5.16 - 5.19B - Antibiotics and antimicrobial practical
Explain why antibiotics can act selectively on bacteria, then design a fair antimicrobial comparison using aseptic cultures. Interpret clear zones cautiously and calculate circular bacterial-colony or clear-zone areas.
5.16 · Why antibiotics target bacteria
An infection can be caused by different kinds of pathogen. Before an antibiotic is used, the important biological question is whether the pathogen is a bacterium and whether it is susceptible to that antibiotic.
Antibiotic
An antibiotic is a medicine that kills susceptible bacteria or inhibits their growth by interfering with bacterial cell processes.
Bacteria are cells. They carry out processes such as building a bacterial cell wall and making proteins on bacterial ribosomes. Different antibiotics inhibit different bacterial processes. Human cells either lack the bacterial target, such as a bacterial cell wall, or have a sufficiently different version of the process. An appropriate antibiotic can therefore inhibit the bacterium without inhibiting host cells in the same way.
Viruses are not cells and do not carry out bacterial cell processes. They reproduce using machinery inside host cells, so an antibiotic that targets a bacterial process has no viral process on which to act. Antibiotics can therefore treat appropriate bacterial infections, but they cannot treat viral infections such as influenza.
5.17B · Keeping a culture uncontaminated
To investigate microorganisms, a known microorganism is grown on a sterile nutrient medium. Aseptic technique means handling the culture in a way that prevents unwanted microorganisms entering it and reduces the chance of cultured microorganisms contaminating the worker or surroundings.
Contamination is both a validity problem and a safety problem. An unwanted species could grow on the agar and change the clear zones, so the result would no longer show only the response of the intended microorganism. Some contaminants could also be harmful. 5.17B-5.19B are Biology-only, but they are required at both Foundation and Higher tier.
| Aseptic technique | What is done | Why it is done |
|---|---|---|
| Sterilise before use | A technician uses an autoclave, which uses steam under pressure, to prepare sterile growth medium and suitable Petri dishes. | Living contaminants are destroyed before the intended microorganism is added. |
| Use a sterile inoculating loop | Heat the loop before transfer, allow it to cool without touching a surface, transfer the microorganism, then sterilise it again. | The loop does not introduce another microorganism; cooling prevents the hot loop killing the intended microorganism; heating afterwards reduces release of the culture. |
| Work near a Bunsen flame | Open the culture only briefly near the flame and keep the lid over the opening as much as possible. | The upward air current reduces the chance of airborne microorganisms falling into the open culture. |
| Keep vessels covered | Replace vial caps promptly. Lift a Petri-dish lid only enough to work, then secure it with two or four small pieces of tape. | Covers reduce contamination entering and reduce escape of the culture. Small tape tabs prevent accidental opening without completely sealing the plate during incubation. |
| Incubate safely | Incubate the covered plate upside down at a teacher-specified school temperature, for example 25 degrees C, and do not reopen it after incubation. | Inverting stops condensation dripping across the agar; using no more than 30 degrees C reduces the chance of growing human pathogens. |
Sterile means that living microorganisms have been removed or destroyed. It is more precise than clean, which may only mean that visible dirt has been removed. The Bunsen flame is used during aseptic transfer; it must be extinguished before ethanol or another flammable solvent is handled.
5.18B · Comparing antimicrobial substances
Core practical 5.18B asks how an antiseptic, antibiotic or plant extract affects a microbial culture. A clear comparison changes one treatment factor, measures bacterial growth in a consistent way and keeps other factors constant.
One valid investigation asks: How does the type of antiseptic affect growth of one bacterial species?
| Variable role | Operational choice |
|---|---|
| Independent variable | Type of antiseptic on the filter-paper disc. A concentration series could instead be the independent variable, but do not change type and concentration in the same comparison. |
| Dependent variable | Mean cross-sectional area of the circular clear zone, in , after the same incubation time. |
| Controlled variables | Same microorganism and starting culture density, equal disc diameter, equal volume and concentration of each antiseptic where type is being compared, same agar composition and depth, and the same incubation temperature and time. |
| Control setup | A disc containing the solvent but no antiseptic. It shows whether the disc or solvent produces inhibition by itself. |
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