Troubleshooting slow growth

Slow growth is a selection problem. You are choosing the next change, and a change made before the evidence is sorted will mix a new variable into an old one. The useful comparison is this line against its own recent record, in the medium and the vessel you actually used. It is not a race against a different line in a paper. The wider routine sits in mammalian cell culture for research labs. This page is the order of checks, and the discipline of leaving a reagent class alone until the check that justifies it has been done. A bottle you have not run side by side has not been shown to be the cause.

What slow means in this flask

Growth here means an increase in cell number, or a clear rise in confluence for an adherent line, over a stated interval. Many continuous lines in a matched bicarbonate medium are expected to need subculture on a cadence of a few days. That cadence is a property of the line, the seed and the formulation. It is not a universal clock. Call the culture slow only when the same line, recently, covered the surface or reached the harvest density sooner under conditions you can name.

Phenol red is a clue, not the measurement. Yellow medium with few cells can mean a contaminant producing acid, or a carbon dioxide setting that is high for that buffer. Purple medium often means the flask has not equilibrated with carbon dioxide, because it sat sealed on the bench or the incubator gas failed. A healthy sparse culture can also look less acid simply because fewer cells are metabolising. Read the colour beside the microscope. Public starting sheets such as the ATCC culture guides often state a medium family and a gas expectation. Your line card wins if they disagree. Many bicarbonate media are written for an elevated carbon dioxide atmosphere, commonly around five percent, sometimes higher. Follow the label on the bottle you opened.

Reagent and equipment classes worth naming

Keep the classes separate so you do not change three at once. Basal medium is the amino acid, vitamin, salt and glucose formulation, plus its buffer system. Serum, when the recipe uses it, is a lot-specific biological mixture. A serum-free supplement pack is a different class and is not a gentle version of serum. Glutamine, or a more stable dipeptide analogue, is a small addition that disappears from warm medium over days. Dissociation enzymes, commonly a trypsin class or a non-enzymatic alternative, are a handling class: too long or too strong, and the survivors grow as if the line had changed. Coatings, such as collagen-class proteins or a charged polymer, are a surface class. Too thick, too old, or rinsed with the wrong solvent, they become toxic rather than adhesive.

Antibiotics and antifungals are optional covers, not nutrients. A dose above the line sheet can slow the cells you meant to protect. The incubator is equipment: temperature, carbon dioxide, and humidity from a pan or a reservoir. The counter is equipment too. A viability dye tells you about the harvest you just made. It does not explain a week of slow cover unless you record it every passage. Cold storage of serum and of frozen stocks is its own chain, sketched in storing biological samples from fridge to freezer.

Checks before a new bottle

Write the last time this line behaved as the sheet describes. Then sort the flasks in front of you.

If one flask is slow and its siblings in the same incubator, same medium bottle and same passage are on schedule, the cause is local. Suspect the seed number, clumps, a coating applied only to that vessel, an extra aliquot of a supplement, or an over-long dissociation that morning. Do not open a new serum lot for a single unhappy flask.

If the whole line slowed in the passage after you changed medium or serum, hold the new lot as a suspect and test it. The fair test is a split of one harvest into the previous lot and the new lot, seeded at the same density, scored on the same day. Until that plate or pair of flasks exists, the enquiry you send should not declare the new bottle defective. Exhausted serum is a real phenomenon between lots. It is demonstrated by the side-by-side, not by frustration.

If several unrelated lines slowed in the same week, look at what they share. That is usually the room's gas, temperature, a water pan gone dry, or a shared supplement. A new serum bought for one line will not repair an incubator that is no longer holding its set point. Record the display for temperature and carbon dioxide when you notice the problem, including after a power flicker.

If the medium stays clear, the morphology is a bit ragged, and the slowdown has lasted more than one passage without a reagent change, put mycoplasma and passage age ahead of a purchase. Mycoplasma often does not cloud medium. Finite and primary cultures also slow because they are senescent. More serum does not rewind a population-doubling limit. Over-trypsinising every passage selects a slower, tougher remnant and looks like a medium problem if nobody recorded the enzyme time.

Change one class after the branch, and keep a flask on the previous condition. Workflow collections such as protocols.io show how other groups write that comparison. Copy the structure, not their volumes.

Moves that make slow growth worse

Seeding still fewer cells because the flask "is not ready" pushes a neighbour-dependent line further from the density it needs. Adding antibiotic on top of antibiotic hides a low-level contaminant until the day you omit them. Mixing two serum lots in one bottle without a label creates a third, undocumented condition. Raising serum, swapping basal medium and recoating the plastic in the same passage means you will never know which class mattered. A toxic coating looks like slow or absent attachment. If cells adhere on an uncoated treated flask and not on the coated one, stop coating until you have the dilution and the rinse the coating protocol states.

A control that fails is a stop, not a prompt to shop. If the side-by-side shows both lots equally slow, the bottle is not your answer. Return to gas, mycoplasma and passage age. If the incubator display is wrong, fix the equipment record before anyone thaws a backup vial into the same box.

Containment does not relax because the cells are quiet

A slow flask is still the biological material your risk assessment named. Do not take it to an open bench to "watch it" and do not give it a richer medium in a shared incubator while you guess. Cabinet class, waste and whether a sick culture stays in the room are institutional decisions. The WHO Laboratory Biosafety Manual is a public reference laboratories use when they write those rules. It does not assign a level to your line. Nothing here is medical advice or a licence to culture a pathogen. A viability kit protocol, such as those collected among Promega protocols, is a measurement method. It is not a diagnosis of why the line slowed.

Heat, a flicker, and a bottle that waited

In a hot building a power flicker is a common shared cause. Temperature and carbon dioxide both move, humidity in a pan shifts, and every line in that box can look "suddenly lazy" the next day. Write the time you noticed, the display you found, and whether the flasks felt warm or cool. Do not invent how many hours the cells should have tolerated. A serum or medium shipment that sat in a warm receiving area is a different clue. Quarantine it from the precious line and ask the supplier's stability statement. Specify a replacement by family, buffer, glutamine need and serum class, and say you will compare it with the current lot. Blame is not a specification. Everyday vessel and liquid classes are listed in the laboratory consumables catalogue. University laboratories framing a broader request can use the academic research reference. Ask through the quote request whether that specification can be discussed. The discussion does not mean a culture service is being operated on your behalf.