Hi there!
This is from my understanding.
There are two ways in detecting transformed bacterium:
1. Via a lacz gene (or another equivalent gene)
2. Via another antibiotic (which is what your teacher has said)
Essentially you have one for antibiotic resistance, and the other a "marker" gene. This "marker" gene has to be disrupted because the foreign, desired gene is inserted meaning it cannot be transcribed.
Thus there are also two ways in creating recombinant plasmids:
1. You can have a plasmid with 1 gene for antibiotic resistance and the other a lacz gene.
2. You can have a plasmid with 2 different genes for antibiotic resistance
The lacz gene is translated into an enzyme that catalyses
colourless X-gal into product that is
blue. (This is very important!)
Then as you know, when creating transformed bacteria we get three possible outcomes.
With two antibiotic-resistant genes[Ampicillin gene and tetracycline gene (marker gene)]
1. No plasmid at all.
> These will be killed in a dish with antibiotics (eg, ampicillin) because they do not have any genes for antibiotic resistance
2. Plasmid but without the foreign gene. Therefore "marker" gene is not disrupted.
> They will not be killed by ampicillin because they have the antibiotic resistance gene. They will also not be killed by tetracycline as this gene is not disrupted. This bacterium is not transformed.
3. Plasmid but with foreign gene. Therefore "marker" gene is disrupted.
> They will not be killed by ampicillin because they have the antibiotic resistance gene. They will be killed by tetracycline as this gene is disrupted. This bacterium is transformed.
With lacz gene and antibiotic-resistant gene[Ampicillin gene and lacZ gene]
1. No plasmid at all.
> These will be killed in a dish with antibiotics (eg, ampicillin) because they do not have any genes for antibiotic resistance
2. Plasmid but without the foreign gene. Therefore "marker" gene is not disrupted.
> They will not be killed by ampicillin because they have the antibiotic resistance gene. LacZ gene is translated as the gene is not disrupted. X gal is formed into a blue product. So these bacteria colonies will be blue.
3. Plasmid but with foreign gene. Therefore "marker" gene is disrupted.
> They will not be killed by ampicillin because they have the antibiotic resistance gene. LacZ gene is not translated as the gene is disrupted. X gal remains colourless. So these bacteria colonies will be colourless.
Therefore, the colourless colonies are those with the transformed plasmid.
TLDR; there are multiple methods in finding out whether or not the bacterium has the recombinant plasmid. You can have one or two antibiotics exposure depending on the genes the plasmid has.