GCSE Biology
GCSE Biology
Embedded Files
Topic 3: Disease
Topic 3: Disease
Culturing microorganisms
Culturing microorganisms
Objectives
Objectives
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.
To describe the practical steps involved in investigating the effect of antiseptics or antibiotics on bacterial growth using agar plates.
Bacterial cell division
Bacterial cell division
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
Aseptic technique
Bacteria can be grown in two main ways:
Nutrient Broth Solution: A liquid medium that provides all the nutrients bacteria need to grow.
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.
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
Testing antibiotics or antiseptics experiment method
Testing antibiotics or antiseptics experiment method
Antibiotics are chemicals produced by microorganisms that can kill bacteria.
Different antibiotics work against different bacteria, so it's important to test their effectiveness.
When an antibiotic is placed on agar jelly, it diffuses outwards and can prevent bacterial growth or kill existing bacteria nearby.
Steps to Compare Different Antibiotics
Prepare the Petri Dish:
Mark the underside of a Petri dish with six spots in a circular pattern and label them A to F.
Spread Bacteria on the Agar:
Pour nutrient agar into the Petri dish using aseptic techniques.
Use a sterilised loop to spread a bacterial species evenly across the agar surface.
Apply the Antibiotics:
Soak small filter paper discs (1 cm diameter) in six different antibiotics, each at the same concentration.
Place each disc on an agar spot (A-F) and note which antibiotic is on each spot.
Incubate:
Incubate the dish at 25°C for two days.
Measure and Calculate:
Measure the diameter of the clear area around each disc where bacteria haven’t grown (this is called the inhibition zone).
Calculate the area of each inhibition zone using the formula: πr², where π is approximately 3.14, and r is the radius (half the diameter).
Experimental setup
Practical skills - Cross-sectional area
Practical skills - Cross-sectional area
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
If the radius of the inhibition zone is 8 mm, the area can be calculated as:
Area = 8 x 8 x 3.14 = 201mm²
The larger the inhibition zone, the more effective the antibiotic is at killing or preventing the growth of that specific bacterial species.
Zones of inhibition.
Maths skills - Population size
Maths skills - Population size
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
Key words
Key words
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 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.
Antibiotics: Chemicals, often produced by microorganisms, that kill or prevent the growth of bacteria.
Nutrient agar: A type of agar gel that provides the nutrients needed for bacterial growth.
Incubate: To keep a culture at a specific temperature for a period to allow microbial growth.
Inhibition zone: A clear area around an antiseptic or antibiotic disc on an agar plate where bacteria have not grown.
Diameter: A straight line passing from side to side through the centre of a circle or sphere.
Radius: A straight line from the centre to the circumference of a circle (half the diameter).
Extension ideas
Extension ideas
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.
Antiseptic vs. Antibiotic Research: Research the difference between antiseptics and antibiotics, including where they are used and why you wouldn't use an antiseptic internally or an antibiotic externally for cleaning surfaces.
Antibiotic Resistance Investigation: Find out what antibiotic resistance is, how it develops, and why it is a major global health concern. You could research specific examples of resistant bacteria.
Related topics
Related topics
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