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⭐⭐⭐⭐⭐ FIVE-STAR SAQs — GENERAL MICROBIOLOGY

(5-Mark Exam Answers)

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Q1. Koch's Postulates with Examples

The Four Postulates:

Classic Examples:

• Anthrax (Bacillus anthracis) - Koch first proved his own postulates with this organism
• Tuberculosis (Mycobacterium tuberculosis) - 1882, Koch's most famous work
• Cholera (Vibrio cholerae)

Limitations / Modern Exceptions:

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Q4. Dark Field Microscope — Principle & Applications

Principle:

• A special dark-field condenser (with a circular opaque disc/stop) blocks all direct light
• Only peripheral oblique rays are allowed to illuminate the specimen from the sides
• These oblique rays do not enter the objective directly
• They enter the objective only if scattered/reflected by the specimen
• Result: specimen appears as bright, luminous objects against a completely dark background
• Can detect organisms as thin as 0.02 µm - far below the resolution of bright-field microscopy (~0.2 µm)

Diagram of Principle:

Light source → Condenser (with central stop) → Oblique rays → Specimen
               ↓                                    ↓
         Direct light blocked            Scattered light enters objective
                                         → Bright image on dark background

Key Advantage:

Applications:

Note: Dark-field is the method of choice for diagnosing primary syphilis because serology (VDRL/TPHA) may be negative in early primary syphilis, and T. pallidum cannot be cultured.

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Q5. Gram-Positive vs. Gram-Negative Cell Wall — Differentiation with Examples

Comparison Table:

Key Structural Components Explained:

• Peptidoglycan backbone: NAG (N-acetylglucosamine) - NAM (N-acetylmuramic acid) repeating units with tetrapeptide side chains cross-linked by pentaglycine bridges (S. aureus) or direct bonds
• Teichoic acids: Glycerophosphate / ribitol phosphate polymers - role in adherence, ion transport, antigenicity

• Thin peptidoglycan in periplasmic space
• Outer membrane: Bilayer with LPS (Lipid A + Core polysaccharide + O-antigen), porins (OmpC, OmpF), and lipoproteins (Braun's lipoprotein anchoring outer membrane to peptidoglycan)
• O-antigen: Highly variable → basis of serotyping (e.g., E. coli O157:H7)

Why Gram-Negative Bacteria are More Resistant to Antibiotics:

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Q7. Bacterial Capsule — Structure, Function & Demonstration

Structure / Composition:

Functions:

1. Anti-phagocytic - Most important virulence function. Hydrophilic capsule repels phagocytes (which have hydrophobic surfaces). Loss of capsule = loss of virulence (e.g., smooth → rough S. pneumoniae)
2. Complement evasion - Interferes with C3b deposition; poorly activates complement
3. Adhesion - Mediates attachment to host tissues and prosthetic devices (biofilm). S. mutans uses dextran/levan capsule to adhere to tooth enamel
4. Desiccation resistance - Capsule is highly hydrated; protects from drying
5. K antigen - Highly antigenic; basis for vaccine development (pneumococcal, meningococcal, Hib polysaccharide conjugate vaccines)

Demonstration Methods:

Key fact: Encapsulated organisms produce smooth (S) colonies (mucoid, glistening). Non-capsulated = rough (R) colonies (dull, irregular). Smooth strains are virulent; rough strains are avirulent.

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Q8. Bacterial Spores — Types, Function, Examples & Demonstration

Characteristics:

• Resistant to: boiling, autoclaving (destroyed only at 121°C/15 psi/15 min), disinfectants, UV radiation, desiccation, extreme pH
• Viable for decades to centuries
• Killed by dry heat at 160–170°C for 1–2 hours

Structure (Layers from inside out):

Position within Cell and Examples:

Clinically Important Spore-Forming Bacteria:

Sporulation (triggered by nutrient depletion):

Germination (3 stages):

1. Activation - Heat, abrasion, acid
2. Initiation - Germinant (L-alanine, adenosine) triggers cortex lysis → water influx, Ca²⁺-dipicolinate release
3. Outgrowth - New vegetative cell emerges with active biosynthesis

Demonstration / Staining:

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Q9. Bacterial Growth Curve

The Four Phases:

Important Formulae:

• Generation time (g) = t / n (where n = number of generations)
• Number of generations: n = (log N - log N₀) / log 2 = 3.3 × log (N/N₀)
• E. coli generation time = ~20 minutes (fastest)
• M. tuberculosis generation time = ~14–20 hours (slow → explains long treatment duration)

Growth Curve Diagram:

Log viable count
     |                    ___________
     |                   /           \
     |                  /             \
     |_________________/               \___________
     |←—Lag—→|←———Log (Exponential)———→|←Stat→|←Death→
                                                   Time →

Clinical Relevance:

1. Log phase cells - most sensitive to antibiotics targeting cell wall synthesis (penicillin, vancomycin) and protein synthesis
2. Stationary phase - antibiotic tolerance; biofilm formation common
3. Chemostat - maintains cells in continuous exponential phase by adding fresh medium and removing spent medium; used in research and fermentation
4. VBNC phenomenon - clinically important; explains persistent infections (e.g., V. cholerae in water)

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Q12. Anaerobic Culture Methods & Media

Classification of Anaerobes:

• Obligate anaerobes - O₂ is lethal (e.g., Bacteroides, Clostridium)
• Aerotolerant anaerobes - O₂ present but not used (e.g., Lactobacillus)
• Microaerophiles - grow best at 5% O₂ (e.g., Campylobacter, H. pylori)

Methods to Achieve Anaerobiosis:

Anaerobic Culture Media:

Incubation:

• Temperature: 35–37°C
• Duration: Minimum 48 hours; hold plates for 5–7 days before reporting negative
• Plates must not be exposed to air during incubation

Clinically Important Anaerobes:

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Q15. Antibiotic Susceptibility Testing (AST) — Classification & Applications

Classification of AST Methods:

A. Phenotypic Methods:

B. Molecular / Genotypic Methods:

Interpretive Categories (CLSI / EUCAST):

Key Definitions:

• MIC (Minimum Inhibitory Concentration) - Lowest concentration of drug that inhibits visible growth after 18–24 h
• MBC (Minimum Bactericidal Concentration) - Lowest concentration that kills ≥99.9% of organisms
• Bacteriostatic - MBC/MIC ratio >4
• Bactericidal - MBC/MIC ratio ≤4

Applications:

1. Guide individual patient therapy - Choice and dose of antibiotic
2. Detect resistant organisms - MRSA, VRSA, ESBL, CRE, MDR-TB
3. Hospital infection control - Track antibiogram trends; isolate resistant strains
4. Antimicrobial stewardship - Prevent overuse; rotate antibiotics
5. New drug development - Breakpoint determination
6. Epidemiological surveillance - WHO/CDC monitoring of resistance patterns

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Q17. PCR and Its Applications

Principle — Three Steps (One Cycle):

• Each cycle doubles the target DNA
• 30 cycles → ~10⁶-fold (2³⁰ ≈ 10⁹) amplification from a single copy
• Products detected by gel electrophoresis or fluorescence

Key Components:

• Template DNA (target)
• Two primers (short oligonucleotides, 18–25 bp; flank the target)
• Taq polymerase (thermostable; from Thermus aquaticus)
• dNTPs (building blocks)
• MgCl₂ (cofactor for polymerase)
• Buffer

PCR Variants:

Applications in Clinical Microbiology:

Advantages over Culture:

• Detects non-culturable organisms
• Results in hours vs. days/weeks
• Extremely sensitive (single copy detection)
• Safe (no live organisms)

Limitations:

• Cannot distinguish live from dead organisms
• Contamination → false positives
• Unknown pathogens missed (unless broad-range 16S PCR)
• Does not provide antibiotic susceptibility

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Q18. Transduction

Mechanism:

1. A bacteriophage infects a donor bacterium and replicates inside
2. During packaging of phage DNA into capsid, a fragment of bacterial chromosomal DNA is accidentally packaged instead of (or along with) phage DNA
3. The resulting transducing phage particle injects this bacterial DNA into a recipient bacterium
4. The transferred DNA is incorporated into the recipient chromosome by recombination

Types of Transduction:

Phage Life Cycles Relevant to Transduction:

Clinical Significance (Most Important for Exam):

Key point: Many major bacterial virulence factors are phage-encoded and spread by transduction - this is how new pathogenic strains emerge.

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Q19. Conjugation

F Plasmid (Fertility Factor):

• Circular, 94.5 kb DNA
• Encodes sex pilus (F pilus) and all transfer functions (~40 genes)
• Cells with F plasmid = F⁺ (male/donor)
• Cells without F plasmid = F⁻ (female/recipient)

Mechanism:

1. F⁺ cell synthesizes F pilus (Type IV secretion system)
2. F pilus contacts F⁻ cell surface and retracts, bringing cells into close contact
3. A mating junction/pore forms between the two cells
4. A plasmid-encoded relaxase enzyme nicks the F plasmid at oriT (origin of transfer)
5. Rolling circle replication begins - one strand is transferred 5'→3' into recipient
6. Complementary strand synthesized in both cells simultaneously
7. Both cells end up with complete F plasmid → F⁻ becomes F⁺

Types of Conjugal Transfer:

R Plasmids (Resistance Plasmids):

• Carry multiple antibiotic resistance genes (encoded by transposons)
• Transfer rapidly between strains, species, and genera
• Clinically important: spread of MDR organisms in hospitals
• Example: E. faecalis transferred vancomycin resistance transposon to S. aureus in co-infected patient → VRSA

Differences from Transformation and Transduction:

Clinical Significance:

• Primary mechanism of MDR spread in clinical settings
• Responsible for outbreaks of MDR Klebsiella, ESBL E. coli, CRE
• Drives plasmid-mediated resistance to carbapenems (NDM, KPC), fluoroquinolones (qnr genes), and polymyxins (mcr genes)

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Q20. Mechanisms of Drug Resistance

Genetic Basis:

Three Major Biochemical Mechanisms:

1. Reduced Drug Accumulation (Exclusion)

• Outer membrane porin mutations reduce drug entry (Gram-negative)
• Example: Loss of OprD porin in P. aeruginosa → carbapenem resistance

• Membrane proteins pump drug out of cell before it reaches target
• Example: MexAB-OprM pump in P. aeruginosa (fluoroquinolones, β-lactams)
• TetA efflux pump → tetracycline resistance
• NorA pump in S. aureus → fluoroquinolone resistance

2. Alteration of Drug Target

3. Enzymatic Inactivation of Antibiotic

Multidrug Resistance (MDR):

• MDR = non-susceptible to ≥1 agent in ≥3 antibiotic categories
• XDR = non-susceptible to all but ≤2 categories
• PDR = non-susceptible to ALL antibiotics

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Q21. Exotoxin vs. Endotoxin — Differentiation with Examples

Comparison Table:

Exotoxin Types with Examples:

Endotoxin — Lipopolysaccharide (LPS) Structure:

• Lipid A (attached to outer membrane) = toxic moiety (causes fever, septic shock)
• Core polysaccharide (R antigen) = structural backbone
• O-antigen (polysaccharide chains) = highly variable; used for serotyping (O157, O139)

Endotoxin Effects (Dose-Dependent):

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Q22. Laboratory Diagnosis of Viral Infections

1. Specimen Collection:

2. Direct Detection Methods:

3. Viral Culture (Gold Standard):

• Cell lines: Vero (HSV), MDCK (influenza), HEp-2 (RSV), MRC-5 (CMV, VZV)
• Embryonated eggs: Influenza (allantoic cavity), Poxviruses (chorioallantoic membrane)
• Laboratory animals: Only for special viruses (suckling mice for arboviruses)

4. Serology:

5. Molecular Methods (Most Sensitive & Specific):

Summary — Method of Choice by Pathogen:

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Q23. Tissue Culture — Methods & Applications

Systems for Virus Propagation (in order of complexity):

1. Laboratory Animals

• Suckling mice (inoculated intracerebrally) - arboviruses, rabies
• Guinea pigs, rabbits - older methods
• Now largely replaced by cell culture for routine diagnosis

2. Embryonated Hen's Eggs

• Allantoic cavity - Influenza A/B, Mumps (large volumes; vaccine production)
• Amniotic cavity - Influenza A (primary isolation)
• Chorioallantoic membrane (CAM) - Vaccinia, HSV, Poxviruses (visible pocks)
• Yolk sac - Chlamydia, Rickettsia, some arboviruses

3. Cell / Tissue Culture (Most Important)

Shell Vial Culture (Modified Centrifugation-Enhanced Culture):

• Specimen centrifuged onto monolayer → DFA at 24–48 h → faster than conventional culture (days vs. weeks)
• Used for: CMV (HEL cells + CMV early antigen DFA; result in 16–24 h vs. 4 weeks), HSV

Evidence of Viral Growth in Culture:

Viral Quantitation Methods:

• TCID₅₀ (Tissue Culture Infective Dose₅₀) = dilution at which 50% of inoculated cultures show CPE; calculated by Reed-Muench method
• PFU (Plaque-Forming Units) = number of plaques per mL = infectious virus particles

Applications of Tissue Culture:

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QUICK REVISION TABLE — ALL 15 FIVE-STAR TOPICS

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