Troubleshooting lawns and satellites

A lawn and a ring of satellites are two different ways a plate stops being a source of single colonies. A lawn means selection did not hold, or you plated a vast excess of cells. An ampicillin satellite is a small colony living in antibiotic that a neighbour has already destroyed. You are deciding whether to dilute, to pour fresh plates, or to stop picking. This is a research application note, not a permit to treat a carpet of growth as a clone.

How a marker is supposed to work, including blue-white colour when the vector uses it, is in antibiotic selection and blue-white screening. The path from a real colony to a verified plasmid is in plasmid cloning from insert to colony.

What the agar is supposed to do

Selection works only while the drug in the agar can stop cells that lack the resistance gene. The concentration belongs to that marker and that medium. Follow the backbone note rather than a number remembered from a different plasmid. Cells that carry the marker form colonies. Cells that do not should remain invisible.

A lawn is growth so dense that separate colonies disappear. Two mechanisms produce it. The drug is absent, wrong, under-dosed, or already inactive, so almost every cell you plated can divide. Or the drug is fine and you spread so many cells that the biomass covers the surface, sometimes as a film of transformants, sometimes as residual growth. Those two lawns ask for opposite fixes. A cells-only plate, with no plasmid, on the same batch of agar, is the split.

Satellites are narrower. On ampicillin, a true resistant colony secretes beta-lactamase. The enzyme leaks into the agar and hydrolyses the drug in a zone around that colony. Nearby cells that never took up the plasmid then divide, and they form a ring of smaller colonies. The ring appears later than the centre. Those satellites are not validated clones. Do not pick them. A plasmid prep or a colony PCR from a satellite is how an empty or mixed result gets a confident label.

Kanamycin does not make that pattern. Its common resistance enzymes act inside the cell that carries the gene. They do not clear a public zone of drug the way secreted beta-lactamase clears ampicillin. Tiny colonies in a halo around a large one are an ampicillin observation. The same picture on kanamycin is a reason to doubt the drug or the pick, not a reason to invoke beta-lactamase.

Plates, cells, and a no-DNA control

You need agar with the antibiotic named on the backbone, poured and cooled, and a plate from the same batch spread with competent cells and no DNA. You need the transformation itself spread so that some region might still show single colonies. A loop, a spreader, and a fresh plate for restreaking are the rest. Antibiotic and plates are catalogue classes in the molecular biology catalogue. The working concentration is the one your plasmid note and your SOP agree on.

Old ampicillin plates are a known failure in a warm room. The drug does not stay useful on a forgotten stack the way a printed protocol assumes. Kanamycin is a different molecule and does not fail by growing satellites, but a kanamycin plate with no drug added still lawns. Date the plate. If you cannot say when the ampicillin went in, treat the plate as untrusted.

Read the pattern, then stop or continue

Look before you pick. Note whether the cells-only control is bare, speckled, or solid. Note whether colonies are isolated or buried. On ampicillin, note whether small colonies sit in rings around large ones.

If the cells-only plate is a lawn, selection is not operating. Remake the agar. Check that the antibiotic was the right one, that it was added after the agar had cooled below the temperature that inactivates it, and that ampicillin plates were not stored warm. Stop colony screening. A PCR of a lawn will amplify whatever was most abundant, including cells with no insert.

If the cells-only plate is bare and the transformation is a solid lawn, you plated a vast excess of cells or the reaction was extremely productive. Dilute the recovery culture and plate again. Stop when you still cannot see an edge of a single colony. Screening inside a lawn does not become valid because the assembly method was Gibson or a ligation. The join is unreadable until a colony is a colony.

If you see large colonies with a halo of small ones on ampicillin, the large colony is the candidate and the halo is satellites. Do not pick the ring. Touch the centre and restreak at once onto a fresh ampicillin plate so the cells that were only borrowing a cleared zone are left behind. Screen and sequence the restreaked isolate. Insert orientation is a question about that isolate's plasmid, not about a satellite that may not have one.

If the plate is a mixture of a few isolated colonies and a late film, pick only well-separated colonies the same day. Long incubation of ampicillin plates is how satellites and a weakening drug turn a readable plate into a lawn. When the film has already merged, stop and replate. Do not fish in it.

Failures that masquerade as clones

Picking a satellite and sequencing it wastes the read. Mixed peaks often mean the toothpick took the centre and the ring together. Restreak until the new plate shows isolated colonies with no halo, then prep.

A lawn that appears only after two days on old ampicillin is the drug fading, not a slow plasmid. Do not call those late colonies validated because they are on the transformation plate. Plate the same recovery on agar poured this week.

Blue-white medium does not rescue a satellite. A white satellite can be a cell with no lacZ fragment and no plasmid, feeding beside a real colony. Colour is a hint on a plate where selection still works. It is not a reason to pick inside a cleared zone.

If every dilution lawns, including a dilution that should have shown tens of colonies, go back to the plates. The transformation volume is not the remaining variable.

Safety

Antibiotic-resistant growth is laboratory waste, lawn or not. Autoclave or discard as your committee requires. The WHO biosafety health topic is background, not permission to propagate a resistant strain you did not mean to archive. Do not rinse plates into a sink. A satellite you accidentally culture can carry no plasmid and still sit in a tube labelled with your construct. That is a record failure and a waste failure together.

Warm rooms and old ampicillin

This is the failure that shows up when plates live on a bench or in a cupboard that never really cools. Ampicillin plates age into lawns. Store them as your SOP says, usually cold, and pour a fresh batch when the room has been hot and the plates are no longer dated. Kanamycin will not draw satellite rings to warn you. Its failure looks like ordinary background. Either way, a warm room is a reason to shorten the life you allow a plate, not a reason to screen harder.

What to put on the request

State the marker, that you need the matching antibiotic for plates, and whether the last batch lawned or grew satellites. Ask for the reagent class, not for a diagnosis by mail. Use the quote request for that sourcing question. Once a restreaked colony exists, a junction read is a separate request. The Sanger sequencing enquiry reference is how to frame it. A sequence specification, if the construct was designed rather than picked from a lawn, belongs with the custom gene synthesis enquiry reference as an enquiry prompt only. Ask whether a quotation is possible. Send the plate pattern in words: cells-only lawn, or ampicillin ring, or kanamycin background. Those three are different orders.