Microorganisms

 

Bacteria reproduce by splitting in half. This process is called binary fission and it can allow a bacterial population to double every 20 minutes with enough food and warmth.  

 

Bacteria are grown in liquid nutrient broth or as visible colonies on agar plates. Agar is a jelly-like substance containing all the nutrients bacteria need to grow. 

 

If we culture microorganisms, this just means that we grow them under laboratory conditions.

 

We can use these methods to test the effectiveness of antibiotics and disinfects and it is therefore important to make sure your culture of bacteria is not contaminated.

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Aseptic Technique

 

How to Prepare an Uncontaminated Culture (Aseptic Technique)

• Sterilise petri dishes and agar before use to kill unwanted microbes.  

• Flame the inoculating loop (metal wire) until red-hot to sterilise it.  

• Work near a Bunsen burner flame to keep air clean.  

• Seal the lid with tape after adding bacteria — prevents contamination.  

• Incubate at 25°C in schools (safe — stops harmful pathogens growing at body temperature, 37°C).

 

You may be asked to calculate bacterial growth. You may need to use one of these formulas:

• Number of divisions = Total time ÷ Division time

• Final population = Initial number × 2ⁿ (n = number of divisions)

 

Worked Example:  

1 bacterium, divided every 10 minutes → how many after 30 minutes?  

30 ÷ 10 = 3 divisions → 1 × 2³ = 8 bacteria

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You may also be asked to calculate the area of a colony:

• First find the radium in mm

• Then use area =  πr²

 

Worked Example:

radius = 6 mm → Area = π × 6² = 113 mm²

Required Practical 2: Grow bacteria on agar and measure zones of inhibition around antibiotic discs.