Microbiology, Immunity and Disease
This domain tests microbial structure and growth, sterilisation, disease transmission and the immune response. Expect growth-curve calculations, comparisons of bacteria, fungi and viruses, and explanations of vaccination and antibody action.
Microbes and growth
Bacteria are prokaryotic, have peptidoglycan walls and divide by binary fission. A batch culture shows lag, log, stationary and decline phases. Fungi are eukaryotic with chitin walls; yeasts bud and moulds form hyphae. Viruses are acellular obligate parasites and are unaffected by antibiotics. Autoclaving uses moist heat at about 121°C to sterilise; pasteurisation reduces spoilage organisms but does not sterilise.
Immunity
Innate defences include skin, mucus, phagocytes and inflammation. Adaptive immunity involves B lymphocytes that become plasma cells and secrete specific antibodies, and T cells, including helper and cytotoxic T cells. Memory cells give a faster, stronger secondary response. Vaccination exposes the immune system to antigens without causing disease, producing memory and, at population level, herd immunity.
Worked example
A culture starts with 100 cells and doubles every 20 minutes. In 2 hours there are 120 ÷ 20 = 6 generations. Final number = 100 × 2⁶ = 100 × 64 = 6400 cells. This assumes unrestricted exponential growth, which applies only in the log phase.
Traps
Antibiotics do not treat viral infections. Not all bacteria are pathogens. Plasma cells are differentiated B cells, not T cells. Do not claim viruses grow on agar; they need host cells.
Disease control
Pathogens spread by contact, droplets, vectors, contaminated food and water. Control includes hygiene, sterilisation, vector control, vaccination and, where appropriate, antibiotics.