Culturing microorganisms allows scientists to investigate how bacteria grow, reproduce and respond to different conditions such as antibiotics or disinfectants.
In school laboratories, only non-pathogenic bacteria are used and strict safety procedures are followed.
Non-pathogenic bacteria are those that are not capable of causing disease, whereas pathogenic microorganisms are those that can cause disease.

Bacteria reproduce asexually by a process called binary fission.
Asexual reproduction involves one parent and produces genetically identical offspring called clones.
During binary fission, the bacterial DNA replicates and the cell divides into two genetically identical daughter cells.
The steps for binary fission are:
- The process begins with one bacterial cell containing a circular loop of DNA.
- The DNA replicates so that two identical copies are present.
- The cell grows slightly larger and the DNA copies move to opposite sides of the cell.
- The cell membrane and cell wall pinch inward, dividing the cell into two identical daughter cells.
Under favourable conditions, some bacteria can divide approximately every 20 minutes.
Repeated binary fission leads to rapid population growth.
Bacterial growth depends on environmental conditions such as nutrient availability and temperature.
Bacteria require nutrients including a carbon source, nitrogen and mineral ions to grow and divide. If nutrients become limited, growth slows down.
Each species has an optimum temperature at which its enzymes work most efficiently.
High temperatures can denature enzymes and kill bacteria, whereas low temperatures slow enzyme activity and reproduction.

Bacteria can be cultured in nutrient broth (a liquid medium) or on agar gel plates (a solid medium).
Culturing bacteria means growing them in controlled laboratory conditions.
Scientists provide suitable nutrients, temperature and environment for bacterial reproduction.
Nutrient broth is a liquid growth medium that contains dissolved nutrients such as sugars, amino acids, vitamins and minerals.
The nutrients within nutrient broth provide energy and raw materials for the bacteria to grow and reproduce.
In nutrient broth, bacteria spread throughout the liquid as they multiply.
Agar is a jelly-like substance made from seaweed that contains nutrients needed for bacterial growth.
When grown on agar, bacteria form visible clusters called colonies.
Each colony usually develops from a single bacterium that has multiplied many times.
Agar plates can be used to test the effectiveness of antibiotics or disinfectants.
Antibiotics are chemical substances that kill bacteria or prevent them from growing.
Small discs soaked in antibiotics, called antimicrobial discs, are placed on the agar surface.
If bacterial growth is prevented, a clear area called a zone of inhibition forms around the disc.
A control plate is used for comparison to show how the bacteria grow when no antibiotic or disinfectant is present.
A solid medium is a nutrient material used to grow microorganisms that has been solidified, usually by adding agar.
A solid medium keeps cells in one place so they grow into separate visible colonies, whereas a liquid medium allows microorganisms to move freely.

Contamination occurs when unwanted microorganisms enter the culture from the air, equipment, or the experimenter.
Contamination can make experimental results unreliable and may allow harmful microorganisms to spread.
Aseptic techniques are used to prevent contamination and ensure reliable and safe results.
Petri dishes and culture media are sterilised before use to destroy unwanted microorganisms.
Inoculating loops are heated until red-hot to kill microbes before transferring bacteria to the culture.
After transferring bacteria, the lid of the Petri dish is secured with tape to reduce contamination.
Plates are incubated upside down to prevent condensation dripping onto the agar surface.
In school laboratories, cultures are incubated at 25°C as a safety precaution.

The effectiveness of an antimicrobial substance can be assessed by measuring the diameter of the zone of inhibition.
The area of a circular zone can be calculated using the formula \text{Area} = \pi r^2, where is the radius of the circle (half the diameter).

To estimate population growth:
- Ensure total time and mean division time are in the same units.
- Divide total time by mean division time to calculate the number of divisions.
- The population doubles with each division.
- The number of bacteria can be calculated using the formula: \text{Final Number of Cells} = \text{Starting Number} \times 2^n (where n is the number of divisions).
Rapid doubling explains why bacterial infections can spread quickly.

Very large or small measurements may need to be expressed in standard form.
The units of area depend on the units used for the radius, typically mm² or cm².
Units must remain consistent throughout calculations to maintain accuracy.

Module Review

Summary

Revision

Glossary

1.1.1 Eukaryotes and Prokaryotes

1.1.2 Cell Specialisation

1.1.3 Cell Differentiation

1.1.4 Microscopy

1.1.5 Culturing Microorganisms