How are microorganisms cultured and counted, and how do bacterial populations grow?
Microbiology: the culturing of microorganisms; aseptic technique; the bacterial growth curve; methods of measuring population growth; and the action of antibiotics.
A focused answer to the Eduqas Component 1 statement on microbiology. Covers culturing microorganisms on agar, aseptic technique, the bacterial growth curve and its phases, methods of counting populations, and how antibiotics act.
Reviewed by: AI editorial process; not yet individually human-reviewed
Have a quick question? Jump to the Q&A page
Jump to a section
What this dot point is asking
Eduqas wants you to describe how microorganisms are cultured, explain aseptic technique, describe the bacterial growth curve and its phases, describe methods of measuring population growth, and explain how antibiotics act. This topic links the energy theme to disease and ecology.
Culturing microorganisms
Microorganisms are grown on a culture medium that supplies nutrients (a carbon source, nitrogen, minerals), water and the right temperature and pH. Nutrient agar is a solid medium poured into Petri dishes; nutrient broth is a liquid. Bacteria are added (inoculated) with a sterile loop or by spreading, then incubated.
Aseptic technique
The bacterial growth curve
In a closed culture (a fixed batch of medium), the population follows four phases:
- Lag phase: bacteria adjust and synthesise enzymes; little increase in number.
- Log (exponential) phase: plentiful nutrients allow maximum division, so numbers rise exponentially.
- Stationary phase: reproduction rate equals death rate (limited nutrients, accumulating toxic waste), so numbers level off.
- Death (decline) phase: nutrients run out and toxins accumulate, so the death rate exceeds reproduction and numbers fall.
Because growth in the log phase is exponential, it is plotted on a logarithmic scale to give a straight line.
Measuring population growth
- Total count: counts all cells, living and dead, for example using a haemocytometer (counting chamber) under a microscope, or by turbidity in a colorimeter.
- Viable count: counts only living cells that can reproduce, by serial dilution and spreading onto agar, then counting the colonies (each colony grows from one cell), and multiplying by the dilution factor.
The action of antibiotics
Antibiotics kill bacteria (bactericidal) or stop them reproducing (bacteriostatic), often by targeting structures bacteria have but human cells do not, such as the peptidoglycan cell wall (for example penicillin prevents cross-links forming, so the wall weakens and the cell bursts). Overuse drives the evolution of antibiotic resistance, which links to natural selection.
Examples in context
Example 1. Why antibiotic resistance spreads. A few bacteria carry a resistance allele; an antibiotic kills the susceptible ones, leaving the resistant ones to reproduce, so resistance becomes common. This is natural selection applied to microbiology and a major public-health issue.
Example 2. Industrial fermenters. Large-scale culturing (for antibiotics, enzymes or food) keeps bacteria in the log phase by continuously adding nutrients and removing waste, an open culture that avoids the stationary and death phases.
Try this
Q1. State two aseptic techniques used when inoculating an agar plate. [2 marks]
- Cue. Any two: flame the inoculating loop and cool it; work near a Bunsen flame; open the lid only slightly; sterilise equipment beforehand.
Q2. Name the phase of the bacterial growth curve in which the population rises exponentially. [1 mark]
- Cue. The log (exponential) phase.
Q3. Explain why a viable count may be lower than the true number of living cells. [2 marks]
- Cue. Cells can clump together, so a group of cells forms a single colony and is counted once, giving an underestimate.
Exam-style practice questions
Practice questions written in the style of WJEC Eduqas exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
Eduqas 20195 marksDescribe the aseptic techniques you would use when culturing bacteria on an agar plate, and explain why each is needed.Show worked answer →
Sterilise the inoculating loop by passing it through a Bunsen flame until red hot, then let it cool, to kill any microorganisms on it and avoid killing the culture.
Work near a Bunsen flame, which creates an updraught of air, to reduce contamination by airborne microorganisms.
Lift the lid of the Petri dish only slightly and at an angle, to limit the entry of airborne microorganisms.
Tape the lid on (but do not seal it completely) and incubate, usually at 25 degrees Celsius in a school, to prevent the growth of human pathogens that grow best at body temperature.
Markers reward flaming the loop, working near a flame, minimal lid opening, and a valid reason such as preventing contamination or limiting pathogen growth.
Eduqas 20214 marksDescribe the phases of a bacterial growth curve grown in a closed culture, explaining what happens to the population in each.Show worked answer →
Lag phase: the bacteria adjust to the new conditions, synthesising enzymes; there is little increase in number.
Log (exponential) phase: nutrients are plentiful and the bacteria divide at their maximum rate, so the population rises rapidly and exponentially.
Stationary phase: the rate of reproduction equals the rate of death, so the population stays roughly constant; this is caused by limited nutrients and the build-up of toxic waste.
Death (decline) phase: nutrients run out and toxins accumulate, so the death rate exceeds the reproduction rate and the population falls.
Markers reward correct naming and explanation of the lag, log, stationary and death phases.
Related dot points
- Respiration: glycolysis, the link reaction, the Krebs cycle and oxidative phosphorylation; the role of NAD and FAD; anaerobic respiration; and respiratory substrates.
A focused answer to the Eduqas Component 1 statement on respiration. Covers glycolysis, the link reaction, the Krebs cycle, oxidative phosphorylation and chemiosmosis, the role of NAD and FAD, anaerobic respiration, and respiratory substrates.
- Population size and ecosystems: factors limiting population size; sampling techniques; succession; the flow of energy through trophic levels; and the carbon and nitrogen cycles.
A focused answer to the Eduqas Component 1 statement on populations and ecosystems. Covers density-dependent and independent factors, sampling with quadrats and transects, succession, energy flow through trophic levels, and the carbon and nitrogen cycles.
- Human impact on the environment: the effects of deforestation, agriculture and pollution; eutrophication; the loss of biodiversity; climate change; and conservation and sustainability.
A focused answer to the Eduqas Component 1 statement on human impact. Covers deforestation and agriculture, eutrophication, the loss of biodiversity, climate change from greenhouse gases, and conservation and sustainability strategies.
- Option A Immunology and disease: pathogens and disease transmission; non-specific defences; the specific immune response; antibodies; active and passive immunity; vaccination; and antibiotics.
A focused answer to the Eduqas Component 3 Option A on immunology and disease. Covers pathogens, non-specific defences, the specific cellular and humoral immune response, antibodies, active and passive immunity, vaccination, herd immunity, and antibiotics.
- Enzymes: their role as biological catalysts; the lock-and-key and induced-fit models; the formation of enzyme-substrate complexes; the effects of temperature, pH, substrate concentration and enzyme concentration; and competitive and non-competitive inhibition.
A focused answer to the Eduqas Biology Core Concepts statement on enzymes. Covers enzymes as catalysts, the lock-and-key and induced-fit models, the four rate factors, denaturation, and competitive and non-competitive inhibition.
Sources & how we know this
- Eduqas A Level Biology Specification (A400) — Eduqas (2015)