Antibiotics on a Petri Dish
In a school lab, a student streaks a bacterial culture evenly across four agar plates and places paper discs soaked in different antibiotics at the centre of each plate. After incubation at 37 °C for 24 h, plate 1 shows a large clear zone around the disc, plate 2 a small clear zone, plate 3 no clear zone, and plate 4 a clear zone with a few isolated colonies growing inside it. The student concludes: 'Plate 3's antibiotic is useless, and plate 4 shows the antibiotic failed.' The teacher notes that the discs were placed on plates that had not been dried, and that the incubation time was longer than the standard protocol.
1. Interpret the results on plates 1–4, and explain why 'no clear zone' on plate 3 does not necessarily mean the antibiotic is useless.
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The clear zone (zone of inhibition) indicates growth inhibition - the larger the zone, the more susceptible the organism (for a given disc load and diffusion).
- Plate 1 - large zone: organism is susceptible to that antibiotic at that concentration.
- Plate 2 - small zone: reduced susceptibility (or poor diffusion / lower potency).
- Plate 3 - no zone: the antibiotic did not inhibit this organism under these conditions. But 'no zone' has several causes besides true resistance:
- the antibiotic may not diffuse well in the agar (large/hydrophobic molecule),
- the disc may be underloaded or degraded,
- the organism may be intrinsically impermeable to it (not the same as acquired resistance),
- the antibiotic's mechanism may not apply (e.g. it targets a structure the organism lacks).
So 'useless' is an overclaim: the correct statement is 'no inhibition observed in this assay'.
2. Explain the colonies growing inside the clear zone on plate 4. What does this indicate, and what confounders could produce a false impression of resistance?
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Isolated colonies inside the inhibition zone are most likely resistant mutants (or a resistant subpopulation) that survived and grew where the rest were inhibited - the classic appearance of acquired resistance in a disc-diffusion assay.
However, before calling it resistance, rule out confounders the teacher flagged:
- Undried plates / excess surface moisture let bacteria spread across the surface, so colonies can appear where inhibition should be - a lawn artefact, not resistance.
- Too-long incubation allows slow-growing or marginally inhibited cells to form visible colonies, and can let resistant subclones emerge.
- Inoculum too dense or uneven streaking can produce satellite colonies.
Interpretation: a few colonies inside a zone suggests resistance but must be confirmed by re-streaking the colonies and re-testing (e.g. MIC determination), not by a single plate. The correct conclusion is 'possible resistant subpopulation; confirm by repeat testing under standardised conditions'.
3. List the standardisation steps that should be controlled to make these four plates comparable, and explain why each matters.
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To compare zones fairly, control:
- Inoculum density - standardise turbidity (e.g. a defined McFarland standard) so the starting cell number is the same; denser inoculum gives smaller zones.
- Agar depth and drying - uniform depth and properly dried plates prevent surface spreading and uneven diffusion.
- Disc antibiotic load - same amount per disc; zone size scales with dose.
- Incubation time and temperature - fixed (e.g. 37 °C, standard hours); longer incubation enlarges zones and lets resistant colonies appear.
- Disc placement and spacing - well separated to avoid overlapping zones and diffusion interference.
- Same medium and pH - affects antibiotic activity and bacterial growth.
Why it matters: zone size is a quantitative readout; if any of these vary between plates, differences in zone size reflect the method, not the organism's susceptibility. Only with all controlled can plates 1–4 be ranked meaningfully, and only then can 'susceptible / reduced / resistant' be assigned.