Testing for mycoplasma in cell culture typically involves one of four approaches: PCR-based detection, biochemical (enzymatic) assays, DNA fluorescence staining, or direct culture on agar and broth media. Each method trades off speed, sensitivity, and cost differently, and many labs use a combination of two methods to catch contamination reliably. With estimates suggesting up to 35% of continuous cell lines carry mycoplasma, routine testing is not optional.
Why Mycoplasma Is So Easy to Miss
Mycoplasma are the smallest self-replicating organisms, and unlike bacterial or fungal contamination, they don’t make your media cloudy or change its color. Cultures can harbor heavy mycoplasma loads for weeks or months with no visible signs, quietly warping gene expression, altering growth rates, and corrupting experimental results. Six species account for more than 95% of all cell culture infections: M. orale (20 to 40% of cases), M. hyorhinis (10 to 40%), M. arginini (20 to 30%), M. fermentans (10 to 20%), M. hominis (10 to 20%), and A. laidlawii (5 to 20%). Three of these are human species found naturally in the mouth and throat, which is why talking over open flasks or pipetting by mouth are classic routes of contamination.
PCR Detection
PCR is the most sensitive rapid method available. It targets conserved regions of the mycoplasma 16S ribosomal RNA gene, which is shared across nearly all mycoplasma species, making a single primer set effective as a universal screen. PCR and related methods can detect fewer than 10 colony-forming units in a sample, far below what culture-based methods require. Results come back within hours rather than weeks, and specificity is essentially 100% when the assay is properly validated.
Most labs either run an in-house PCR protocol or send samples to a commercial service. If you run your own, a “hot start” step is important to prevent nonspecific amplification from other bacterial DNA that may be present. Commercial kits simplify this by including optimized primer mixes and positive controls. One practical consideration: PCR detects mycoplasma DNA whether the organisms are alive or dead. If you’ve recently treated a culture with an antimycoplasma agent, a positive PCR result may reflect dead organisms rather than an active infection. In that scenario, pairing PCR with a viability-based method helps clarify the picture.
Biochemical (Enzymatic) Assays
The MycoAlert assay from Lonza is the most widely used biochemical test. It works by detecting enzymes that mycoplasma produce but mammalian cells do not. You add a substrate to your cell culture supernatant, take a luminescence reading, then add a second reagent, wait 10 minutes, and take a second reading. The ratio of the second reading to the first determines your result.
A ratio below 1 is negative. A ratio above 1 indicates mycoplasma contamination. Borderline results, roughly 1.0 to 1.3, should be quarantined and retested after 24 hours. The entire test takes under 20 minutes from start to finish. Sensitivity sits around 10 to 50 colony-forming units per milliliter depending on the species, which is less sensitive than PCR but more than adequate for catching established infections. Because the assay measures active enzyme activity, it only detects live mycoplasma, which is an advantage when monitoring the success of a decontamination treatment.
DNA Fluorescence Staining
Staining with fluorescent DNA dyes like Hoechst 33258 or DAPI is one of the cheapest and fastest screening options. You fix your cells on a coverslip, apply the dye, and examine them under a fluorescence microscope. Mycoplasma appear as tiny bright dots or a haze of fluorescence clustered around or between cells, distinct from the large, round nuclear staining of your host cells.
The limitation is sensitivity. Mycoplasma genomes are tiny, so the fluorescent signal they produce is faint compared to the nuclear DNA of your cultured cells. Low-level contamination can be difficult to distinguish from background fluorescence or cellular debris. A newer approach uses enzymatic nick-translation to incorporate labeled nucleotides directly into mycoplasma DNA, then visualizes them by immunofluorescence. This method produces a stronger, more specific signal that doesn’t intensely stain host nuclei and doesn’t cross-react with other bacteria. Formaldehyde fixation followed by permeabilization gives the best results with this technique.
Direct Culture on Agar and Broth
Microbiological culture remains the regulatory gold standard. The USP <63> protocol calls for inoculating samples into both broth and onto solid agar, then incubating at 35 to 37°C with 5% CO₂. Hayflick’s medium is the traditional formulation, though some species (M. hyorhinis, M. pneumoniae, and others) require SP4 medium supplemented with arginine to grow at all. Colonies are identified under a dissecting microscope at 20x magnification, where they show a characteristic “fried egg” morphology.
The major drawback is time. The full USP protocol runs 28 days. Even under ideal conditions, colonies take 2 to 6 weeks to appear, and the method requires more than 1,000 colony-forming units to reliably detect contamination. That’s orders of magnitude less sensitive than PCR. Culture does have the unique advantage of recovering live organisms for species identification and antibiotic susceptibility testing, which matters if you need to characterize an outbreak across multiple lines. For routine screening, though, most labs have moved to faster methods and reserve culture for regulatory submissions or confirmatory testing.
How to Collect a Good Sample
Test results are only as reliable as the sample you collect. To maximize sensitivity, grow your cells to 80 to 90% confluence and let the culture sit in the same media without refreshing for three days before testing. This gives mycoplasma time to shed enough DNA and enzymes into the supernatant to be detectable. Collect 500 microliters of supernatant from the flask. For suspension cell lines, stand the flask upright for about 30 minutes to let cells settle before drawing off the supernatant.
One critical rule: remove antibiotics before testing. Penicillin and streptomycin are ineffective against mycoplasma, but they can suppress mycoplasma metabolic activity enough to push enzymatic assays below the detection threshold or reduce colony counts in culture-based methods. The antibiotics mask contamination without eliminating it. Ideally, passage your cells at least once in antibiotic-free media before testing. This also reflects best practice more broadly, since routine antibiotic use in cell culture is one of the major reasons mycoplasma contamination goes undetected for so long.
Choosing a Testing Strategy
No single method is perfect for every situation. PCR offers the best combination of speed and sensitivity and works well as a primary screening tool. Enzymatic assays like MycoAlert are ideal for quick, same-day checks when onboarding new cell lines or spot-checking existing ones. DNA staining is useful as a visual confirmation but shouldn’t be relied on alone due to its lower sensitivity.
Many experienced labs use a two-method approach: a rapid test (PCR or enzymatic assay) for routine screening every 2 to 4 weeks, paired with a second orthogonal method to confirm any positive or borderline results. This strategy covers the blind spots of each individual test. PCR catches low-level contamination that enzymatic assays might miss, while enzymatic assays confirm viability that PCR cannot distinguish.
For labs producing cell-based therapies or other clinical-grade products, the FDA recommends mycoplasma testing at the master cell bank, working cell bank, and final product stages. Culture-based testing per USP <63> is the default requirement, though alternative assays with comparable sensitivity are accepted if properly validated. The 28-day culture timeline creates obvious challenges for products with short shelf lives, which is driving wider adoption of validated rapid PCR methods in regulated settings.