What you need to know
- 6.1Principles and techniques of culturing microorganisms using aseptic techniqueMethods that stop unwanted microorganisms contaminating a culture and stop cultures escaping, such as flaming loops and using sterile equipment..
CP9 used aseptic method to test plant extracts. This lesson is the culture principles themselves. CP13 applies them to a liquid growth curve. Do not copy a branded SOP.
By the end, you should be able to…
- Define aseptic technique as keeping unwanted microbes out and cultured microbes in.
- Describe sterilising loops, media and waste.
- Explain Bunsen updraught, lid angle and why dishes are not fully sealed.
Build your understanding
Two directions of contamination
aseptic techniqueMethods that stop unwanted microorganisms contaminating a culture and stop cultures escaping, such as flaming loops and using sterile equipment. uses sterile equipment and working methods so that unwanted microorganisms do not contaminate the culture, and so that cultured organisms do not escape to infect people or the room. Both directions matter. A “clean-looking” bench is not enough.
Work with the non-pathogenic speciesA group of organisms that can interbreed to produce fertile offspring. the school issues. Incubate plates at about 25 °C, not 37 °C, so human-body pathogens are less likely to be selected. Never open an incubated plate to sniff it.
Kill, then transfer
An autoclaveA machine that sterilises equipment and media with steam under pressure, at about 121 °C. uses high-pressure steam to sterilise media and, after the practical, to kill cultures before disposal. Nutrient agar is autoclaved so that later colonies come from the inoculum, not from the agar. An inoculating loop is flamed until red-hot, then cooled in the air without touching a surface, then used.
Flame the neck of a bottle as you open and close it. Hold a Petri-dish lid as a shield rather than placing it on the bench. Work in the updraught of a lit Bunsen so rising sterile air makes it harder for microbes to fall into the open dish.
Incubate and finish
Do not tape a dish completely shut: a small air gap lets aerobes obtain oxygen and reduces selection for anaerobes that can include pathogens. Incubate inverted if that is the local method, then autoclave or disinfect all contaminated material. Wash hands.
A full kitchen recipe with brand names is not the specification. Principles: sterile media, sterile tools, protected transfer, safe incubation temperature, safe kill at the end.
NOT: aseptic technique is not only for the scientist. Contaminants also make a growth measurement meaningless.
- Work only with the non-pathogenic species the school issues
- A completely sealed plate favours some dangerous anaerobes
- Do not open an incubated plate to smell it
- The Bunsen updraught is one school method, not a different principle
- 25 °C reduces selection for human-body pathogens
Two traps this diagram blocks
- Do not incubate the school plate at body temperature.
- Do not seal the lid all the way around.
From the diagram alone
Question
Explain two aseptic precautions when inoculating an agar plate, and state the incubation temperature expected in school. [4]
Show a model answer
The loop is flamed and cooled in air, and the lid is held as a shield so contaminants do not fall in. Bottle necks may be flamed. The plate is incubated near 25 °C, not at 37 °C, and it is not sealed completely.
| Action | What it prevents |
|---|---|
| Autoclave the medium | Colonies that did not come from your inoculum |
| Flame and cool the loop | Killing the sample, or picking up bench microbes |
| Bunsen updraught and lid as shield | Airborne contaminants falling in |
| Incubate ~25 °C, not fully sealed | Selecting human pathogens or anaerobes |
| Autoclave waste | Living cultures leaving the lab |
Make your reasoning visible
Say both jobs of aseptic technique
Protect the culture and protect people.
Cool the loop
A red-hot loop sterilises itself and then cooks the inoculum if you do not wait.
Key explanations
- Do not seal a dish completely. A small air gap supplies oxygen and avoids selecting for anaerobes.
- All contaminated material is autoclaved or disinfected. Hands are washed.
Avoid this misconception
The claim: aseptic techniqueMethods that stop unwanted microorganisms contaminating a culture and stop cultures escaping, such as flaming loops and using sterile equipment. is only to keep the scientist safe.
The correction: It also stops contaminants spoiling the culture, which would make any growth measurement meaningless.
Order aseptic technique
Aseptic technique keeps the culture pure and keeps microorganisms off you. The order matters. Put the steps of streaking a plate in order.
Order the steps
What to do
- Reorder the cards below: drag a card (on a touch screen, drag its number), tap a card then tap where it goes, or use the ↑ and ↓ arrows at its right.
- Press Check under the cards to mark each one green (right place) or red; the count shows how many are right.
- Stuck? Hint first outlines the first wrong place, then gives a clue, then places that step for you.
- Shuffle mixes the cards again; Start again returns them to the starting order. Keyboard: Space picks up, arrow keys move, Space drops.
You are streaking a plate from a broth culture. Put the steps in order.
0 of 7 in the right place
Work near a lit Bunsen burner throughout; the updraught carries microbes away from the plate. Disposal (autoclaving the cultures) comes after the investigation and is not ordered here.
Check your understanding
Try each question before opening the suggested answer.
Check your understanding (continued)
Practice question
Retrieval and explanation · suggested answer
State what is meant by aseptic technique when culturing microorganisms.
Show suggested answer
Sterile equipment and methods so that unwanted microbes neither contaminate the culture nor escape from it.
Practice question
Retrieval and explanation · suggested answer
How is an inoculating loop sterilised before a transfer?
Show suggested answer
Heat it in a Bunsen flame until red-hot, then allow it to cool without touching any surface.
Practice question
Retrieval and explanation · suggested answer
Why autoclave nutrient agar before pouring plates?
Show suggested answer
High-pressure steam kills microbes and spores in the medium, so later colonies come from the inoculum.
Practice question
Retrieval and explanation · suggested answer
Why should a Petri dish not be completely sealed?
Show suggested answer
A small air gap supplies oxygen for aerobes and reduces the chance of selecting anaerobic pathogens.
Take the idea further
Apply the same specification knowledge to an unfamiliar problem. Any extra assumptions are supplied in the question.
Stretch question
A student measures turbidity and therefore says aseptic sampling is unnecessary because “we are not plating”. Correct the reasoning.
Show a hint
Both directions of contamination still apply.
Explore the reasoning
Each sample can contaminate the remaining culture and the bench. Contaminants also scatter light, so turbidity from mixed species is not a growth curve of the intended organism. Aseptic sampling still matters.
Recall this lesson
Lesson recall questions
Question 1 of 5: List two reasons for using aseptic technique when culturing microorganisms.
Aseptic culture of microorganisms
Practice question
Retrieval and explanation · suggested answer
List two reasons for using aseptic technique when culturing microorganisms.
Lesson: 6A 4: Microbial Techniques
Try this from memory first.
Put your biology to work
Explore where these ideas appear in the world. Enrichment, not extra required recall.
The same principles as a professional bench
6A · Microbial TechniquesThe Microbiology Society’s public explanation of microbes is that they are everywhere people, air and surfaces are. School aseptic rules are a local version of that fact: assume…Specification 6.1Read & use the scienceClose article
Why this matters
The Microbiology Society’s public explanation of microbes is that they are everywhere people, air and surfaces are. School aseptic rules are a local version of that fact: assume contaminants, then use heat, steam and barriers. CLEAPSS and a teacher’s risk assessment still govern what you actually do; this page is not a licence to culture unknowns. A clean result is never proof that no contaminant entered; it is evidence within a detection limit. Professional laboratories also use controls and documented procedures so an unexpected colony can be investigated.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.1 understand the principles and techniques involved in culturing microorganisms, using aseptic technique
Use the science
Why is “I washed my hands, so the plate can sit open” incomplete?
Explore a possible response
Hands are only one source. Air, lids and loops can still add or release microbes. Aseptic technique is a set of barriers, not one wash.
Explore the source
- Microbiology Society: what is microbiologyMicrobiology Society
A cleanroom is aseptic technique at scale
6A · Microbial TechniquesThe FDA’s guidance for sterile drug production treats cleanroom air, equipment, operators and materials as potential sources of contamination. The principle is the same as school aseptic transfer:…Specification 6.1Read & use the scienceClose article
Why this matters
The FDA’s guidance for sterile drug production treats cleanroom air, equipment, operators and materials as potential sources of contamination. The principle is the same as school aseptic transfer: reduce the chance that an unwanted microorganism enters the culture or product. Professional teams also monitor the environment and validate the process rather than assuming that a white coat guarantees sterility. This connection is about risk control, not instructions for culturing unknown organisms. For a school practical, the teacher’s approved safety procedure remains essential. A clean-looking sample can still contain invisible contaminants.
Back to the specification
Pearson Edexcel IAL Biology · Issue 2, February 2021 · Unit 4, pp. 29–32. The application extends beyond required recall.
- 6.1 understand the principles and techniques involved in culturing microorganisms, using aseptic technique
Use the science
Why would sterilising a transfer loop be insufficient if the vessel were left open in unfiltered air?
Explore a possible response
Microorganisms or particles could enter from the air, so asepsis requires control of several contamination routes, not only the loop.
Explore the source
- Sterile drug products produced by aseptic processingUS Food and Drug Administration