22 OCT 2026

To test the isolates that grew on tetracycline for ampicillin resistance by streaking them onto the amp plates poured on October 21.

• Tetracycline streak plates with growth (streaked Oct 20)

• Ampicillin (amp) selection plates (poured Oct 21)

• Sterile inoculation loops

Isolation Streaking

• Labelled 36 amp plates to match their source isolates on the tetracycline plates.

• In a fumehood, use a fresh sterile loop/stick for every quadrant, picked an isolated colony from each tetracycline plate and streaked it onto its matching amp plate (36 plates total).

• Let the plates dry on the bench before inverting.

• Inverted all 36 plates and incubated them overnight at 37°C.

Why this test?

Streaking the tetracycline-positive isolates onto ampicillin tests whether their resistance extends to another β-lactam antibiotic (see Oct 21). Each amp plate was labelled to match its source isolate, using the unique-ID system introduced after the Oct 08 labelling error.

What I will look for?

Isolates that grow on ampicillin as well as on IP or MP and tet are the most promising clones. A typical next step for those clones would be to extract the plasmid and sequence the DNA insert, to identify the gene responsible for the resistance.

Next step

Read the plates after overnight incubation.