CULTURING microorganisms means growing them in controlled laboratory conditions so we can study them.
Culturing bacteria is essential for:
Testing the effectiveness of antibiotics and antiseptics.
Studying the growth and behaviour of pathogens.
Producing medicines, foods (yoghurt, bread, cheese) and industrial chemicals.
Researching new drugs and treatments.
Bacteria and fungi are grown on or in a CULTURE MEDIUM โ a substance containing all the nutrients the microorganism needs:
Carbon source (glucose or other sugar) for energy.
Nitrogen source (nitrates or amino acids) for protein synthesis.
Minerals and vitamins.
The most common culture medium is AGAR โ a gel made from seaweed. Nutrients are dissolved in warm liquid agar which is then poured into PETRI DISHES where it sets into a firm gel. Bacteria grow on the surface, forming visible colonies.
Preventing Contamination โ Aseptic Technique
Contamination means unwanted microorganisms getting into the culture โ this would give false results and could introduce dangerous pathogens.
ASEPTIC TECHNIQUE is the set of procedures used to prevent contamination:
STERILISING EQUIPMENT:
Petri dishes and culture media are sterilised using an AUTOCLAVE (high-pressure steam at 121ยฐC) โ kills all microorganisms including spores.
Glass equipment (beakers, loops) can be sterilised by heating in a Bunsen flame.
PROCEDURE:
Work near a Bunsen burner โ hot rising air carries contaminants away from the work area.
Flame the inoculating loop until red hot before AND after use โ kills all bacteria on it.
Flame the neck of the culture bottle or test tube before opening.
Lift the Petri dish lid only slightly and briefly โ never fully open it.
Seal Petri dishes with tape after inoculation โ prevents airborne contamination.
TEMPERATURE OF INCUBATION:
In school laboratories, cultures are incubated at a MAXIMUM of 25ยฐC โ not at body temperature (37ยฐC).
This is a safety precaution โ bacteria that thrive at 37ยฐC are more likely to be harmful human pathogens.
In industry and research, 37ยฐC may be used under strict controlled conditions.
Investigating Antibiotic and Antiseptic Effectiveness
Agar plates are used to test which antibiotics or antiseptics are most effective against bacteria.
METHOD:
1. Spread bacteria evenly over the surface of an agar plate (lawn of bacteria).
2. Place discs of filter paper soaked in different antibiotics/antiseptics on the agar surface.
3. Incubate at 25ยฐC for 24โ48 hours.
4. Observe and measure the CLEAR ZONES (inhibition zones) around each disc.
INHIBITION ZONE:
As the antibiotic/antiseptic diffuses outward from the disc, it kills or inhibits nearby bacteria.
A LARGER inhibition zone = MORE EFFECTIVE antibiotic/antiseptic at that concentration.
NO inhibition zone = the bacteria are RESISTANT to that antibiotic.
CALCULATING ZONE AREA:
Area = ฯ ร rยฒ (where r = radius of clear zone)
Compare zones to determine which substance is most effective.
CONTROL DISC: a disc soaked in distilled water only โ confirms that any inhibition is due to the substance, not the disc itself.
โ ๏ธ Common Mistake
Petri dishes in school are incubated at 25ยฐC โ NOT at 37ยฐC (body temperature). This is deliberately to reduce the risk of growing dangerous human pathogens. A larger inhibition zone means MORE effective antibiotic โ the bacteria cannot grow in that area. A disc with no clear zone = resistant bacteria.
๐ Key Equations
Area of inhibition zone = ฯ ร rยฒ
๐ Key Note
Culture medium: agar + nutrients in Petri dish. Aseptic technique: sterilise equipment, flame loops, work near Bunsen. Incubate at max 25ยฐC in school (safety). Inhibition zone: clear area around antibiotic disc โ larger = more effective.
๐ฏ Matching Activity โ Match the Aseptic Technique Step
Match each step to why it is done. โ drag the symbols on the right to match the component names on the left.
Autoclave Petri dishes and media
Drop here
Flame the inoculating loop
Drop here
Work near a Bunsen burner
Drop here
Seal Petri dish with tape
Drop here
Incubate at 25ยฐC not 37ยฐC
Drop here
Larger inhibition zone
Drop here
More effective antibiotic โ bacteria cannot grow in a wider area
Prevents airborne microorganisms from entering after inoculation
Sterilises the loop before and after use โ prevents contamination
Rising hot air carries airborne contaminants away from the work surface
Kills all microorganisms including spores before use
Safety โ reduces risk of growing dangerous human pathogens
โฝ FIFA Worked Examples
Inhibition Zone Area
An antibiotic disc produces a clear zone with a diameter of 18 mm. Calculate the area of the inhibition zone.
F
Area = ฯ ร rยฒ
I
Diameter = 18 mm โ radius = 9 mm. Area = ฯ ร 9ยฒ
F
Area = ฯ ร 81 = 254.47...
A
Area โ 254 mmยฒ
๐งช Required Practical
๐ฌ RP6 โ Required practical: investigate the effect of antiseptics or antibiotics on bacterial growth using agar plates and measuring inhibition zones.
Know the method, variables, equipment and how to analyse results.
๐ฏ Test Yourself
Question 1 of 3
1. Why are school bacterial cultures incubated at 25ยฐC rather than 37ยฐC?
2. What does a large inhibition zone around an antibiotic disc indicate?
3. Why is an inoculating loop flamed before and after transferring bacteria?
โญ How Well Do You Understand This Topic?
Be honest with yourself โ this helps you know what to revise!
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