Marking plates so a negative control stays negative

A negative control that grows colonies ends the transformation. You may not pick from the sample plates, because the one dish that should have stayed bare has told you that cells can form colonies without the DNA you claim to have added. The decision this troubleshooting page supports is whether that growth came from a lid swap, a dead antibiotic, or DNA that was never negative. Culture, recovery and the meaning of a clean no-DNA plate are in bacterial culture and transformation. How the drug is supposed to police the colony is in antibiotic selection and blue-white screening.

The hero photograph shows shake flasks, which is where a dense culture starts. The failure this page is about happens after a fraction of that culture, or of a transformation recovery, meets an agar surface whose identity has been allowed to drift. Plate and antibiotic classes are in the reagents and chemicals catalogue. The cloning path is the molecular biology pathway. Design questions about the plasmid can sit on the custom gene synthesis reference as an enquiry, not as a running synthesis. A note about pre-labelled plates goes through the quote request.

How a false transformation is born

You plate cells with no DNA, cells with a known plasmid, and cells with a new ligation, onto what you believe is one selective batch. Next morning the no-DNA plate has as many colonies as the ligation. If you ignore it and pick "transformants", every downstream digest becomes a riddle, because many of those colonies never received a plasmid. They grew because the agar did not select, or because the plate under that lid was not the plate you think, or because the tube marked no-DNA contained plasmid already.

The marking habit is the cheap control on that story. Write on the base, the dish half that contains the agar. Write the antibiotic, the date, and the strain or sample. Write on the side wall as well if stacks are stored, so a label is visible without opening a lid and tempting a swap. A lid label is a convenience and a liar. Lids migrate in every fridge that holds more than one person's plates.

A notebook map should use the same words as the plastic. "Amp, 12 March, no DNA, strain initials" on the base and "plate 3" in the book is how a true event becomes unreadable a week later. The Addgene bacterial transformation protocol shows where a no-DNA plate sits in the workflow. The words on your base are what make that position real.

What the base has to carry

Antibiotic name, not a private abbreviation that only one student uses, unless the laboratory has published the abbreviation on the wall. The date the agar was poured, because age is part of beta-lactam performance. The strain, because a kanamycin-resistant chromosome looks like a failed negative control on kanamycin plates that are actually perfect. The sample identity: no DNA, control plasmid, ligation name. If two concentration classes of the same drug exist in the room, the class goes on the base too. Follow the plasmid map for which class is selective. This page does not print a dosing table.

Pen choice is part of marking. Ink that runs when a cold plate sweats will erase the only record. Use a marker the laboratory has already found survives condensate and a wipe with the disinfectant you use on the outside of dishes. Write after the agar has set and the outside is dry, so the pen is not skating on a wet wall. Do not write only on a piece of tape that peels in the incubator.

Pre-poured plates you did not watch being dosed still need a base label you trust. If the manufacturer's mark is on a sleeve and the sleeve is thrown away, copy the drug, the lot sense and the date onto the base before the sleeve goes. A sleeve is a lid that fits fifty plates. It migrates even more easily.

Troubleshooting a negative control that grew

Read the base that is in your hand. Ignore the lid that happened to be on it. If the base is blank or names a different drug, the control is void. Discard the set or quarantine it, and repeat on plates whose bases you label before you spread anything. Do not try to reconstruct identity from position in the stack. Position is not a label.

If the base names the right drug and the right date, fetch the stock bottle. Confirm the name, the opened date, the storage, and that a precipitated antibiotic was resuspended the way the sheet says. A stock that is water, or that sat warm, produces a perfect-looking negative plate full of the empty host. Streak that empty host from an old trusted colony, using the habit in the Addgene streak plate protocol, onto one plate from this pour and onto one plate from a new pour. Growth on both means the strain is resistant or both pours are bad. Growth only on today's pour means today's drug addition failed. No growth on a fresh pour, and colonies only on the original negative plate, pushes you back to a swap or a splash.

Look at the pattern of the colonies. A uniform lawn matches absent selection or a huge inoculum of resistant cells. A cluster of colonies on one side matches a drip or a spreader that touched a sample plate and then the control. Colonies only around the edge can be a lid that dripped condensate from a neighbouring dish. In the splash case, a repeat no-DNA plate, spread in a clean area with a fresh spreader, is the test. If it stays bare and the known plasmid still transforms, the method is intact and the first control was physically seeded.

Then audit the "negative" DNA. A tube from a shared box may already contain a plasmid. Cells taken from a selective plate of a previous transformation are not a no-DNA control. They are a positive control with the name crossed out. The host for a true negative plate is a colony or a glycerol stock that has never seen the marker, or competent cells from a lot you have shown cannot grow on the drug. Handling context for strains that arrived as collection material is in the ATCC culture guides. Local vials deserve the same suspicion until the base and the notebook agree.

Lid, base, and a plate with no name

A known plasmid that fails on a base-labelled plate, while the empty host also fails, is a different troubleshooting path: dead cells or dead DNA, not a lying negative control. Keep the paths apart. The negative control answers only one question: can this host make a colony on this agar without new DNA?

Other ways a control is ruined without a bad lot of agar

The spreader was not changed. The pipette tip touched the sample and then the control because the order of plating was reversed in a hurry. Condensate from a lid that had been on a lawn dripped onto a fresh negative plate. The incubator water tray, in a machine that has one, was contaminated and lids were wet. None of these require the antibiotic to have failed. The marking still matters, because a labelled base lets you throw away one plate and keep the rest of a good pour. An unlabelled stack forces you to throw away the pour.

A negative control that stays bare, beside colonies only where DNA was added, is the result that allows picking. It is still not proof the insert is correct. It is proof that, on this base, this host needed the DNA to grow.

Research use and a plate you do not trust

Do not pick colonies off a failed negative control "in case some are real". You will select for whatever grew, including a contaminant. Cloning-strain plates remain inside the institutional waste rule. An odd morphology is a stop, not a side project. This page is not medical advice and not a labelling scheme for clinical specimens. Recording practice of the kind method repositories illustrate, including protocols.io, is a habit. Your safety office still owns the organism.

Ink, humidity, and a warm bench overnight

In a humid cold room, lids sweat and ink written on them smears into unreadability. The base, once dry, keeps a laboratory marker better, which is another reason the base is the record. Plates left on a warm bench overnight before a transformation are both a drug-stability problem for ampicillin and a labelling problem if the date is missing. You cannot retire "yesterday's plates" if yesterday is not written down. A power cut that warms a fridge full of lids stacked loose will finish a swap that was only half likely. After any such event, trust only bases you can still read, and pour or open a new labelled set for the controls.

What to ask for if plates arrive pre-poured

Ask that the antibiotic name, the concentration class and a date survive on the base of each dish, not only on an outer bag. Ask how the receiving lab is expected to run an empty-host check. The marking method can be discussed so a negative control stays tied to a known pour. Send that with the strain and the marker through the quote request. If the DNA on the "positive" side of the control pair still has to be designed, keep custom gene synthesis as a reference beside the plate specification, not as a claim that the DNA already exists.