Culturing Microorganisms (AQA GCSE Biology): Revision Note
Exam code: 8461
Binary fission
Bacteria multiply by a type of simple cell division known as binary fission
In the right conditions, a bacterial cell prepares to divide by replicating its genetic material before it increases in size
A copy of each piece of circular DNA moves to each end of the cell before the cytoplasm divides, and new cell walls form around each daughter cell
Growing bacterial cultures in the lab
The effect of disinfectants and antibiotics on microorganisms can be investigated using cultures of bacteria grown in the lab
Under the right conditions, some species of bacteria can multiply as frequently as once every 20 minutes.
This is ideal for microbiologists as large cultures of bacteria for study can be grown in relatively short periods of time
To multiply this quickly, bacteria require an adequate supply of nutrients and an appropriate temperature in which to grow
Warmer temperatures promote faster growth, but in a school lab the maximum safe temperature for growth is 25 °C
Above this temperature, more harmful pathogens are likely to grow
Bacteria can be grown in a nutrient broth solution or as colonies on an agar gel plate
Uncontaminated cultures & aseptic techniques
It is vital that contamination free cultures of microorganisms are grown in the lab
Competing species can affect the growth of cultures, as well as the validity of any study performed
Some important aseptic techniques are outlined in the table below:
Technique | Explanation |
|---|---|
All work should be carried out in front of a lit Bunsen burner | Hot air rises, creating a convection current that prevents airborne microorganisms from landing on the plate |
Hot agar jelly is poured into a sterilised Petri dish. The agar is left to cool and set | The Petri dish and culture medium are heated to a high temperature to kill any microorganisms that could contaminate the experiment |
An inoculating loop is passed through a hot flame before it is used to transfer bacteria to the culture medium | Any microorganisms on the loop are killed by the high temperature |
The Petri dish lid should only be partly lifted when transferring cultures | This reduces the risk of airborne microorganisms landing on the plate |
The lid of the Petri dish should be secured with tape at intervals around the edges | This prevents the lid from falling off while still allowing oxygen to enter |
The dish should be stored upside down | Condensation from the lid does not drip onto the agar |
Cultures should not be incubated above 25 °C in a school laboratory | This restricts the growth of harmful pathogens, which are more likely to grow at higher temperatures |
Calculating inhibition zone area
The effectiveness of different antibiotics, antiseptics or disinfectants can be determined by calculating the area of an inhibition zone around a disc of the substance being tested
To calculate the area of an inhibition zone you should use the equation:
area of a circle = πr2
Examiner Tips and Tricks
It is far more accurate to measure the diameter of an inhibition zone than the radius, but remember to halve it before using the area equation above.
If the zone of inhibition is not perfectly circular then you will need to take at least two diameter measurements and find the mean.
Calculating bacteria in a population
The average length of time taken for a bacterial cell in a population to divide is the mean division time
It is possible to determine the number of divisions that have taken place as follows:
no. of divisions = time for which division has been taking place ÷ mean division time
The number of cells that have been produced can then be determined using the formula 2n, where n = number of divisions
Worked Example
If a bacterial cell has a mean division time of 24 minutes and has been dividing for 4 hours, how many cells will it have produced?
Answer
Step 1: convert units
4 hours = 240 minutes
Step 2: calculate the number of divisions
time for which division has been taking place ÷ mean division time
240 ÷ 24 = 10 divisions
Step 3: calculate the number of cells
number of cells = 210
= 1024 cells
Examiner Tips and Tricks
Check that both the mean division time and the time for which the cell has been dividing have the same units (either hours or minutes).
Calculations in standard form
Higher Tier Only
If you are calculating the number of bacteria present in a population, you are likely to be handling very large numbers
You should be able to express the number of bacteria in standard form
Worked Example
A bacterial cell has a mean division time of 1260 seconds and has been dividing for 6 hours.
Calculate the number of bacterial cells produced during this time. Give your answer in standard form.
Answer
Step 1: convert units to match
6 hours = 360 minutes = 21 600 seconds
Step 2: use formula to calculate number of divisions
time for which division has been taking place ÷ mean division time
= 21 600 ÷ 1260
= 17.1
= 17 divisions
Step 3: calculate the number of cells
number of cells = 217
= 131 072
Step 4: convert to standard form
131 072 = 1.31 x 105
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