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Kulture Scientific · Technical Guide

Contamination troubleshooting for mammalian cell culture

Find It. Stop It. Trace It.

All cell types Bacteria · Yeast · Mold · Mycoplasma Updated Oct 2026 Get the PDF

Something in your culture looks wrong, your cells have been acting off, or contamination keeps coming back no matter what you change. This guide walks through the questions to answer, in order: What am I looking at? What do I do right now? Where did it come from? And how do I test for the contaminants I cannot see? Each step works as a quick reference, so you can jump straight to the one you need.

Purpose and Scope

This guide covers identification, immediate response, and source tracing for microbial, viral, chemical, and cross-contamination in mammalian cell culture. It applies to adherent and suspension cultures in standard vessels. It is a companion to your institution's biosafety procedures, not a replacement for them.

Step 1: Read the Media

Phenol red is your first warning. Check the color every day, then confirm under the microscope. A color change alone does not tell you what happened. It tells you where to look next.

Three T-25 flasks showing phenol red color: red-orange control, yellow acidic, and magenta alkaline medium
Phenol red color guide. Left to right: normal (red-orange), acidic (yellow), alkaline (magenta).
Media colorWhat it meansPossible causesNext step
Red-orangeNormal pHHealthy culture, or a contaminant that does not change pH (mycoplasma, virus, another cell line)Routine checks; test on schedule
YellowAcidicBacterial contamination, or cells overgrown and media exhaustedCheck for turbidity, then the microscope
Magenta / pinkAlkalineLoss of CO2 (flask left out, cap too tight, incubator CO2 drift) is common; contamination is possibleCheck incubator CO2 and cap, then the microscope
◆ Rule of thumb: if the media changed faster than your cell density can explain, assume contamination until the microscope says otherwise.

Step 2: Identify What You See

Under the microscope, look in the spaces between your cells. Compare what you see to the reference images below.

Bacterial contamination in cell culture medium showing dense short rods and chains
Bacteria (40x)
Tiny dots or short rods, often in chains, filling the spaces between cells. Media is usually cloudy and acidic. Live samples may show movement.
Baker's yeast Saccharomyces cerevisiae as a reference for yeast contamination in cell culture
Yeast (10x; reference: baker's yeast)
Round to oval cells, noticeably larger than bacteria, single or clustered. Look for budding at higher magnification. Shown: S. cerevisiae for reference; culture contaminants look similar.
Fungal mold contamination showing branching hyphae radiating from a dense center
Mold (10x)
Thin, branching filaments (hyphae) radiating from a dense center. Often visible to the eye as fuzzy clumps floating at the surface before the media clouds.
A single unbranched cotton fiber in cell culture media, often mistaken for fungal hyphae
Not contamination: a cotton fiber
One long strand, uniform width, no branching, frayed at the tip. This is cotton, not mold. Hyphae branch; fibers do not, and they unravel at the ends.

Before you call it contamination

Not everything cloudy or floating is alive. Rule out these look-alikes first:

Dying cells. A bad thaw, the wrong plasticware or coating, or over-confluence can fill the media with floating cells and debris. Debris does not multiply.
Fibers and particles. Lint, plastic fragments, and dust (see the fiber image above).
Precipitate. Media components can precipitate after freeze-thaw or temperature swings, giving fine, non-moving granules.
The 24-hour test. If you are unsure, mark the spot and look again tomorrow. Contaminants grow; look-alikes do not.

Contaminant quick reference

ContaminantOnsetMediaUnder the microscopeFirst action
BacteriaHours to 2 daysCloudy; rapid shift to yellow; sometimes a surface filmTiny dots or rods between cells; may moveRemove and discard; check neighbors
YeastDaysCloudy; pH shift, usually slower than bacteriaRound or oval cells larger than bacteria; budding or clustersRemove and discard; check neighbors
MoldDaysFuzzy floating clumps, often before cloudingBranching filaments; spore clustersRemove and discard; clean incubator and pan
MycoplasmaPersistent, often unnoticedUsually clear, no color changeNothing obvious; possible debris, granularity, slower growthQuarantine and test (Step 5)
VirusVariableUsually no changeOften nothing; some cause cytopathic effectsReview reagent sources and documentation
Another cell lineGradual takeoverNo changeMorphology or growth rate drifts from expectedSTR profile; check the ICLAC register (see FAQ)
ChemicalVariableUsually no changePoor growth; stressed or vacuolated cellsCheck water, glassware rinsing, reagent age, endotoxin
ⓘ A clear flask does not rule out the bottom four rows. Mycoplasma can reach about 108 organisms per mL without clouding the media.

Step 3: Respond Immediately

1

Do not open the vessel in the biosafety cabinet

Close it, remove it from the incubator, and decontaminate according to your institution's biosafety procedures.

ⓘ Moving fast matters. Spores and aerosols spread to neighboring vessels and shared equipment.
2

Inspect neighboring cultures

Check every vessel on the same shelf and every culture handled in the same session.

3

Quarantine reagents

Set aside and label the media, serum aliquot, dissociation reagent, buffers, and supplements used with the affected culture. Do not discard them yet. They are your evidence for Step 4.

4

Decontaminate equipment

Biosafety cabinet, incubator interior and humidity pan, water bath, pipette controller, and centrifuge buckets.

5

Restart from your bank

Thaw a vial from your working bank, frozen before the event and ideally with a documented negative mycoplasma test. If the working bank itself is suspect, go back to your master bank.

6

Record the event

Date, cell line, passage number, reagent lot numbers, operator, and recent changes.

◆ One event can be bad luck. Two events sharing a lot number or a piece of equipment is a pattern, and you will only see it if the first one was written down.
⚠ Rescue is a last resort. If you have a master or working bank, restart from it. It is faster, cheaper, and safer than any rescue. A rescued culture has been through stress and treatment that can change its biology, so its data may no longer match your earlier work, and every day it stays in the incubator risks spreading contamination to other cultures. Consider rescue only when the line is truly irreplaceable: no frozen stock anywhere, including collaborators and cell repositories. See the FAQ for how to approach it.

Step 4: Find the Source

When contamination keeps coming back, work through the suspects in this order. Most labs start by blaming technique and check the frozen stock last. Reverse that. The stock and the reagents are the easiest to test and the most often missed.

Suspect 1: Your frozen stock

A contaminated bank produces a recognizable pattern:

Thawed cultures grow normally for one to two weeks, then crash
Every affected culture traces back to the same freeze date or bank
Other cell lines using the same media and reagents stay clean
Antibiotics in the media stretch out the delay before contamination shows
1

Test a vial from the suspect bank

Thaw one vial into antibiotic-free media, and add a small aliquot of the thawed suspension to a broth tube (see Suspect 2). Run a vial from a different bank alongside it for comparison.

2

Check what went into the freeze

Note the lots of freezing medium, serum, and DMSO used for that bank. A contaminated cryoprotectant contaminates every vial frozen with it.

3

Replace the bank, then test the new one

Discard the suspect bank and expand from your master bank or an earlier clean bank. Test a vial from the new bank before you rely on it.

◆ This is why the master bank exists. A working bank can be replaced; a master bank you never work from is your insurance.

Suspect 2: Your reagents

Test each component on its own. If you quarantined your reagents in Step 3, you already have what you need.

1

Set up one tube or vessel per component

Basal media, serum, each supplement, dissociation reagent, buffer, and antibiotic, each labeled separately.

◆ For a faster read, add an aliquot of each component to tryptic soy broth (TSB), which supports a wide range of bacteria and fungi, and incubate for 3 to 7 days. Media alone in a flask also works but shows growth more slowly.
2

Add a handling control

Open a tube of sterile broth or fresh media in the cabinet and handle it exactly as you normally would, adding nothing.

3

Add an unopened control

Incubate a sealed, unopened container of the same media. If it grows, the problem started before the bottle reached you.

4

Incubate and observe

Inspect daily for turbidity, color change, and growth under the microscope. For a definitive answer, the compendial sterility test (USP <71>) runs 14 days, at two temperatures, in two media types.

⚠ Broth and media tests detect bacteria, yeast, and mold. They do not detect mycoplasma or viruses.
5

Read the result and act

Discard the contaminated component and every open container of the same lot. Notify the supplier with the lot number, and keep a retained sample.

ⓘ A clean result from one aliquot does not clear the whole bottle when contamination is low-level.

Suspect 3: Hidden reservoirs

If the stock and reagents test clean, the source is usually somewhere nobody cleans:

WhereWhat to check
Incubator internalsFan, fan cover, HEPA filter, door gasket, and shelf supports. Check your manual for which parts a high-heat cycle reaches and which must be removed, cleaned, or replaced separately.
Humidity panWater age and type (sterile distilled), visible growth, residue in the pan itself.
Biosafety cabinetCertification date, the spill tray under the work surface, the front grille. Building HVAC work or room airflow changes can disrupt cabinet performance.
Pipette controllerThe internal filter, especially if liquid was ever drawn up into it.
Aspiration line and trapShared lines and traps that are rarely disinfected or replaced.
Water bathWater level relative to bottle caps, cleaning schedule, visible growth. Dry bead baths remove the water entirely.
Reusable filters and glasswareMicrocracks in bottle-top filter units; sterilization cycles that are not verified.
ConsumablesDamaged packaging, cracked flasks or serological pipettes, boxes that were crushed or got wet.
Culture roomCardboard boxes (fungal growth when damp), vents near the cabinet, foot traffic.
Antibiotic useRoutine antibiotics suppress susceptible organisms and hide low-level contamination. Is there an antibiotic-free control culture?

Suspect 4: Technique

Check technique last, not first. If the handling control grows while the stock, every reagent, and the unopened control stay clean, the problem is how and where cultures are handled. Watch one full session from start to finish: hands crossing open vessels, lids placed face up or face down, how long containers sit open, and what gets touched between the cabinet and the incubator.

ⓘ Experienced, careful people get contamination too. A recurring problem is far more often a contaminated bank, reagent, or reservoir than a sudden lapse in technique.

Step 5: Test for What You Cannot See

No single mycoplasma test catches everything. ECACC recommends at least two methods for a reliable result, because sensitivity varies by species.

MethodTime to resultSensitivityLimitations
Direct culture (broth and agar)About 28 daysHighSlow; some strains do not grow on media
Indirect DNA stain (indicator cells + Hoechst or DAPI)3 to 5 days incubation, plus stainingHighReading takes practice
Direct DNA stainSame dayLowDifficult, subjective reading
PCR / qPCRSame dayHighCannot tell live from dead; inhibitors cause false negatives; carryover causes false positives
ELISASame dayModerateLimited range of species
Enzymatic luminescenceFastest availableVaries by kitConfirm positives with a second method

When to test: the realistic minimum

Most labs will never test every line on a fixed calendar. If you test at only three points, make it these. Each one protects something you cannot easily replace.

1

When a new line arrives

Before it shares an incubator with anything else. A line from a collaborator or another lab is the most common way mycoplasma walks in the door.

2

Before you freeze a bank

Your frozen stock is what you restart from after a contamination event. If the bank is infected, every restart is infected too.

3

When cells start acting off

Slower growth, more debris, failed transfections, or results that drift. Test before you change media, protocols, or reagents.

◆ If you can do more: test after any contamination event in the same room, and before you publish or send cells to a collaborator. A recent negative result protects your data and your reputation.
◆ Grow cells without antibiotics before testing. Testing laboratories such as Bionique ask for at least one, preferably three, antibiotic-free passages to improve detection sensitivity.

When to Use UV in the Biosafety Cabinet

Many labs run the cabinet UV lamp out of habit. NIH, CDC, NSF/ANSI, and the American Biological Safety Association agree that UV is not recommended or necessary for decontaminating a biosafety cabinet. Chemical disinfection is the primary method, every time. For the full story, see Your Hood's UV Lamp Is Mostly Mood Lighting.

Why UV falls short

It only works in a direct line of sight. Shadows, cracks, the front grille, and the spill tray under the work surface never see it, and that is exactly where fungal spores and hardy bacteria hide.
Mold is its weak spot. At a typical lamp intensity, Penicillium needs about 30 minutes for a 90% kill, so a 15-minute run barely touches it.
Dust and dried media shield microbes from it.
High humidity (above about 70%) sharply reduces its effect.
Lamp output fades with age. A bulb can glow long after it stops killing anything.
It degrades some plastics and tubing, and it can burn your eyes and skin.

If your lab uses UV anyway

1

Disinfect first

Wipe every surface with your chemical disinfectant. UV is an extra layer after cleaning, never instead of it.

2

Empty cabinet, sash closed, nobody working

Never run UV while anyone is at the cabinet. Post a sign: turn off UV before working.

3

Keep it short

About 15 minutes is a common institutional limit.

4

Maintain the lamp

Wipe the bulb with 70% alcohol weekly and check its output or replace it on a schedule, often yearly.

ⓘ Lamps contain mercury. Dispose of them through your institution's hazardous waste program.

Prevention Checklist

Inspect cultures daily, by eye and under the microscope
One cell line in the biosafety cabinet at a time
A dedicated media bottle for each cell line; never share bottles between users
Quarantine new lines until mycoplasma testing and STR profiling (an identity check) are complete
Keep a master bank you never work from and a working bank you do
Test a vial from every new bank before you rely on it
Avoid routine antibiotics, or run an antibiotic-free control culture in parallel
Replace humidity pan water with sterile distilled water on a set schedule (at least weekly is a common recommendation)
Keep water bath levels below bottle caps; clean on a schedule
No cardboard in the culture room
Review supplier certificates of analysis for test methods, not just "Pass"
Log every contamination event with lot numbers

Notes

Copper incubator interiors kill microbes on contact: copper ions damage the cell membrane and generate reactive oxygen species. Warmth, humidity, and tarnish improve the effect. Copper only works on what touches it, and organic soil such as a media spill blocks it. Treat it as a backup to cleaning, not a substitute.
Alloys need more than 60% copper to be effective. Copper-enriched stainless steel performed no better than plain stainless steel in manufacturer testing.
A high-heat decontamination cycle does not always reach every part of an incubator. Check the manual for the HEPA filter, fan, and fan cover, and replace filters on the manufacturer's schedule.
Not every filament is mold and not every dot is bacteria. Fibers, debris, and precipitates are common look-alikes. Contaminants grow; debris does not. When in doubt, check again in 24 hours.
This guide applies to mammalian cell culture. Primary cultures may carry contaminants from source tissue and often justify short-term antibiotic use.

Frequently Asked Questions

Yes. Mycoplasma can reach about 108 organisms per mL without causing turbidity or a color change. Viruses and cross-contaminating cell lines also leave the media unchanged. Only testing confirms a clean culture.
No. Penicillin targets the bacterial cell wall, which mycoplasma lacks. According to ATCC, streptomycin inhibits only about half of mycoplasma strains, and gentamicin is generally ineffective at routine concentrations.
ATCC estimates roughly 15 to 35% of continuous cell cultures are affected, with a minimum of 1% in primary cultures. A 2015 survey of RNA-seq data in NCBI's archive found mycoplasma in 11% of the 884 projects examined.
That delay is the classic sign of a contaminated frozen stock. A small number of organisms survive freezing, and antibiotics in the media slow them down without eliminating them. Once they outgrow the antibiotic, the culture crashes. Test a vial from the suspect bank in antibiotic-free media and broth, and check the freezing reagents used for that bank.
Rarely, and only as a last resort. If you have a master or working bank, restart from it. If the line is truly irreplaceable, with no frozen stock anywhere: quarantine the culture away from everything else, send a sample to a microbiology lab to identify the organism and test which antibiotics it is susceptible to, treat based on those results, and retest until clean. Document the history. Treated cells may behave differently than before, so confirm they still match earlier data, and freeze a new bank once they test clean.
Yeast cells are noticeably larger, round to oval, and often budding or clustered. Bacteria are much smaller dots or rods, and some are motile. Both cause turbidity and a pH shift, so the microscope is the tiebreaker.
Mold hyphae branch and usually radiate from a dense center. Fibers are single, unbranched strands of uniform width that fray at the ends. If you are unsure, check the same spot again in 24 hours. Mold grows; lint does not.
Not necessarily. Alkaline media is often caused by CO2 loss: a flask left out of the incubator, a cap closed too tightly on a non-vented flask, or incubator CO2 drift. Check those first, then look under the microscope.
STR stands for short tandem repeat. STRs are short stretches of DNA that repeat a different number of times in different people, so a panel of them works like a genetic fingerprint for a human cell line. You send a small sample to a testing service, and the resulting profile is compared to reference databases to confirm the line is what the label says. It is the standard identity test for human cell lines.
ICLAC is the International Cell Line Authentication Committee, a volunteer group of scientists working on cell line identity. Its Register of Misidentified Cell Lines is a free online list of lines known to be something other than what they claim, usually because another line, often HeLa, took over the culture years ago. As of its February 2026 update it lists 608 lines. Check it before you start working with a line, and before you publish.
Not as your disinfection method. NIH, CDC, NSF/ANSI, and ABSA agree UV is not recommended or necessary for cabinet decontamination. If you use it, disinfect chemically first, run it only in an empty, closed cabinet, and maintain the lamp.

References

  1. ATCC. Mycoplasma Contamination. atcc.org/the-science/authentication/mycoplasma-contamination
  2. Olarerin-George AO, Hogenesch JB. Assessing the prevalence of mycoplasma contamination in cell culture via a survey of NCBI's RNA-seq archive. Nucleic Acids Res. 2015;43(5):2535-2542.
  3. Sigma-Aldrich. Cell Culture Contamination Troubleshooting.
  4. Sigma-Aldrich. Mycoplasma Clearance Performance of Stericup and Steritop Filters.
  5. Merck / ECACC. Testing for Mycoplasma.
  6. Mycoplasma detection methods comparison, Table 3. pmc.ncbi.nlm.nih.gov/articles/PMC4453835
  7. ALTEX. 2013;30(3):275-291. Mycoplasma detection methods, Table 7.
  8. Bionique Testing Laboratories. M-250 Multi-media Direct Culture and DNA Fluorochrome Test.
  9. ICLAC Register of Misidentified Cell Lines, v14 (Feb 2026). iclac.org/databases/cross-contaminations
  10. Lab Manager. CO2 Incubators: Contamination and Key Operational Issues; Cell Culture Contamination, Part 3.
  11. University of Ottawa. Guideline for Use of UV Lamps in Biological Safety Cabinets.
  12. Burgener J. Applied Biosafety. 2006;11(4):228-230.
  13. University of Utah OEHS. UV Lamps in Biological Safety Cabinets Fact Sheet.
  14. UC San Diego EH&S. Biosafety Cabinet Usage.
  15. Eppendorf. White Paper 066: CellXpert C170 CO2 Incubator Copper Interior (manufacturer data).
  16. Kaur H, et al. Antibacterial activity of solid surfaces is critically dependent on relative humidity, inoculum volume and organic soiling. FEMS Microbes. 2023. doi:10.1093/femsmc/xtad022