Mycoplasma Testing in Cell Culture

In cell culture, mycoplasma means a member of the class Mollicutes — most often Mycoplasma orale, M. arginini, M. fermentans, M. hyorhinis or Acholeplasma laidlawii. Between them these few species account for the great majority of cell-culture contaminations, and every one of them is a laboratory nuisance rather than anything you would encounter outside a tissue-culture room.

Four properties make mycoplasma harder to catch than any other biological contaminant:

• Size. Mycoplasma cells are roughly 0.3–0.8 µm in diameter and highly pleomorphic. They deform enough to pass through 0.2 µm and 0.22 µm sterilising-grade membranes, so filter-sterilising a supplement is not a reliable barrier against them.
• No cell wall. Mollicutes have no peptidoglycan wall. Every antibiotic that works by blocking cell-wall synthesis — penicillin first among them — has no effect whatsoever. A culture kept permanently on penicillin–streptomycin is not protected against mycoplasma.
• No visible signature. Mycoplasma reaches 10⁷–10⁸ organisms per mL without clouding the medium, without a phenol red colour shift you would notice, and without particles visible under a routine inverted phase-contrast microscope. There is nothing to see.
• They are metabolically active. At those densities mycoplasma consumes arginine, thymidine, glucose and other nutrients directly from the medium, so it competes with your cells even while the flask looks normal.

Published prevalence estimates are consistently uncomfortable. Continuous cell lines are reported contaminated at rates of roughly 15–35%, and an independent survey of public RNA-seq data found mycoplasma reads in about 11% of submitted experiment series — evidence that contaminated cultures routinely reach publication.

Mycoplasma rarely kills a culture. That is precisely the problem: it changes the culture instead, and the changes are silently absorbed into your results.

Documented effects include depletion of arginine and other amino acids from the medium, depletion of nucleotide precursors, altered rates of proliferation and altered saturation density, changes in membrane composition and surface antigen presentation, chromosomal aberrations after prolonged carriage, altered response to cytokines and to drugs, and global shifts in host gene expression. A contaminated line can produce internally consistent, reproducible data that is simply not about your biology.

The practical consequences are the ones that cost money: transfection and transduction efficiencies drop, viral titres change, cytotoxicity IC₅₀ values shift, antibody yields from hybridomas fall, and results stop reproducing between labs. Several journals and most funders now expect a documented mycoplasma status for every cell line used in a submitted manuscript.

Knowing the source is what makes prevention tractable, because the sources are few and they map cleanly onto the species found.

• Other contaminated cultures in the same lab. This is the dominant route by a wide margin. Aerosols from pipetting, shared media bottles, shared waterbaths and shared incubators move mycoplasma between flasks. A single contaminated line in a shared incubator will eventually seed its neighbours.
• Laboratory personnel. M. orale, M. salivarium and M. fermentans are human oral commensals. They arrive by talking over an open flask, by an unmasked cough, or on hands.
• Animal-derived raw materials. M. arginini and A. laidlawii are associated with bovine serum; M. hyorhinis with porcine-derived trypsin. Modern serum and trypsin are filtered and tested, so this route is much less common than it once was, but it is not zero — which is one reason animal-origin-free reagents reduce risk.
• Incoming cell lines. Any line received from a collaborator, rather than from an authenticated repository, should be treated as positive until tested.

The pattern that follows is simple: quarantine everything incoming, do not share bottles between lines, and test.

No single assay is right for every situation. The trade-off is between sensitivity, turnaround and how much the result can be relied on for release of a critical batch.

PCR and qPCR amplify conserved regions of the 16S rRNA gene (or the 16S–23S intergenic spacer) shared across Mollicutes. Real-time assays reliably detect down to about 1–10 CFU/mL and return a result within a few hours. They are the default routine screen in most labs. Two cautions: PCR detects DNA from dead as well as live organisms, and both false negatives from PCR inhibitors in conditioned medium and false positives from amplicon carryover are real risks. Run an internal amplification control and a no-template control every time.

Enzymatic luminescence assays exploit mycoplasma-specific enzymes that convert ADP to ATP; the ratio of luminescence readings before and after substrate addition gives the result in about 20 minutes. They are the fastest option and need no thermal cycler, but they detect only viable organisms and are less sensitive than PCR.

Fluorescent DNA staining with Hoechst 33258 or DAPI shows mycoplasma DNA as small extranuclear specks over the cytoplasm and between cells. Read directly on the test culture it is insensitive — around 10⁶ CFU/mL. Read indirectly, after co-culturing the test supernatant on a clean indicator line such as Vero or 3T6 for 3–6 days, sensitivity improves to roughly 10² CFU/mL. It is inexpensive and needs only a fluorescence microscope, but reading it well takes experience.

Direct culture in mycoplasma broth and on agar, incubated aerobically and anaerobically for up to 28 days, is the compendial reference method and the most sensitive for organisms it can grow. Its weakness is that several important contaminants — M. hyorhinis strains in particular — are difficult or non-culturable, so culture alone can miss them. Pharmacopoeial testing therefore pairs direct culture with an indicator-cell DNA stain.

Sequencing-based screening falls out of RNA-seq or whole-genome data you already have: reads mapping to Mollicutes genomes are strong evidence of contamination. It is not a test you would commission, but it is a free retrospective check on any dataset.

A defensible testing schedule for a research lab:

1. On receipt, before the line enters the general incubator. Quarantine it in a separate incubator, or at minimum a separate shelf with dedicated media, until it clears.
2. After thawing a new vial from a working bank, if that bank has not itself been tested.
3. Monthly for every line in active culture. Weekly is common in labs running cell therapy or vaccine work.
4. Before freezing any master or working cell bank. Banking a contaminated line multiplies the problem across years.
5. Before any critical experiment whose result you intend to publish or use for a release decision.
6. After any incident — a spill, an incubator failure, a suspected contamination elsewhere in the room.

Test the culture at the point of greatest sensitivity: medium that has been on the cells for at least 48–72 hours, from a flask that is at least 70–80% confluent, with no antibiotics present for at least two passages. Antibiotics in the medium suppress the organism to below the detection limit without eliminating it, and turn a real positive into a false negative.

Act on the assumption that the positive is real and that it has spread.

1. Stop passaging the culture. Do not open it in the hood again except inside a containment plan.
2. Confirm with a second, mechanistically different method. A PCR positive confirmed by indirect DNA stain, or by an enzymatic assay, is a result you can act on. A single PCR line on a gel is not.
3. Test every other line in the same incubator, and every shared reagent bottle. Assume horizontal spread until you have data.
4. Decontaminate. Empty and disinfect the incubator including the water pan, wipe the hood, discard shared media and supplement bottles that were open during the period in question, and replace the waterbath water.
5. Log it. Record the line, the passage, the date, the assay and the disposition. Contamination that is not logged recurs.
6. Restart from a clean frozen stock wherever one exists. This is almost always faster, cheaper and more defensible than treatment.

Discarding is the default, and for good reasons. Treatment is slow, incompletely effective, and leaves you with a line whose provenance you can no longer fully vouch for.

Elimination is worth attempting only when the line is genuinely irreplaceable — a unique primary isolate, a clone that took months to generate, a line with no clean frozen stock anywhere.

If you do treat, the realistic picture from the published literature is this: no single agent clears 100% of cases; formulations combining two agents with different mechanisms (typically a quinolone with a pleuromutilin or a macrolide) outperform any single agent; reported clearance rates for the better protocols sit in the region of 70–85% of treated lines; and courses run for about two to three weeks. Treatment is normally carried out on an isolated culture, in a separate incubator, with dedicated media.

The step most often skipped is the one that matters most: after treatment, withdraw the agent completely and culture the line antibiotic-free for at least two to three weeks, then re-test at least twice by two different methods before returning it to the general incubator. Suppressed mycoplasma reappears when the pressure is removed, and a line released on a single post-treatment negative frequently turns positive again a month later.

Prevention is cheaper than any test regime and vastly cheaper than a retracted result.

• Source cell lines from an authenticated repository (ATCC, ECACC, DSMZ, JCRB) rather than from a collaborator's freezer. Repository stocks are tested and authenticated.
• Quarantine every incoming line in a separate incubator until it has tested negative.
• One bottle, one line. Never return a pipette to a shared medium bottle after it has entered a flask. Aliquot supplements into single-use volumes.
• Work quietly and keep your face out of the hood. Most human-origin mycoplasma arrives by aerosol from the operator.
• Do not run routine prophylactic antibiotics. They cannot touch mycoplasma, and they hide the bacterial contamination that would otherwise have alerted you to a technique problem.
• Bank early and bank clean. A tested master bank at a low passage is the insurance policy that makes discarding a contaminated culture a minor inconvenience rather than a disaster.