Class 11 Micro-biology — Exercise Solutions
Key Concepts Quick Recap
| Pioneer | Contribution | Year |
|---|
| Leeuwenhoek | First to observe microbes (animalcules) | 1674 |
| Pasteur | Disproved spontaneous generation; pasteurisation; germ theory | 1857-1865 |
| Koch | Postulates; isolated M. tuberculosis | 1876-1882 |
| Fleming | Discovered penicillin (Penicillium notatum) | 1928 |
| Jenner | First vaccine (smallpox using cowpox) | 1796 |
| Gram | Gram staining technique | 1884 |
I. Choose the Best Answer (MCQs)
1. The technique used to distinguish bacteria into two groups based on cell wall structure is: (a) Acid-fast staining (b) Gram staining (c) Sudan staining (d) Schiff staining
Answer(b) Gram staining Gram staining (Hans Christian Gram, 1884): crystal violet + iodine → decoloriser → safranin. Gram-positive (+): thick peptidoglycan cell wall → retain crystal violet → purple. Gram-negative (−): thin peptidoglycan + outer lipopolysaccharide membrane → decolorised → counterstained red/pink.
2. Which of the following is an obligate intracellular parasite? (a) E. coli (b) Staphylococcus aureus (c) Virus (d) Penicillium
Answer(c) Virus Viruses: acellular (not cells); contain nucleic acid (DNA or RNA) + protein coat (capsid). Cannot replicate independently; require host cell machinery → obligate intracellular parasites. Non-living outside host cell; living inside. E. coli and Staphylococcus: bacteria (free-living, can be cultured outside host).
3. Pasteurisation is carried out at approximately: (a) 100°C for 5 minutes (b) 72°C for 15 seconds (HTST) or 63°C for 30 minutes (LTLT) (c) 37°C for 1 hour (d) 121°C at 15 psi for 15 minutes
Answer(b) 72°C for 15 seconds (HTST) or 63°C for 30 minutes (LTLT) Pasteurisation: kills pathogens without sterilising. HTST (High Temperature Short Time): 72°C/15 sec. LTLT (Low Temperature Long Time): 63°C/30 min. Used for: milk, juice, beer, wine. Does not kill all organisms (non-sterile; has shelf life, must be refrigerated). 121°C/15 psi/15 min = autoclave (complete sterilisation).
4. The bacterial growth phase in which the rate of cell division equals the rate of cell death is: (a) Lag phase (b) Log/Exponential phase (c) Stationary phase (d) Decline/Death phase
Answer(c) Stationary phase Bacterial growth phases: 1. Lag phase: adaptation, no growth. 2. Log/Exponential: maximum growth rate, doubling time constant. 3. Stationary: growth rate = death rate; nutrient depletion, waste accumulation. 4. Decline/Death: death rate > growth rate. Most antibiotics work best during log phase.
5. The antibiotic penicillin was discovered by: (a) Pasteur (b) Koch (c) Fleming (d) Jenner
Answer(c) Fleming Alexander Fleming (1928): observed that Penicillium notatum mould inhibited Staphylococcus growth (zone of inhibition). Named the substance penicillin. Fleming, Florey, and Chain shared Nobel Prize (1945). Penicillin works by inhibiting bacterial cell wall synthesis (peptidoglycan cross-linking).
6. Which of the following is a method of physical sterilisation? (a) Penicillin treatment (b) Autoclave (c) Chemical disinfectant (d) Antiseptic solution
Answer(b) Autoclave Physical sterilisation methods: 1. Heat: dry heat (hot air oven 160-180°C/1-2h), moist heat (autoclave 121°C/15psi/15min), pasteurisation. 2. Radiation: UV light (DNA damage), gamma rays. 3. Filtration: removes bacteria (Millipore filter 0.22 μm). Chemical methods: disinfectants (chlorine, formaldehyde), antiseptics (iodine, alcohol, hydrogen peroxide).
7. The first vaccine ever developed was against: (a) Tuberculosis (b) Polio (c) Smallpox (d) Influenza
Answer(c) Smallpox Edward Jenner (1796): observed that milkmaids who contracted cowpox (Vaccinia) were immune to smallpox (Variola). Inoculated 8-year-old James Phipps with cowpox → exposed to smallpox → no disease. World's first vaccine. Word "vaccine" from Latin "vacca" = cow.
II. Short Answer Questions
Q1. Explain the lytic and lysogenic cycles of bacteriophage.
Answer
Bacteriophages: viruses that infect bacteria. Two replication cycles:
Lytic Cycle (virulent phage): 1. Attachment: Phage attaches to specific receptor on bacterial surface. 2. Penetration: Phage injects DNA (or RNA) into the host; empty capsid remains outside. 3. Replication: Host cell machinery used to replicate phage DNA; phage proteins synthesised. 4. Assembly: New phage particles assembled from replicated components. 5. Lysis: Enzyme (lysozyme) breaks cell wall → cell bursts → releases 100-200 new phage particles.
Lysogenic Cycle (temperate phage): 1. Phage DNA integrates into host chromosome → called prophage. 2. Prophage replicated with host DNA each cell division → all daughter cells carry prophage. 3. Host cell survives (not lysed). 4. Induction: stress (UV radiation, chemicals) → prophage excises → enters lytic cycle.
Example: Phage lambda (λ) of E. coli exhibits both cycles.
Q2. What is the difference between innate and adaptive immunity?
Answer
| Feature | Innate Immunity | Adaptive Immunity |
|---|
| Nature | Non-specific, present from birth | Specific, acquired during life |
| Response time | Immediate (minutes-hours) | Slow (days-weeks, first exposure) |
| Memory | No immunological memory | Has memory (faster on re-exposure) |
| Components | Skin, mucus, fever, phagocytes (neutrophils, macrophages), NK cells, complement | B cells (antibodies), T cells (cell-mediated) |
| Specificity | Same response regardless of pathogen | Tailored response to specific antigen |
| Diversity | Limited (pattern recognition) | Highly diverse (antigen-specific receptors) |
Key point: innate immunity buys time while adaptive immunity mounts a specific response. Vaccines exploit immunological memory of adaptive immunity.
Exam Checklist
- Leeuwenhoek: father of microbiology; first to see bacteria (animalcules).
- Pasteur: disproved spontaneous generation; pasteurisation; germ theory.
- Koch's postulates: 4 criteria to link microorganism to disease.
- Fleming: penicillin (1928) from Penicillium notatum.
- Jenner: first vaccine (smallpox, 1796).
- Gram stain: G+ = purple (thick PG); G− = pink/red (thin PG + outer membrane).
- 4 growth phases: Lag → Log → Stationary → Death.
- Lytic: kills host cell. Lysogenic: integrates DNA as prophage.
- Autoclave: 121°C, 15 psi, 15 min = sterilisation.
