Topic 6: Microbiology and Pathogens — Edexcel A-Level Biology B

Aseptic technique keeps cultures pure and protects the worker: sterilise loops and glassware, flame the neck of containers, work near a Bunsen updraught, and never open a plate more than needed. Culturing needs a sterile nutrient supply, correct temperature and a controlled atmosphere. Media differ: broth is liquid and mixes the whole population, agar gives surface colonies, and selective media contain agents that suppress unwanted microbes so only chosen types grow. Growth can be measured by direct cell counts, dilution plating to count viable colonies, dry mass, or turbidity, which uses light scattering as an indirect optical estimate. A growth curve shows lag, log, stationary and death phases; during log phase, exponential growth rate constants are calculated from population sizes at two times.

Bacteria cause disease in three main ways. They can invade and destroy host tissues, as Mycobacterium tuberculosis does in lung tissue. They can release exotoxins, proteins secreted by living bacteria such as Staphylococcus spp., which damage host cells. They can also release endotoxins, components of the bacterial cell wall released when bacteria such as Salmonella spp. die, which trigger fever and inflammation. Understanding these mechanisms explains why different bacterial infections produce different symptoms and why treatment and prevention must target the specific mechanism.

Antibiotics act on bacteria, not on viruses, and are classified by what they do to a bacterial population. A bactericidal antibiotic kills bacteria, so the number of viable cells falls; penicillin is the classic example because it blocks the cross-linking of peptidoglycan in the growing cell wall, so a bacterium with a weakened wall bursts by osmosis. A bacteriostatic antibiotic does not kill outright but stops bacteria dividing, so the population stops growing and the immune system can clear the cells; tetracycline is the classic example because it binds the bacterial 30S ribosomal subunit and prevents tRNA binding, so protein synthesis is inhibited. Note that a bacteriostatic drug can still allow recovery if it is removed, and that the two labels describe typical action against a given species at a given dose, not a permanent property of the drug.

Antibiotic resistance arises by mutation in a bacterial population, and the resistant cells survive when the antibiotic is present. Because bacteria reproduce rapidly and can pass plasmids between cells by conjugation, resistance alleles spread horizontally as well as vertically, so a resistant strain can appear in a patient who has never taken that antibiotic. Control methods include prescribing antibiotics only when needed, completing the prescribed course, using narrow-spectrum drugs where possible, maintaining good hygiene and infection control in hospitals, and restricting antibiotic use in agriculture. Control is difficult because resistance genes move between species on plasmids, because patients may not complete courses, because antibiotics are used widely in farming, and because few new antibiotic classes are being developed.

Pathogens need not be bacteria: this specification requires three very different agents. Puccinia graminis, a fungus, releases airborne urediniospores that land on wheat leaves; hyphae invade tissue and feed, producing rusty pustules that reduce photosynthesis and yield. Influenza virus is transmitted by airborne respiratory droplets and contact; its RNA is replicated inside host epithelial cells, damaging the respiratory tract and causing fever and coughing. Plasmodium spp. are protoctists transmitted when an infected female Anopheles mosquito bites and injects sporozoites; the parasites multiply in the liver and then in red blood cells, which burst, causing cyclical fever. For each agent, link transmission route, entry into host cells and the resulting damage. Detailed life cycles are not required.

Endemic malaria persists where the Plasmodium parasite, Anopheles vectors and human hosts coexist. Control methods include insecticide-treated bed nets, indoor residual spraying, draining or treating standing water, antimalarial drugs and vaccination. Each has social effects (bed nets depend on household behaviour and sleeping arrangements), economic effects (nets, sprays, drugs and vaccination cost money and may reduce workforce illness) and ethical effects (spraying homes requires consent; testing drugs or vaccines in affected communities requires informed consent and fair benefit-sharing). The scientific community validates methods by peer-reviewed trials, statistical analysis of incidence data and replication, so that policy rests on evidence rather than anecdote.

This section details the immune response. Macrophages engulf pathogens via phagocytosis, displaying antigens to become antigen-presenting cells. Neutrophils are phagocytic, destroying pathogens early in infection. Lymphocytes provide specific immunity. In the humoral response, antigen presentation to T cells occurs (covering the specification's 'antigen presenting T cells'), activating T helper cells. Clonal selection occurs when an antigen binds to a B cell with a complementary receptor. Cytokines from T helper cells then stimulate this B cell to divide by mitosis into plasma cells, which secrete antibodies. In the cell-mediated response, antigen-presenting cells activate T helper cells. These release cytokines stimulating T killer cells to destroy infected or abnormal host cells.

After an infection or vaccination, some cloned T and B cells become memory cells. In a secondary immune response, memory B cells rapidly produce plasma cells that secrete antibodies, while memory T cells rapidly differentiate into active T helper and T killer cells. Immunity can be natural or artificial, and active or passive. Natural active immunity arises from infection, while natural passive involves maternal antibodies. Artificial active immunity comes from vaccination, whereas artificial passive involves injected antibodies. Vaccination introduces antigens to stimulate active artificial immunity and can produce herd immunity when enough people are immune. If a proportion of a population chooses not to vaccinate, herd immunity may be lost and disease outbreaks become more likely.

In this core practical, you grow bacteria in liquid broth and track population change over time, for example by measuring turbidity with a colorimeter or by dilution plating. You must plan for safety: use aseptic technique, sterilise equipment, flame loops and container necks, keep cultures covered, and incubate at a temperature that limits risk to humans. Ethical use for microbes means choosing a non-pathogenic organism, ensuring safe containment, and disposing of cultures by autoclaving. You record readings at timed intervals, plot a growth curve, and use log-phase data to calculate an exponential growth rate constant using base 10 logarithms. Controls and repeats improve reliability, and you evaluate limitations such as turbidity not distinguishing live from dead cells.

Streak plating separates a mixed bacterial culture into single colonies so that an individual species can be isolated. A sterile loop is flamed, cooled, then streaked across one sector of a sterile agar plate; the loop is re-flamed and a fresh streak is drawn from the first sector into a second, then a third, each time reducing cell density. After incubation, well-separated colonies arise from single cells. Safe and ethical practice means aseptic technique, correct disposal and autoclaving, and treating all cultures as potential pathogens.