4.5Unit 4: Energy, Environment, Microbiology and Immunity

Microorganisms & Immunity — IAL Edexcel Modular Revision Notes & Topical Questions

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4.5 Microorganisms and Immunity — IAL EDEXCEL MODULAR notes: Explain how microorganisms are cultured, counted and controlled, compare bacteria and viruses, and describe pathogens, the immune response, immunity, vaccines and antibiotics.Culturing — growing enough microorganisms to…

Requirements
Inoculation
Incubation
Aseptic technique
Pure culture
Haemocytometer

Section 1 of 13Culturing microorganisms and aseptic technique

1Culturing

Culturing growing enough microorganisms to make measurements, such as bacteria that form visible colonies of identical clones on agar.

2Requirements

Requirements nutrients, oxygen for aerobes, an optimum pH and a favourable temperature.
- (carbon, nitrogen, minerals)

3Media

Media a liquid broth or solid nutrient agar, a gel from seaweed, kept sterile until use.
A selective medium suits one type of microorganism and identifies mutants, antibiotic-resistant or genetically modified strains.
An indicator medium gives a colour change to distinguish the desired colonies.

4Inoculation

Inoculation a sterilised loop transfers microorganisms between agar and broth.
Cover dishes with a lid and seal flasks with sterile cotton wool, not airtight for aerobes.

5Incubation

Incubation about 20–25 °C prevents growth of human pathogens, which grow best at 37 °C.

6Safety

Safety sterilise equipment by heating or antimicrobial chemicals, keep cultures in the laboratory and sterilise sealed waste at high temperature and pressure.
Mutation could create pathogenic strains and the air could contaminate cultures.

7Aseptic technique

Aseptic technique flame loops and bottle necks, work near a Bunsen flame so rising air carries microbes away, disinfect surfaces, tape lids lightly and invert plates.

8Pure culture

Pure culture isolate one microorganism using aerobic or anaerobic conditions and a selective medium.

Section 2 of 13Measuring the growth of microorganisms

Haemocytometer

Haemocytometer a slide with a chamber of 0.1 mm³ marked by grid lines used to count single-celled organisms.
Dilute the broth with trypan blue, which stains dead cells blue so only living cells are counted.
Count cells in the four corner squares, counting cells on two borders and ignoring the others, and take the mean.
Cells per ml mean count × dilution factor × 10⁴.

Dilution plating

Dilution plating diluted broth is spread on agar so each colony arises from one viable cell.
Total viable count number of colonies × dilution factor, and the mean of several plates is used.
Each 1 in 10 step multiplies the dilution factor by 10.

Fungi

Fungi growth is measured by the diameter or area of the mycelium on agar, or by the dry mass of mycelium filtered from broth, dried and weighed.

Turbidimetry

Turbidimetry a colorimeter measures cloudiness, and more cells absorb more light.
A calibration curve of turbidity against haemocytometer counts estimates the cells in unknown samples.

Section 3 of 13Growth curve and the yeast practical

Binary fission
- the circular DNA and any plasmids replicate, then the cell divides into two identical daughter cells.

Phase	Description
Lag	Slow increase while the population adjusts to the new environment
Exponential (log)	Nutrients and space are plentiful and the population doubles with each division
Stationary	Births equal deaths because of limited resources and toxic waste
Death (decline)	Death rate exceeds reproduction as nutrients run out and waste builds up

Calculations

N = N₀ × 2ⁿ, where n is the number of generations.
- (time ÷ generation time)
More generally N_t = N₀ × 2^kt, where k is the growth rate constant and k = (log N_t − log N₀) ÷ (log 2 × t).

Log scales

Log scales they fit the wide range of numbers on one graph, and the intervals on the axis are uneven.

Sterilise the work area, then add 250 cm³ glucose solution to a conical flask as the medium.

Inoculate with 1.25 g dried yeast using aseptic technique near a Bunsen flame and seal with a cotton wool stopper.

Swirl and place on a magnetic stirrer, loosely cover with foil and incubate at 20 °C.

Zero the colorimeter using a cuvette of plain glucose solution.

Transfer about 3 cm³ of suspension to a cuvette and record absorbance against time.

Repeat over 12 hours, for example every 30 minutes at first, then every 2–3 hours, and plot absorbance against time.

Extension

Extension temperature or glucose concentration can be varied,
- and a haemocytometer can back up turbidity because dead cells and particles also affect turbidity.

Safety

Safety wear eye protection, use aseptic technique, incubate at 20 °C not 37 °C and disinfect afterwards.

Section 4 of 13Bacteria and viruses

Bacterium

Bacterium a single-celled prokaryote with a murein cell wall, no membrane-bound organelles, 70S ribosomes and a single circular chromosome.
Optional structures are plasmids, a slime capsule (protects against drying and immune attack), flagella, pili (attachment and gene transfer) and mesosomes.

Virus

Virus a non-cellular infectious particle with a nucleic acid core (DNA or RNA) in a protein capsid made of capsomeres.
Viruses have no plasma membrane, cytoplasm or ribosomes.
Some have a lipid envelope from the host membrane and attachment proteins that bind host cells.
They reproduce only inside living host cells.

Virus type	Features	Examples
DNA virus	Viral DNA is the template for new DNA and mRNA	Smallpox, adenoviruses, bacteriophages
RNA virus	Usually single-stranded RNA with no DNA; more mutations	Tobacco mosaic virus, Ebola
Retrovirus	RNA virus using reverse transcriptase to make DNA that integrase inserts into host DNA	HIV

1Lysogenic pathway

Lysogenic pathway viral DNA (provirus) is inserted into host DNA and a repressor stops expression, so it is copied with the host (latency).
Damage or low nutrient levels can activate it into the lytic pathway.

2Lytic pathway

Lytic pathway viral genes are expressed, new viruses assemble and the host cell bursts (lysis), releasing viruses and usually causing disease.

Section 5 of 13Tuberculosis and HIV

Pathogen

Pathogen an organism that causes disease, and infectious diseases are transmissible between individuals.

Tuberculosis

Tuberculosis caused by Mycobacterium tuberculosis, spread in droplets from coughs and sneezes and faster in overcrowded conditions.
Phagocytes in the lungs engulf the bacteria, which survive and reproduce inside them.
In a healthy person the infected phagocytes are encased in tubercles where bacteria stay dormant (primary infection).
Active TB develops if the immune system is weakened, for example by HIV, with fever, fatigue, cough and lung damage that can cause respiratory failure.

HIV

HIV the human immunodeficiency virus, a retrovirus passed in body fluids by sex, blood donation, shared needles and from mother to child across the placenta, at birth or in breast milk.

Tuberculosis and HIV

HIV attaches to a receptor on a helper T cell (CD4 cell) and the capsid releases RNA into the cell.

Reverse transcriptase makes complementary DNA from the RNA, which is made double-stranded and inserted into host DNA.

The host cell makes viral components that assemble into new viruses, which bud off and infect more helper T cells.

Stages

Stages acute flu-like illness, then a symptom-free chronic stage lasting years, then symptoms as helper T cells fall.
B cells are no longer activated and no antibodies are made.
AIDS is the final stage, with opportunistic infections such as TB that cause death.
Progression depends on existing infections, HIV strain, age and access to healthcare.

HIV evasion

HIV evasion it kills helper T cells and mutates its antigens, so memory cells for one strain do not recognise another.
It prevents infected cells presenting antigens on their surface.

TB evasion

TB evasion the bacteria stop lysosomes fusing with the phagocytic vacuole and disrupt antigen presentation.

Section 6 of 13Routes of entry and barriers

Routes

Routes vectors, inhalation (flu, measles, TB), ingestion (cholera, Salmonella), indirect contact via objects, direct contact (HIV, Ebola, syphilis) and inoculation through broken skin (hepatitis B, tetanus, rabies).
- (mosquitoes carrying malaria)

Skin

Skin a physical barrier, and blood clotting seals breaks although a few pathogens enter before a clot forms.

Gut and skin flora

Gut and skin flora harmless microorganisms compete with pathogens for resources.

Stomach acid

Stomach acid hydrochloric acid is unfavourable to many pathogens in food and drink.

Lysozyme

Lysozyme an enzyme in tears, saliva and mucus that damages bacterial cell walls so the bacteria lyse.

Section 7 of 13Non-specific immune response

Antigens

Antigens surface molecules, such as proteins and glycoproteins, that identify a cell as self or non-self.

Inflammation

Inflammation mast cells release histamine at damaged tissue.
Vasodilation increases blood flow and capillary walls become leaky, causing swelling.
Phagocytes leave the blood to engulf particles and cells release cytokines to signal for an immune response.

Interferons

Interferons virus-infected cells make these antiviral proteins,
- which stop viral protein production, protect uninfected cells and activate white blood cells.

Phagocytosis

Phagocytosis chemicals from pathogens and histamine attract phagocytes, which recognise non-self antigens.

Non-specific immune response

The phagocyte membrane extends around the pathogen and engulfs it in a phagocytic vacuole (endocytosis).

Lysosomes fuse with the vacuole and release digestive enzymes including lysozyme.

The phagocyte presents the pathogen's antigens on its surface as an antigen-presenting cell, which starts the specific response.

Section 8 of 13Specific immune response

Specific immune response

Antigen-presenting cells

Antigen-presenting cells macrophages and other cells show antigens to lymphocytes, which bind them with specific receptors.

Antibody (immunoglobulin)

Antibody (immunoglobulin) a Y-shaped protein of four polypeptide chains, two heavy and two light, joined by disulfide bonds.

Constant regions are the same within a class, and five classes have different roles.
Variable regions differ between antibodies and form the antigen binding site that gives specificity.
A hinge region gives flexibility in some classes.
Plasma cells secrete antibodies, and membrane-bound antibodies sit on lymphocytes.

Antibody functions

Antibody functions they block pathogen receptors so host cells are not infected, neutralise toxins (anti-toxins) and cause agglutination,
which makes phagocytosis easier.

Cell	Origin and role
T helper	Matures in the thymus and releases cytokines that activate B cells and T killer cells and label pathogens for phagocytosis
T killer	Binds and destroys infected cells that display the specific antigen, for example with perforin pores
T memory	Stays in the blood for a faster response to the same pathogen
B cell	Matures in bone marrow with antibody receptors; when activated it divides into plasma cells and memory cells

Clonal selection
- a T or B cell binding its specific antigen divides by mitosis into clones with the same receptor.

Section 9 of 13Developing immunity and vaccines

Primary response

Primary response the first encounter is slow because few cells have the right receptors, so symptoms appear before plasma cells make antibodies.
T and B memory cells are made and remain after infection.

Secondary response

Secondary response memory cells respond faster and in larger numbers, producing more antibodies and often eliminating the pathogen before symptoms.

Active immunity

Active immunity the body makes its own antibodies and memory cells after antigen exposure.
Natural active immunity follows infection and artificial active immunity follows vaccination.

Passive immunity

Passive immunity antibodies come from another source, so there are no memory cells and protection is short-lived.
Natural passive immunity is antibodies across the placenta or in breast milk.
Artificial passive immunity is an injection of antibodies, such as the tetanus antitoxin.

Vaccine

Vaccine it contains antigens to induce artificial active immunity.
It may contain dead or weakened pathogens, less harmful strains, antigens alone or genetic material coding for antigens.
It is given by injection or by mouth and produces memory cells for long-term immunity.
Antigenic variation by mutation means vaccines may need to be updated.
Herd immunity protects unvaccinated people when enough of the population is immune.

Section 10 of 13Antibiotics and resistance

Antibiotics

Antibiotics chemicals that damage bacterial cells with little or no harm to human tissue, such as penicillin discovered by Fleming in 1928.
Bactericidal antibiotics kill bacteria and bacteriostatic antibiotics inhibit growth.
Penicillin blocks enzymes forming cell wall bonds, so the weakened wall bursts as water enters by osmosis.
Others bind ribosomes to stop protein synthesis, damage membranes or stop DNA coiling.

Selective action

Selective action human cells have no cell walls and have different enzymes and ribosomes, and viruses have none of these structures,
so antibiotics do not work on viruses.

Disinfect the work area and use a Bunsen flame for an updraft.

Spread bacterial culture over an agar plate with a sterile spreader or loop.

Place equal-sized discs soaked in different antibiotics, or different concentrations, on the plate with sterile forceps.

Add a negative control disc soaked in distilled water.

Tape the lid lightly, invert the plate and incubate at 25 °C for 24–48 hours.

1Interpretation

Interpretation a larger clear zone means a more effective antibiotic, area = πr², and no clear zone means resistance.

2Hospital-acquired infections

Hospital-acquired infections poor hygiene and weakened patients increase spread.
Hospitals use hand washing, isolation wards and disinfection of surfaces and equipment.

3Antibiotic resistance

Antibiotic resistance hospital antibiotic use is a selection pressure for resistant strains such as MRSA,
- which spread by horizontal gene transfer on plasmids.
Measures: no antibiotics for minor infections or viral diseases, no preventative use, narrow-spectrum antibiotics, rotating different antibiotics and completing courses.

4Evolutionary race

Evolutionary race hosts evolve immune defences while pathogens evolve ways to evade them.

Section 11 of 13More from the specification

Section 12 of 13Key terms

Term	Meaning
Pathogen	An organism that causes disease.
Aseptic technique	Methods that prevent contamination of cultures.
Antigen	A molecule on a cell surface that identifies it as self or non-self.
Antibody	A Y-shaped protein secreted by plasma cells that binds a specific antigen.
Memory cell	A lymphocyte that remains after infection to give a faster secondary response.
Retrovirus	An RNA virus that uses reverse transcriptase to make DNA.
Vaccine	A preparation containing antigens that induces artificial active immunity.
Horizontal gene transfer	Exchange of genes, such as resistance genes, between bacteria.

Section 13 of 13Exam technique

Saying antibiotics treat viral infections — viruses have no cell wall, ribosomes or enzymes of their own for antibiotics to target.

Calling it the HIV virus — HIV is the virus and AIDS is the disease.

Confusing active and passive immunity — only active immunity involves the body making memory cells.

Incubating cultures at 37 °C in school — use about 20–25 °C to avoid growing human pathogens.

Last updated: 15 October 2026