Antibiotic selection and blue-white screening

Antibiotic selection and blue-white screening are two different questions that happen to share a Petri dish. Selection asks whether a cell is keeping a plasmid that confers resistance. The colour screen asks whether a short peptide encoded on that plasmid is still intact enough to complement a host enzyme. Neither question is "is this the DNA I designed?". This page is for people plating non-pathogenic E. coli K-12 cloning strains after a transformation. The culture and the competence step itself are in bacterial culture and transformation. Reagent classes sit in the reagents and chemicals catalogue, the surrounding methods on the molecular biology pathway, and any design question can be framed with the custom gene synthesis reference as an enquiry, not as a service this site claims to perform. Specifications go through the quote request.

The decision the plate is allowed to make

Use antibiotic agar to discard cells that did not take up a resistant plasmid, or that cannot express the marker. Use a blue-white overlay only when the vector and the host were built for alpha-complementation. Then stop. A clear no-DNA plate and a white, isolated colony are permission to test that colony. They are not a map of the insert. Laboratories that skip the test discover the error after a week of expression from the wrong plasmid.

The organisms in view are the same laboratory cloning strains as on the parent page: K-12 derivatives chosen for plasmid work. This is not guidance for plating clinical specimens or any organism your institution has not already accepted for the bench.

How antibiotic selection works

A resistance gene on the plasmid encodes a protein that modifies or destroys the drug, or that replaces a sensitive target. Without that protein, a sensitive host stops dividing at the dose written for that marker. With it, the transformant forms a colony. The host must actually be sensitive. A strain that already carries a chromosomal kanamycin cassette will grow on kanamycin whether or not your plasmid arrived. Read the genotype before you pour.

The drug has to survive preparation. Many antibiotics are heat-labile. They are mixed into agar or broth only after sterilisation and cooling. A plate poured with the drug already boiled is a rich medium with a label. Working concentration is specific to the marker, the copy number and the strain. Take it from the plasmid map and the antibiotic note you trust. Do not borrow a single number for every drug and every vector. Public cloning notes at Addgene show how laboratories record that pairing. Enzyme and reagent classes used to build the plasmid are surveyed in catalogues such as NEB's product list; those pages are orientation, not a transcribed protocol.

Ampicillin deserves a separate sentence because it fails in a distinctive way. The enzyme that confers resistance can leak into the agar and clear a halo. Neighbouring cells that never received the plasmid then form small satellite colonies. They look like a successful dense transformation. They are unselected growth. Pick only a well-isolated large colony, restreak onto fresh antibiotic agar if you are unsure, and do not interpret a halo of pinpricks as extra clones. Markers that inactivate the drug inside the cell, rather than destroying a cloud of it outside, give cleaner plates. They still need the right dose and plates that have not been stored until the drug is gone.

Plates age. Warm storage, light and time reduce effective dose. A plate that "used to work" can become a lawn. Date them, store them as the antibiotic requires, and keep a no-DNA control on the same batch. That control is the only honest test of the plate in front of you.

How blue and white colour is made

Many cloning vectors carry a short lacZ alpha fragment in the multiple-cloning site. The alpha peptide does not hydrolyse substrate by itself. The host supplies the rest of beta-galactosidase from a lacZ delta M15 allele. The two pieces associate, enzyme activity returns, and 5-bromo-4-chloro-3-indolyl-β-D-galactopyranoside, usually called X-gal, is cleaved to a blue insoluble pigment. An insert that interrupts the alpha reading frame usually abolishes that activity, so the colony stays the colour of the cells, which readers call white.

An inducer such as IPTG is often added because the lac promoter is repressed without it. Some vectors express the alpha fragment from a promoter that does not need IPTG. Adding inducer "because the paper did" can be unnecessary or, on a repressed system, essential. Know which promoter you have. The sequence of the cassette, if you need to check a named accession, can be looked up in GenBank. The accession does not tell you that your particular miniprep matches it.

X-gal is poorly soluble in water and is usually dissolved in a compatible organic solvent before it is spread or poured. The solvent volume should be small enough not to poison the lawn. The chromogenic substrate is light-sensitive and is not a dye you add after colonies have already been picked. Put it in or on the agar before you interpret colour.

A workflow with branch points

Pour or spread antibiotic agar and, if you are screening colour, X-gal and inducer as the vector requires. Plate a no-DNA control and, when you can, an empty-vector control beside the ligation or assembly. Incubate inverted. Read antibiotic selection first. If the no-DNA plate has colonies, stop. Colour on a non-selective plate is not selection.

If the no-DNA plate is clear and the empty vector is blue, then white colonies on the insert plates are candidates. If the empty vector is also white, the host may lack delta M15, the X-gal may be missing or degraded, or the plates were read too soon. Do not pick "whites" from a screen that cannot turn blue. If everything is blue, including wells you expected to be disrupted, consider an in-frame insert, a ligation that never accepted the insert, or an incubation so short that you are looking at background colour. Restreak a few colonies onto fresh screening agar before you trust a pale result.

Failure modes

White colonies that miniprep as empty vector usually mean the backbone was damaged in lacZ alpha, or you picked a satellite, or the colour had not developed. Blue colonies that nevertheless contain an insert are the in-frame case; design the screen knowing your insert length, or do not use colour as the decision. A lawn means selection failed: drug omitted, cooked, expired, or the strain was resistant already. Mixed blue-white colonies that were picked with a thick loop are two clones in one tube. Restreak. Toxic inserts can disappear entirely, so a plate of only blues can mean the recombinant never formed a colony. That is a cloning result, not a cue to incubate for three days and pick tiny satellites.

Confirm with a method that sees the DNA. A diagnostic restriction digest, a PCR that spans the junction, or a Sanger read will contradict a colour call often enough that the contradiction is the point of the assay. The colour got you to a shorter list.

Safety and research-use limits

Plates of K-12 cloning strains are still biological waste. Inactivate them by the method your institution specifies before disposal. Antibiotics and X-gal solvent are chemical hazards; the solvent in particular does not belong in a biosafety cabinet as if the cabinet were a fume hood. ATCC culture guides are a public reminder that culture material comes with handling conditions. This article is not medical advice, not a diagnostic identification of an organism, and not permission to plate an isolate that has not been assessed.

Climate and the life of a plate

In a hot, dry incubator, thin agar cracks and colonies stay small. In humid weather, plates weep and X-gal can crystallise on the lid. Store antibiotic stocks and poured plates as their chemistry requires, protected from a warm bench. A power cut that warms a cold room full of ampicillin plates is a reason to include a no-DNA control with the next session, not a reason to trust last month's pour. Develop colour at the temperature the strain grows, then move plates somewhere cooler if you want the pigment to finish without the colonies overgrowing. Write the hour you scored them. "White" without a time is an incomplete observation.

What to send with an enquiry

Name the antibiotic, the need for X-gal or inducer, the host genotype, and the orthogonal check you will run after the screen. Ask for grade and specification. Use the quote request and the reagents and chemicals catalogue. The custom gene synthesis reference is only a prompt if the insert itself must be designed. It does not mean a synthesis run is being offered as a performed service on the strength of this paragraph.