Microbiology and Immunology
What this domain tests
Microbiology and Immunology covers microbial structure, growth, control, host defenses, and vaccination. Questions often test whether you can distinguish innate from adaptive immunity and choose the correct interpretation of a growth curve or Gram stain.
Core rules and concepts
- Bacteria are prokaryotes: no membrane-bound nucleus; peptidoglycan cell wall; Gram-positive has thick peptidoglycan, Gram-negative has thin peptidoglycan plus outer membrane.
- Binary fission growth: N = N0 × 2^n, where n is number of generations.
- Antibiotics target bacterial structures such as cell wall, ribosome, or DNA gyrase and do not treat viral infections.
- Innate immunity is fast and nonspecific, including barriers, phagocytes, and inflammation; adaptive immunity is slower, specific, and has memory via B and T cells.
- Antibodies neutralize or agglutinate; vaccines create memory without causing disease.
- Passive immunity transfers antibodies, as in maternal transfer or antivenom, and is short-lived.
Worked example
A culture starts with 100 cells and doubles every 30 minutes. After 3 hours, n = 6 generations. N = 100 × 2^6 = 100 × 64 = 6,400 cells. Reasoning: each generation doubles the population; the lag phase is ignored in this idealized calculation. If the question gives a lag phase, subtract it before calculating n.
Traps that cost marks
Do not say antibiotics kill viruses. Do not confuse Gram-positive purple with Gram-negative pink or red. Do not equate innate immunity only with lack of memory; it also lacks specificity. Do not say passive immunity creates memory. Do not assume all bacteria are pathogens; most are commensal or beneficial.