Culturing microorganisms

Bacteria can multiply rapidly through a process called binary fission, where a single cell divides into two identical cells. If they have enough nutrients and the right temperature, bacteria can double in number as often as every 20 minutes.

Aseptic technique

Bacteria can be grown in two main ways:

1. Nutrient Broth Solution: A liquid medium that provides all the nutrients bacteria need to grow.

2. Agar Gel Plate: A solid surface in a Petri dish where bacteria grow in visible colonies.

When growing bacteria, it is essential to use aseptic techniques to avoid contamination by other microorganisms. Here’s how to ensure an uncontaminated culture:

• Sterilise Petri dishes and culture media before use to kill any unwanted microorganisms.

• Sterilise inoculating loops (used to transfer bacteria) by passing them through a flame to eliminate contaminants.

• Secure the Petri dish lid with adhesive tape to prevent other microorganisms from entering.

Recommendations to achieve aseptic technique

• Store the Petri dish upside down to stop condensation from dripping onto the culture.

• Incubate cultures at 25°C in school laboratories to reduce the risk of growing harmful pathogens.

Additional maths skills

Calculating Population Size: Using the mean division time, you can determine the number of bacteria in a population after a set period. Since bacteria double with each division, they grow exponentially. For example, starting with 1 cell:

• After 20 minutes: 2 cells

• After 40 minutes: 4 cells

• After 1 hour: 8 cells

• After 2 hours: 64 cells

• After 5 hours: 32,768 cells

Cross-sectional Area: When measuring bacterial colonies on an agar plate, calculate the area of colonies or clear zones around them using the formula: Area=πr²

Additional maths skills (Higher tier only)