Cell Culture Contamination

It is worth separating cell culture contamination into two groups, because they demand completely different lab behaviour.

The visible group — bacteria, yeast, and filamentous fungi — grows fast, changes the medium, and is obvious within one to five days. You find these by looking. They are painful but self-announcing, and the useful question is not "what is it" but "how did it get in", because the answer is almost always a specific, correctable break in aseptic technique.

The invisible group — mycoplasma, viral contamination, and cross-contamination with a misidentified cell line — produces no turbidity, no colour change and nothing you can see. These are found only by deliberate testing, and they are far more damaging to your results precisely because they are silent. A misidentified line or a mycoplasma-positive line will keep producing clean-looking, reproducible data that is about the wrong thing.

There is a seventh category worth naming: chemical contamination — endotoxin, detergent residue, plasticiser leached from tubing, metal ions, or a medium component out of specification. It is not an organism, it is invisible, and it produces exactly the symptoms most often blamed on "the cells being unhappy": slow growth, poor attachment, failure to reach confluence.

Appearance. The medium goes cloudy or turbid, usually within 24–72 hours of the introducing event. With phenol red in the medium, a heavy aerobic or facultative population drops the pH and turns the medium from red-orange to yellow. Under phase contrast at 200–400×, bacteria appear as very small rods, cocci or coccobacilli, typically 0.5–1 µm across and up to about 20 µm long, moving between cells with obvious Brownian or, in motile species, directed movement. A shimmering, granular background between adherent cells that was not there yesterday is the classic first sign.

Common sources. Operator technique — hands, sleeves, talking over open vessels, passing an unsterilised item over an open flask. Contaminated waterbaths and incubator water pans. Unfiltered or incorrectly filtered supplements. A contaminated stock bottle used across multiple lines.

Response. Discard. Autoclave or disinfect the vessel before it leaves the hood. Do not attempt to rescue a bacterially contaminated flask with antibiotics: even when the visible growth is suppressed, low-level survivors and their endotoxin load persist, and you have converted an obvious problem into a hidden one.

Appearance. Yeast appears under the microscope as discrete ovoid or spherical particles, typically 3–5 µm and therefore noticeably larger than bacteria, often visibly budding — a smaller daughter cell attached to a larger mother. Budding is the single most useful identifying feature.

The medium turns turbid, but usually more slowly than with bacteria and often only once contamination is well established. The pH behaviour is distinctive and worth knowing: yeast produces little pH change until the contamination is heavy, at which point the pH typically rises rather than falls. A culture that has gone cloudy while the phenol red has gone pink-purple rather than yellow is more likely yeast than bacteria.

Common sources. Airborne — yeasts are abundant in the general environment, so a hood with a compromised airflow, a poorly maintained HEPA filter, or work performed outside the hood is the usual explanation. Skin and hair. Contaminated water baths.

Response. Discard. Then check the hood: certify the airflow, check the filter, and check that nobody is working with the sash at the wrong height.

Appearance. Filamentous fungi are the most visually obvious of all contaminants. Under the microscope, mycelia appear as thin, wisp-like branching filaments (hyphae) crossing the field, sometimes with denser clumps of spores. To the naked eye, contamination starts as small white, yellow, grey or black fuzzy dots, either floating in the medium or attached to the flask wall at the liquid surface, and develops within a few days into large furry patches. The medium often stays clear for longer than with bacteria, and the pH may rise slightly as the culture progresses.

The "stringy" or "cotton-wool" appearance people describe in a flask is almost always filamentous fungus.

Common sources. Airborne spores — fungal spores are hardy, ubiquitous and survive on surfaces for long periods. Building work, damp, poorly maintained incubators (the water pan is a classic reservoir), and contaminated incubator shelving. Cardboard packaging brought into the culture room is an underrated source.

Response. Discard, and treat it as an incubator-level event rather than a flask-level one. Empty the incubator, remove and autoclave the shelves and the water pan, run the incubator's decontamination cycle or wipe down with an approved sporicidal agent, and replace the water pan contents with fresh sterile water containing an approved anti-fungal water treatment. Fungal spores that have colonised an incubator will keep reinfecting new cultures until the incubator itself is cleaned.

Appearance. None. This is the entire problem. Mycoplasma reaches 10⁷–10⁸ organisms per mL without turbidity, without a visible pH change and without anything detectable under a routine inverted microscope. Cells at 0.3–0.8 µm with no cell wall pass through 0.2 µm filters and are intrinsically resistant to penicillin.

Indirect signs are non-specific and easy to explain away: gradual slowing of growth, lower saturation density, medium exhausting faster than expected, falling transfection efficiency, a drifting drug response. Any of these should prompt a test rather than a change of serum lot.

Common sources. Other contaminated cultures in the same lab, spread by aerosol and shared bottles; laboratory personnel, since several contaminating species are human oral commensals; animal-derived serum and trypsin, historically important and now much reduced; and incoming cell lines from non-repository sources.

Response. Confirm by a second method, quarantine, and normally discard — restarting from a clean frozen stock is faster and more defensible than treatment. Full detection method comparison, testing frequency and the elimination-versus-discard decision are covered in the mycoplasma testing reference.

Appearance. Usually none in the producing culture. Many viruses replicate in a host cell line without cytopathic effect, so the culture looks and grows normally. Where cytopathic effect does occur, it shows as rounding, detachment, syncytium formation or plaque-like clearings in the monolayer that are easy to mistake for over-confluence or a bad trypsinisation.

Why it matters. Viral contamination is rarely a problem for routine research cultures, but it is a serious one for any material intended for biologics or cell therapy manufacture, and it is a laboratory biosafety consideration when the source material is human or primate.

Common sources. The original tissue or donor material; animal-derived raw materials, principally serum and trypsin; and other cultures in the facility.

Response. Viral testing is specialist work — PCR panels for specific agents, and in vitro adventitious agent assays on indicator cell lines for a broad screen. For routine research, prevention is the practical control: use serum and trypsin from suppliers that publish virus screening data, or use animal-origin-free (ADCF) reagents and recombinant dissociation enzymes, which remove the route entirely.

The most consequential contamination in cell biology is not an organism at all: it is one cell line overgrowing another, or being mislabelled at some point in its history.

Large-scale authentication surveys have repeatedly found that a significant minority of cell lines in active laboratory use are not what their label says. HeLa is the best-known culprit — fast-growing, robust and historically responsible for displacing many other lines — but any vigorous line will overgrow a slower one if they are ever mixed.

Appearance. None, or a slow drift in morphology, growth rate or marker expression that is easy to attribute to passage number.

Sources. Handling two lines in the hood at the same time. Shared media bottles. Mislabelled vials. Poor freezer records. Receiving a line from a collaborator whose own chain of custody is unknown.

Response and prevention. Short tandem repeat (STR) profiling is the standard authentication method for human lines, and mouse STR panels exist for murine lines. Profile a line when you receive it, when you bank it, and periodically thereafter. Never have two cell lines open in the hood at once. Never share a medium bottle between lines. Most journals now require an STR-verified identity statement, and the cost of a profile is trivially small against the cost of a repeated experiment.

Not everything that looks wrong is alive. Before discarding a culture, rule out the non-biological explanations:

• Endotoxin from water, serum or raw materials activates immune-competent cells and alters growth and differentiation without any visible sign. It is heat-stable, survives autoclaving and is not removed by 0.2 µm filtration.
• Detergent or disinfectant residue on glassware or from an incompletely rinsed hood surface.
• Plasticiser or extractable leaching from tubing, storage bottles or non-tissue-culture-grade plastics.
• Precipitates — calcium phosphate crystals, precipitated serum protein, or crystallised medium components after cold storage — look like particulate contamination under the microscope but are refractile, geometric, do not move under their own steam, and do not multiply between observations. Re-examine after four hours: contamination grows, precipitate does not.
• Cell debris and apoptotic bodies from a stressed or over-confluent culture can resemble bacterial contamination at low magnification. Raising the magnification usually settles it.

Ranked roughly by how much contamination each measure prevents per unit of effort:

1. Aseptic technique and hood discipline. Work at the correct sash height, keep hands and materials downstream of critical openings, never pass anything over an open vessel, wipe every item into the hood with 70% ethanol, and do not talk into the cabinet. This one item prevents most contamination.
2. One line open at a time, one bottle per line. Eliminates cross-contamination and most horizontal microbial spread in a single stroke.
3. Quarantine incoming cultures in a separate incubator until tested.
4. Maintain the incubator. Weekly water pan change with an approved treatment, monthly wipe-down, scheduled decontamination cycle, and HEPA filter changes on the manufacturer's interval.
5. Certify the hood annually and check the airflow alarm works.
6. Bank early, bank clean, and test the bank. A tested master bank at low passage turns a contamination event into an afternoon's inconvenience.
7. Do not use prophylactic antibiotics as a substitute for any of the above. They hide low-level bacterial contamination, do nothing about mycoplasma, fungi or cross-contamination, and cause labs to stop noticing that their technique has drifted.