Culturing microorganisms: growth and contamination are separate
הוצג| English | משמעות |
|---|---|
| aseptic technique/æˈseptɪk tekˈniːk/ | טכניקה אספטית |
| binary fission/ˈbaɪnəri ˈfɪʃn/ | חלוקה בינארית |
| antibiotic/ˌæntɪbaɪˈɒtɪk/ | אנטיביוטיק |
| disinfectant/ˌdɪsɪnˈfektənt/ | חומר חיטוי |
What would explain this observation?
- An extra organism on an agar plate can change a test result. Aseptic technique 无菌操作 reduces contamination so the effect of the tested substance can be interpreted.
Build the model
- Bacteria divide by binary fission 二分裂.
- With enough nutrients and suitable temperature, some can divide every twenty minutes, but a supplied mean division time should be used for a particular calculation.
- They can grow in nutrient broth or form colonies on agar.
- An uncontaminated culture is needed to compare antibiotic 抗生素 or disinfectant 消毒剂 effects.
- binary fission: Division of a bacterial cell into two cells.
- aseptic technique: Methods that reduce contamination of a culture.
- antibiotic: A medicine that kills bacteria or inhibits their growth.
- disinfectant: A substance used to kill or reduce microorganisms on surfaces.
Why are school cultures generally incubated at 25 °C?
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
Pathogens of humans grow best near 37 °C, so school incubation at 25 °C makes dangerous growth less likely while still allowing culture growth. It kills nothing and guarantees no fixed division time.
Match each technical term to its precise meaning.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Use the definitions to distinguish related quantities and processes.
Choose evidence that can test it
- Sterilized dishes and media reduce unwanted microorganisms.
- Sterilizing an inoculating loop prevents it transferring contaminants.
- The lid is secured with small pieces of tape rather than sealed all around; the plate is stored upside down to reduce condensation falling onto the agar.
- School cultures are generally incubated at 25 °C to reduce the chance of growing harmful human pathogens.
In the lab: Use only the school’s approved non-pathogenic organism, apparatus and supervised protocol. Students explain the aseptic controls and record labelled plate observations; staff manage approved sterilization and disposal. Do not reopen incubated cultures. Keep organism, medium, temperature and exposure time the same in comparisons.
Which two habits make the investigation or model in this case more defensible?
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
Stating the conditions and assumptions lets others repeat the work exactly, and recording the supporting evidence lets them check the conclusion. Hiding disagreeing results removes the evidence that could challenge a wrong conclusion, and one measurement can never establish a wider claim.
Work from known quantities
- Known: a model starts with 40 bacteria and has a mean division time of 30 min.
- After 90 min there are three complete divisions.
- Population = 40×2³ = 320.
- This ideal model assumes every cell divides on schedule without death or nutrient limitation.
- In this specification, expressing this culture-growth result in standard form is a Higher-tier extension.
Your turn: A model starts with 25 bacteria and completes four division rounds. Find its population.
A model starts with 25 bacteria and completes four division rounds. Find its population.
רמת קושי 2/5 · 140 מטבעות + בונוס קטן בהשלמה הראשונה היום
The result is 400 cells. Known: 25 bacteria; four division rounds, each doubling the population. Population = 25 x 2^4 = 25 x 16 = 400 cells. This ideal model assumes no death and unlimited nutrients.
Check the conclusion and its limits
- Twenty minutes is not a universal division time.
- An apparently clear plate is not proof of sterility, and a contaminated culture cannot be made a fair comparison by ignoring the extra colonies.
- Practical precautions protect people and the validity of the evidence.
Not true: Every bacterial species divides once every twenty minutes under all conditions. Division time depends on the species and the conditions - temperature, nutrients, oxygen. Twenty minutes is a best-case figure for some bacteria, not a universal constant.
- Bacteria divide by binary fission.
- Sterilized dishes and media reduce unwanted microorganisms.
- Watch for: twenty minutes is not a universal division time.
Division time depends on the species and the conditions - temperature, nutrients, oxygen.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
True. Division time depends on the species and the conditions - temperature, nutrients, oxygen. Twenty minutes is a best-case figure for some bacteria, not a universal constant.
Division of a bacterial cell into two cells: write the technical term.
רמת קושי 1/5 · 100 מטבעות + בונוס קטן בהשלמה הראשונה היום
binary fission means Division of a bacterial cell into two cells.