Culturing microorganisms

To describe the process of bacterial cell division by binary fission and understand the rate of bacterial reproduction.
To explain the importance of aseptic technique when growing bacterial cultures and list key steps to achieve it.
To apply mathematical skills to calculate bacterial population size and understand exponential growth.

This topic is for students studying the full biology course only

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.

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:
1. Sterilise Petri dishes and culture media before use to kill any unwanted microorganisms.
2. Sterilise inoculating loops (used to transfer bacteria) by passing them through a flame to eliminate contaminants.
3. Secure the Petri dish lid with adhesive tape to prevent other microorganisms from entering.
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.

Recommendations to achieve aseptic technique

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

Express large numbers, like bacterial population sizes, in standard form for simplicity.

Bacteria: Single-celled prokaryotic microorganisms that reproduce by binary fission.
Binary fission: The process by which a single bacterial cell divides into two identical daughter cells.
Nutrient broth: A liquid medium used to grow bacteria.
Agar gel plate: A solid medium in a Petri dish used to grow bacterial colonies.
Aseptic technique: Methods used to prevent contamination by unwanted microorganisms when working with cultures.
Uncontaminated culture: A culture containing only the desired microorganism, free from other microbes.
Sterilise: To make something free from bacteria or other microorganisms.
Inoculating loop: A tool used to transfer microorganisms to a culture medium.
Exponential growth: Growth that occurs at an increasingly rapid rate, as seen in bacterial populations doubling regularly.
Colony: A visible mass of bacteria growing on an agar plate, originating from a single bacterium or a few bacteria.

Bacterial Growth Calculation Challenge: Practice calculating bacterial population size after different time periods, given a starting number of cells and a division time (e.g., every 20 minutes). Use standard form for very large numbers.
Aseptic Technique Demonstration Video Analysis: Find online videos demonstrating aseptic technique in a laboratory setting. Watch carefully and list every step taken to prevent contamination, explaining why each step is important.