immunosuppression
- Complication: post-herpetic neuralgia; ophthalmic zoster threatens the eye
The same virus causes a widespread rash first and a localised one later. Feature Chickenpox Shingles Type Primary Reactivation Rash Generalised Dermatomal Age Children Elderly Applied
- Treatment: aciclovir (early); varicella vaccine available
- A patient with shingles can transmit chickenpox to a susceptible person
🔑KEY POINTS TO REMEMBER- VZV causes chickenpox (primary) and shingles (reactivation).
- Latency in dorsal root ganglia; shingles is dermatomal.
- Post-herpetic neuralgia is the main complication; treat with aciclovir.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Smallpox, caused by the variola virus, was the first disease eradicated from the world, declared eradicated by the WHO in 1980.
Features Favouring Eradication
- No animal reservoir or carrier state
- Characteristic rash → easy clinical diagnosis
- Effective, heat-stable vaccine (freeze-dried vaccinia)
- Subclinical cases did not transmit
- Short period of infectivity
Strategy & Clinical Note
- Surveillance and containment (ring vaccination) rather than mass vaccination alone
- Last natural case: Somalia, 1977; eradication declared 1980
- Rash was centrifugal, all lesions at the same stage (unlike chickenpox)
Surveillance and ring vaccination broke transmission chains worldwide. Feature Smallpox Chickenpox Rash Centrifugal Centripetal Stage Same Different crops Deep scars Yes No Applied
- Model for polio and measles elimination programmes
- Virus retained only in two designated laboratories
🔑KEY POINTS TO REMEMBER- Smallpox was eradicated in 1980 — the first such disease.
- Aided by no animal reservoir, obvious rash, stable effective vaccine.
- Strategy: surveillance and ring vaccination.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Antiviral agents inhibit specific steps of viral replication with minimal damage to host cells.
By Mechanism
- Nucleoside analogues — aciclovir (herpes), ganciclovir (CMV), zidovudine (HIV)
- Neuraminidase inhibitors — oseltamivir (influenza)
- Protease inhibitors — ritonavir (HIV)
- Reverse transcriptase inhibitors — tenofovir, efavirenz
- Interferons — hepatitis B and C
Principles & Problems
- Most are virustatic, not virucidal
- Selectivity: aciclovir needs viral thymidine kinase
- Resistance develops → combination therapy (HAART)
- Best given early in illness
Drugs slow replication while host immunity finishes the job. Virus Drug Herpes Aciclovir Influenza Oseltamivir HIV HAART combination Applied
- Vaccination remains more effective than treatment for most viruses
- HIV requires lifelong combination therapy
🔑KEY POINTS TO REMEMBER- Antivirals inhibit specific replication steps and are mostly virustatic.
- Aciclovir depends on viral thymidine kinase for selectivity.
- Combination therapy prevents resistance in HIV.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).