What blue and white colonies can and cannot mean

Colour on a cloning plate is a report about one peptide, not a report about your insert. Blue and white screening applies alpha-complementation of beta-galactosidase in a non-pathogenic E. coli host that can support it. The decision this page supports is which claim a colour allows, and which claim has to wait for a digest or a sequence. The antibiotic underneath the colour is explained in antibiotic selection and blue-white screening. The path from a candidate colony to a verified plasmid is plasmid cloning from insert to colony. Growing the host at all is bacterial culture and transformation.

The hero photograph shows Erlenmeyer flasks of amber culture on a shaker. Those flasks are what you start after you have picked a candidate, and they are a poor place to discover that the colony was white only because the plate had no substrate. Pick from a plate you can still interpret. Reagent classes, including the substrate and the inducer as classes rather than as a house recipe, are in the reagents and chemicals catalogue. The workflow is the molecular biology pathway. If the insert sequence is still being designed, custom gene synthesis is a reference for that design question, not a statement that synthesis is running. Specifications go through the quote request.

The claim colour can support

You may claim that, on this batch of plates, this colony did or did not restore beta-galactosidase activity through alpha-complementation. You may claim that, if the empty-vector control turned blue and the no-DNA control stayed bare. You may not claim that a white colony contains the intended insert, in the intended orientation, with the intended junctions. You may not claim that a blue colony is empty if a small in-frame insert would have left the peptide working. Colour is a screen that throws away some of the empties when the system is healthy. The Addgene molecular biology reference discusses these cloning screens as classes. The colony is still a candidate until a restriction digest or a sequence says otherwise.

How alpha-complementation produces a pigment

The host carries a lacZ allele, commonly written lacZ delta M15, that makes an inactive enzyme missing a small N-terminal piece. The vector carries that missing piece, the alpha peptide, with a multiple cloning site inside the peptide's coding sequence. When the peptide is produced and is intact enough, the pieces complement and enzyme activity returns. X-gal is a chromogenic substrate. Cells that cleave it go on to a blue pigment. No concentration table belongs on this page. Follow the vector and the substrate sheet for how the reagent is applied. Some methods mix it into cooled agar. Some spread it on the surface. The sheet you have is the method.

IPTG is an inducer of the lac promoter on many of these vectors. Some vectors make enough alpha peptide without it. If your vector needs the inducer and the plate has none, the blue path may never start, and empties look white. If the plate has no X-gal, nothing turns blue for a different reason. If the host is not a complementation host, the peptide has nothing to complement, and the whole plate stays white while the antibiotic, if it is real, still selects for the plasmid. Genotype is part of the screen. A beautiful cloning strain with the wrong lac region will not run it.

A background account of the lac system as cell biology, not as a protocol, is in the NCBI Bookshelf text of Molecular Biology of the Cell. The transformation that put the plasmid into the cell follows the shape in the Addgene bacterial transformation protocol.

What blue usually means, and what white usually means

Blue usually means the alpha peptide was restored well enough to complement. The common reason is that the cloning site was not disrupted, so the vector is empty of insert. Another reason, which fools people who clone short fragments, is an insert that landed in frame and left a peptide the enzyme can still use. Blue is then a colony that contains foreign DNA and looks like a disappointment. If your design expects that outcome, do not use colour as the filter. Use the digest.

White usually means the peptide was disrupted or was not made. A successful disruptive insert does that. So does a deletion or a frameshift in the alpha gene of a vector that never received an insert. So does a colony scored before pigment developed. So does a satellite that grew after a beta-lactam faded and never held the plasmid. So does a failed reagent: inactive X-gal, omitted IPTG, or a plate incubated so briefly that even the empty-vector control has not turned. White is a candidate when, and only when, the empty vector on the same batch is blue.

Pale colonies are a third call. Pigment is not instant. Many protocols allow a further interval, sometimes in the cold, for blue to finish. Follow that interval. A pale colony picked as white will often be blue in the morning and empty in the digest. Record colour at a stated time. Do not record a feeling.

Controls before you score the ligation

Plate the empty vector on the same X-gal batch. It should be blue if the screen is alive. Plate no DNA. It should be bare if the antibiotic is alive. If you have a known disruptive clone, it should be white, and it tells you the white end of the scale is reachable. Then look at the ligation. Prefer well-isolated colonies. A white halo around a blue colony, or a blue centre with a white edge, is a mixed or a late-developing mound. Restreak it before you trust the colour. Pick the centre of a colony that is one colour, and still run the digest.

If the empty vector is white, stop scoring. Fix the screen: host genotype, substrate, inducer, time. Antibiotic colonies can still be picked as unscreened candidates if you are willing to test more of them. Pretending the whites are informative will fill a gel with empty vectors.

Reagent classes for the digest you will run afterwards appear among Promega protocols. They do not replace the map.

Colour and the honest claim

Failures that repaint the plate

A substrate stock that has precipitated and was not mixed gives patchy colour and a false mix of white and blue that follows the streak of the spread, not the clones. An incubator opened early gives a plate of pales. A beta-lactam plate stored warm grows satellites that are white because they have no plasmid and no reason to make the peptide. Those satellites are not insert clones. The antibiotic article describes that halo. Score only colonies that match the strain and that sit away from a satellite field.

A vector that does not carry lacZ alpha will not screen, however much X-gal you add. Read the map before you buy the substrate. A host and a vector that do not share the complementation system waste a day and produce a confident, uniform white plate of empties and inserts together.

Research use

The screen is for laboratory cloning strains under the institutional assessment. It is not a diagnostic test and not a way to identify an unknown organism. Waste plates are inactivated as the safety office requires. This page is not medical advice. It deliberately does not list dye concentrations. Those live on the product sheet you are following.

Time, heat, and a sweaty cold room

Colour is slow, and a power cut that cools the incubator halfway through the night leaves pales that are not a genotype. Record the interruption and give the plates the protocol's remaining development time before you score. A warm room does not speed the enzymology in a way you should use to justify an early call. Humidity in a poorly sealed cold room, where some people park plates so the blue deepens, can flood the lid and smear colonies into each other so a white and a blue become one pale streak. Develop colour with plates inverted and lids dry. Write the substrate batch and the date on the base. A sleeve label that says X-gal, sitting on a different stack, is how a no-substrate plate gets scored as all-insert.

What to put in an enquiry

Name the host genotype class, including whether it is a lacZ delta M15 complementation host. Name the vector class and whether it needs an inducer. Name X-gal as a reagent class and the antibiotic as a separate line. Ask for the written specification and the storage class. Do not ask this page, or a supplier note, to stand in for a digest. The screening method can be discussed against the vector sheet. Send the fields through the quote request. If the insert is still a sequence to be designed, keep custom gene synthesis as the design reference, and keep the colour screen labelled as a later candidate filter, not as proof the designed sequence is in the colony.