Microbiology
High-yield MBBS Microbiology question bank — bacteriology, immunology, virology, mycology and parasitology, with laboratory diagnosis, at Ananthanarayan & Paniker / Jawetz depth.
12chapters144questions82High-Yield
Definition
A bacterium is a prokaryotic unicellular organism lacking a nuclear membrane, with characteristic cell-wall and cytoplasmic structures.
Essential Structures
- Cell wall — peptidoglycan; gives shape and rigidity
- Cell membrane — selectively permeable
- Cytoplasm — ribosomes (70S), inclusions
- Nucleoid — single circular DNA, no nuclear membrane
Non-Essential (accessory) Structures
- Capsule — antiphagocytic, virulence
- Flagella — motility
- Pili / fimbriae — adhesion, conjugation
- Spore — resistant dormant form
- Plasmids — extrachromosomal DNA (resistance)
Essential envelope and nucleoid, plus accessory structures for virulence. Structure Function Cell wall Shape, rigidity Capsule Antiphagocytic Flagella Motility Pili Adhesion, conjugation Applied
- Cell wall is the target of β-lactam antibiotics
- Capsule is the basis of polysaccharide vaccines
🔑KEY POINTS TO REMEMBER- Bacteria are prokaryotes: no nuclear membrane, 70S ribosomes.
- Essential: cell wall, membrane, cytoplasm, nucleoid.
- Accessory: capsule, flagella, pili, spore, plasmid.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Sterilisation is the complete destruction of all microorganisms including spores; disinfection destroys most pathogens but not necessarily spores.
Physical Methods
- Moist heat — autoclave (121°C, 15 psi, 15 min)
- Dry heat — hot air oven (160°C, 2 h); flaming, incineration
- Radiation — gamma rays (disposables), UV (surfaces)
- Filtration — membrane filters for heat-labile fluids
Chemical Methods
- Alcohols, halogens (iodine, chlorine)
- Aldehydes — formaldehyde, glutaraldehyde
- Ethylene oxide gas (heat-sensitive equipment)
- Phenols, chlorhexidine
Sterilisation uses physical or chemical agents to remove all microbes. Method Condition Use Autoclave 121°C, 15 min Instruments, media Hot air oven 160°C, 2 h Glassware Filtration 0.22 μm Serum, antibiotics Applied
- Spores are the most resistant form — the test of sterilisation
- Sterilisation control: Bacillus stearothermophilus spore strips
🔑KEY POINTS TO REMEMBER- Sterilisation kills all microbes including spores; disinfection does not.
- Autoclave: 121°C, 15 psi, 15 minutes (moist heat).
- Heat-labile fluids are sterilised by filtration.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Bacterial growth is the orderly increase in cell number by binary fission, requiring specific nutritional and environmental conditions.
Nutritional Requirements
- Carbon, nitrogen, water, energy source
- Minerals and growth factors
- Autotrophs — use simple inorganic sources
- Heterotrophs — need organic carbon (all pathogens)
Growth Conditions
- Temperature — mesophiles grow at 37°C (pathogens)
- pH — usually 7.2–7.4
- Oxygen — aerobes, anaerobes, facultative, microaerophilic
- Moisture, carbon dioxide
Growth Curve (4 phases)
- Lag → log (exponential) → stationary → decline
Bacteria pass through four phases as nutrients are used up. Phase Feature Lag Adaptation, no division Log Rapid multiplication Stationary Growth = death Applied
- Antibiotics act best in the log phase
- Toxins and spores form in the stationary phase
🔑KEY POINTS TO REMEMBER- Bacteria multiply by binary fission; generation time ~20 min for E. coli.
- Growth needs suitable temperature (37°C), pH (7.2–7.4), oxygen.
- Growth curve: lag, log, stationary, decline.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Culture media are nutrient preparations used to grow, isolate and identify bacteria in the laboratory.
Classification by Purpose
- Simple / basal — nutrient broth, nutrient agar
- Enriched — blood agar, chocolate agar
- Selective — MacConkey, TCBS, LJ medium (inhibit others)
- Enrichment (liquid) — selenite F, alkaline peptone water
- Differential / indicator — MacConkey (lactose fermenters)
- Transport — Cary-Blair, Stuart’s medium
Media are designed either to select organisms or to tell them apart. Medium Type Organism Blood agar Enriched Streptococci MacConkey Selective/differential Enterobacteria LJ medium Selective M. tuberculosis TCBS Selective V. cholerae Applied
- Blood agar shows haemolysis patterns (α, β, γ)
- Transport media preserve organisms during transit
🔑KEY POINTS TO REMEMBER- Media types: simple, enriched, selective, enrichment, differential, transport.
- MacConkey is both selective and differential (lactose fermentation).
- LJ medium for tuberculosis; TCBS for cholera.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Bacterial genetics concerns the bacterial genome and the mechanisms by which genetic material is transferred between bacteria.
Genetic Material
- Single circular chromosome (nucleoid)
- Plasmids — extrachromosomal, carry resistance/virulence genes
- Transposons — jumping genes
Mechanisms of Genetic Transfer
- Transformation — uptake of free (naked) DNA
- Transduction — transfer by bacteriophage
- Conjugation — direct contact via sex pilus (plasmid transfer)
Mutation
- Spontaneous or induced change in DNA → new traits, resistance
Three mechanisms spread genes — including resistance — between bacteria. Mechanism Vehicle Transformation Free DNA Transduction Bacteriophage Conjugation Sex pilus Applied
- Conjugation is the main route of antibiotic-resistance spread
- Basis of recombinant DNA technology
🔑KEY POINTS TO REMEMBER- Bacterial genome: circular chromosome + plasmids + transposons.
- Transfer: transformation (naked DNA), transduction (phage), conjugation (pilus).
- Conjugation spreads multidrug resistance.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
The Gram stain is the most important differential stain in bacteriology, dividing bacteria into Gram-positive and Gram-negative.
Steps (in order)
- Crystal violet (primary stain) — 1 min
- Gram’s iodine (mordant) — 1 min
- Acetone / alcohol (decolouriser) — few seconds
- Safranin / carbol fuchsin (counterstain)
Principle & Result
- Gram-positive — thick peptidoglycan retains the dye → violet
- Gram-negative — thin wall, lipid outer membrane loses dye → pink/red
Wall thickness decides whether the violet complex is retained. Type Colour Example Gram-positive Violet Staphylococcus Gram-negative Pink E. coli Applied
- Guides empirical antibiotic choice
- Over-decolourisation is the commonest error
🔑KEY POINTS TO REMEMBER- Gram stain steps: crystal violet, iodine, decolouriser, safranin.
- Gram-positive violet (thick peptidoglycan); Gram-negative pink.
- Guides initial antibiotic selection.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
A bacterial spore (endospore) is a highly resistant, dormant form produced by certain bacteria under adverse conditions.
Features
- Formed by Bacillus and Clostridium (Gram-positive bacilli)
- Contains dipicolinic acid and calcium → heat resistance
- Very low water content; thick multilayered coat
- Metabolically inactive (dormant)
- Resistant to heat, drying, chemicals, radiation
Spores let bacteria survive harsh conditions and revive later. Feature Detail Organisms Bacillus, Clostridium Key chemical Dipicolinic acid Resistance Heat, chemicals Applied
- Spores are the benchmark for sterilisation (autoclave)
- Tetanus and gas gangrene spread by soil spores
🔑KEY POINTS TO REMEMBER- Spores are dormant resistant forms of Bacillus and Clostridium.
- Dipicolinic acid + calcium give heat resistance.
- Their destruction defines successful sterilisation.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
The bacterial capsule is a well-defined polysaccharide layer outside the cell wall that is an important virulence factor.
Features & Functions
- Usually polysaccharide (polypeptide in B. anthracis)
- Antiphagocytic — main virulence role
- Protects against drying and complement
- Antigenic — basis of typing and vaccines
- Demonstrated by negative staining (India ink) or Quellung reaction
The slippery capsule prevents phagocytes from engulfing the bacterium. Organism Note S. pneumoniae Quellung reaction K. pneumoniae Mucoid colonies B. anthracis Polypeptide capsule Applied
- Capsular polysaccharide vaccines (pneumococcal, meningococcal, Hib)
- Loss of capsule → loss of virulence
🔑KEY POINTS TO REMEMBER- Capsule is a polysaccharide layer outside the cell wall.
- Main role is antiphagocytic (virulence).
- Basis of Quellung reaction and polysaccharide vaccines.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Flagella are long filamentous appendages of protein (flagellin) responsible for bacterial motility.
Arrangement
- Monotrichous — single polar flagellum (V. cholerae)
- Lophotrichous — tuft at one pole
- Amphitrichous — at both poles
- Peritrichous — all over the surface (Salmonella, E. coli)
Features
- Composed of flagellin protein
- H antigen — used in serotyping
- Demonstrated by hanging-drop preparation or special stains
Rotating flagella propel the bacterium and carry the H antigen. Arrangement Example Monotrichous V. cholerae Peritrichous Salmonella Applied
- Motility distinguishes Klebsiella (non-motile) from E. coli
- H antigen used in Widal test
🔑KEY POINTS TO REMEMBER- Flagella are made of flagellin and provide motility.
- Arrangements: mono-, lopho-, amphi-, peritrichous.
- Carry the H antigen used in serotyping.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
The autoclave sterilises by moist heat under pressure, the most reliable and widely used method of sterilisation.
Principle & Conditions
- Steam under pressure raises the boiling point
- Standard: 121°C, 15 psi, 15 minutes
- Alternative: 134°C, 30 psi, 3–5 minutes
- Kills by coagulation of microbial proteins
- Destroys spores (unlike boiling)
Uses & Controls
- Surgical instruments, dressings, culture media, gloves
- Biological control — Bacillus stearothermophilus spores
- Chemical control — Browne’s tube, autoclave tape
Pressurised steam reaches temperatures that destroy even spores. Parameter Value Temperature 121°C Pressure 15 psi Time 15 minutes Applied
- Air must be fully displaced or sterilisation fails
- Not suitable for heat-labile materials (use filtration)
🔑KEY POINTS TO REMEMBER- Autoclave: moist heat, 121°C at 15 psi for 15 minutes.
- Kills by protein coagulation; destroys spores.
- Controlled by B. stearothermophilus spore strips.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
The Ziehl–Neelsen stain demonstrates acid-fast bacilli, whose high mycolic acid content resists decolourisation by acid.
Steps
- Carbol fuchsin with heating — 5 min
- Decolourise with 20% sulphuric acid (and alcohol)
- Counterstain with methylene blue
Result & Principle
- Acid-fast bacilli — bright red rods
- Background and other bacteria — blue
- Mycolic acid in the wall retains carbol fuchsin
Only mycolic-acid-rich organisms hold the red stain against acid. Result Organism Red (acid-fast) M. tuberculosis, M. leprae Blue Other bacteria Applied
- Diagnosis of pulmonary tuberculosis (sputum smear)
- Modified ZN stain for Cryptosporidium, Nocardia
🔑KEY POINTS TO REMEMBER- ZN stain uses carbol fuchsin with heat, acid decolourisation, methylene blue.
- Acid-fast bacilli appear red because of mycolic acid.
- Used for tuberculosis and leprosy diagnosis.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).Definition
Disinfectants are chemicals applied to inanimate objects, while antiseptics are applied to living tissue to destroy or inhibit microbes.
Common Agents
- Alcohols — 70% ethanol (protein denaturation)
- Halogens — chlorine (water), povidone-iodine (skin)
- Aldehydes — formaldehyde, glutaraldehyde (endoscopes)
- Phenols — cresol, lysol (floors, excreta)
- Chlorhexidine, hydrogen peroxide, quaternary ammonium compounds
Levels of Disinfection
- High level — kills all except large numbers of spores (glutaraldehyde)
- Intermediate — kills mycobacteria (alcohols, iodine)
- Low level — kills most vegetative bacteria
The same chemistry is applied to surfaces or to skin. Agent Use 70% alcohol Skin, surfaces Glutaraldehyde Endoscopes Phenol Floors, excreta Applied
- Ideal disinfectant: broad spectrum, non-toxic, non-corrosive, cheap
- Organic matter reduces disinfectant activity — clean first
🔑KEY POINTS TO REMEMBER- Disinfectants for objects; antiseptics for living tissue.
- Alcohols, halogens, aldehydes, phenols are the main groups.
- Activity reduced by organic matter — clean before disinfecting.
📚SOURCES: Textbook of Microbiology (Ananthanarayan & Paniker); Jawetz, Melnick & Adelberg’s Medical Microbiology; Textbook of Microbiology (Baveja).