Chapter 2 — Bacteria and Viruses
Key Concepts Quick Recap
| Term | Definition |
|---|
| Coccus | Spherical bacteria (Staphylococcus, Streptococcus) |
| Bacillus | Rod-shaped bacteria (E. coli, Bacillus anthracis) |
| Spirillum | Spiral-shaped bacteria (Spirillum, Campylobacter) |
| Vibrio | Comma-shaped (Vibrio cholerae) |
| Binary fission | Asexual reproduction in bacteria |
| Generation time | Time for bacterial population to double; E. coli ≈ 20 min |
| Plasmid | Extrachromosomal circular DNA in bacteria |
| Capsid | Protein coat surrounding viral nucleic acid |
| Bacteriophage | Virus that infects bacteria |
I. Choose the Best Answer (MCQs)
1. Gram-positive bacteria appear purple after Gram staining because they: (a) Have thin peptidoglycan layer (b) Have thick peptidoglycan layer that retains crystal violet (c) Have an outer membrane (d) Are smaller than Gram-negative bacteria
Answer(b) Have thick peptidoglycan layer that retains crystal violet Gram-positive: thick peptidoglycan (20-80 nm) retains crystal violet-iodine complex after decolourisation. Gram-negative: thin PG + outer LPS membrane; crystal violet lost after alcohol → takes up counterstain (safranin) → pink.
2. Which phase of bacterial growth curve shows maximum growth rate? (a) Lag phase (b) Log (exponential) phase (c) Stationary phase (d) Decline phase
Answer(b) Log (exponential) phase Log phase: cells dividing at maximum constant rate; all cells are metabolically active; most sensitive to antibiotics (attacking cell wall synthesis or protein synthesis requires actively dividing cells).
3. The genetic material of bacteriophage T4 is: (a) Single-stranded RNA (b) Double-stranded RNA (c) Single-stranded DNA (d) Double-stranded DNA
Answer(d) Double-stranded DNA Bacteriophage T4: dsDNA phage; infects E. coli. Used extensively in molecular biology research (Hershey-Chase experiment proved DNA is the genetic material).
4. In the lytic cycle of a bacteriophage: (a) Phage DNA integrates into bacterial chromosome (b) Host cell is lysed and new phages released (c) Bacteria become lysogenic (d) No new phages are produced
Answer(b) Host cell is lysed and new phages released Lytic cycle: phage attaches → DNA injected → phage DNA replicates → new phage assembled → host cell lysed → 100-200 new phages released.
5. HIV is a: (a) DNA virus (b) Retrovirus (RNA → DNA) (c) Bacteriophage (d) Prion
Answer(b) Retrovirus (RNA → DNA) HIV: Human Immunodeficiency Virus. Single-stranded RNA virus. Carries reverse transcriptase → RNA → DNA → integrates into host genome (provirus). Destroys CD4+ T cells → AIDS.
6. The bacterial structure that helps in attachment to surfaces is: (a) Flagellum (b) Pili (fimbriae) (c) Capsule (d) Plasmid
Answer(b) Pili (fimbriae) Pili (fimbriae): hair-like appendages on bacterial surface. Functions: attachment to host cells (pathogenicity), gene transfer (sex pili in conjugation). Flagella: locomotion; Capsule: protection from phagocytosis.
7. Endospores are formed by which of the following? (a) E. coli (b) Bacillus and Clostridium (c) Staphylococcus (d) HIV
Answer(b) Bacillus and Clostridium Endospores: highly resistant dormant structures formed by Bacillus (anthrax, Bacillus anthracis) and Clostridium (tetanus, C. tetani; botulism, C. botulinum) under adverse conditions. Resistant to heat, desiccation, radiation, disinfectants.
II. Short Answer Questions
Q1. Describe the bacterial growth curve with phases.
Answer
| Phase | Description |
|---|
| Lag phase | Adaptation period; cells synthesise enzymes; no net increase in count |
| Log (exponential) phase | Rapid binary fission; population doubles every generation time; E. coli: ~20 min |
| Stationary phase | Growth rate = death rate; nutrients depleted, metabolites accumulate |
| Decline (death) phase | Death rate > growth rate; population falls; cells lyse |
Q2. What is the difference between lytic and lysogenic cycles?
Answer
| Feature | Lytic Cycle | Lysogenic Cycle |
|---|
| Outcome | Host cell lysed; new phages released | Phage DNA integrated (prophage); host survives |
| Phage state | Virulent | Temperate |
| New phage production | Immediate | Delayed |
| Example | T4 phage | Lambda (λ) phage |
| Trigger to lytic | Always lytic | UV, chemicals can induce lysis |
Exam Checklist
- Bacterial shapes: coccus, bacillus, spirillum, vibrio, spirochete.
- Gram staining: thick PG (G+, purple) vs thin PG + outer LPS membrane (G–, pink).
- Bacterial growth: lag → log (max growth) → stationary → decline.
- Lytic: lyse host; lysogenic: phage DNA integrates (prophage).
- HIV: retrovirus; RNA → DNA (reverse transcriptase); destroys CD4+ T cells.
- Endospores: Bacillus and Clostridium; extremely resistant.
- Pili: attachment; Flagella: motility; Capsule: anti-phagocytosis.
