Microbiology, immunity and forensics
Infectious diseases and antibiotics
Cholera (bacterium Vibrio cholerae): contaminated water; toxin causes severe diarrhoea; controlled by clean water, sanitation, oral rehydration. Malaria (Plasmodium, a protoctist): spread by female Anopheles mosquitoes; controlled by nets, insecticides, drugs, draining water. Tuberculosis (Mycobacterium tuberculosis): airborne droplets; BCG vaccine, long antibiotic courses, contact tracing. Measles (virus): droplets; MMR vaccine. HIV/AIDS (retrovirus): body fluids; destroys T-helper cells; antiretroviral drugs, condoms, screening. Antibiotics target features of bacteria absent in human cells; penicillin inhibits enzymes building peptidoglycan cross-links, so cells burst by osmosis. Antibiotics don't affect viruses. Resistance arises by random mutation and spreads by natural selection and plasmid transfer, worsened by overuse and unfinished courses.
≈ 45 minImmune response and vaccination
Phagocytes (neutrophils, macrophages) engulf pathogens; macrophages become antigen-presenting cells. T-helper cells with complementary receptors are activated and release cytokines. B cells with complementary receptors undergo clonal selection and expansion, forming plasma cells (secrete antibodies) and memory cells. Cytotoxic T cells kill infected cells. Antibodies: four polypeptide chains (2 heavy, 2 light), variable regions with two antigen-binding sites, held by disulfide bonds; cause agglutination, neutralisation and opsonisation. The secondary response is faster and larger because of memory cells. Active immunity (own antibodies; natural infection or vaccination) is long-lasting; passive (given antibodies; placenta, milk, injection) is short-term. Herd immunity protects the unvaccinated. Monoclonal antibodies target specific antigens for diagnosis and treatment.
≈ 50 minMicrobial growth and forensics
Microorganisms are cultured on agar or in broth using aseptic technique. A growth curve has a lag phase (enzymes being made), log (exponential) phase (doubling each generation), stationary phase (births = deaths; nutrients limited, waste builds up) and death phase. . Counting: serial dilution then plate count (colonies × dilution factor ÷ volume plated); haemocytometer; turbidity (colorimeter). Forensics: time of death is estimated from body temperature (falls ~1.5 °C per hour to surroundings), rigor mortis (starts 2–6 h, lasts ~36 h, as ATP runs out), decomposition by bacteria and enzymes, and forensic entomology (blowfly larvae stages), plus a predictable succession of insects on the body. DNA profiling identifies victims and suspects.
≈ 45 min