Fundamental Techniques in Cell Culture Laboratory Handbook-2nd Edition
Flow Scheme for Detection of Mycoplasma by Indirect DNA Stain
DNA staining methods such as indirect Hoechst staining techniques are quick with results available within 72 hours which compares favourably with 4 weeks for detection by culture. Staining of cultures directly with a DNA stain can provide results in 24 hours, however, in a much-reduced sensitivity (~106cfu/ml). This may be improved by co-culturing the test cell line in the presence of an indicator cell line such as Vero. This enrichment step results in a sensitivity of 104 cfu/ml of culture and is the preferred method used by ECACC. This step also improves sensitivity by increasing the surface area upon which mycoplasma can adhere. As with detection by culture, DNA staining methods are suitable for the detection of mycoplasma from cell cultures or cell culture reagents.
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- Place sterile coverslips in tissue culture dishes/plates e.g. 12 well plates.
- Inoculate indicator cells into the prepared dishes e.g. 2 x 104 Vero cells per well of 12 well plate.
- Incubate at 37°C in 5% CO2 for 2 – 24 hrs to allow the cells to adhere to the cover slips.
- Bring attached test cell lines into suspension using a cell scraper. Suspension cell lines may be tested directly.
- Remove 1ml of culture supernatant from duplicate dishes and add 1ml of test sample to each. Inoculate 2 dishes with 100 cfu of each positive control organism.
- Leave duplicate tissue culture dishes un-inoculated as negative controls.
- Incubate dishes at 37°C in 5% CO2 for 1-3 days.
- After 3 - 5 days fi x cells to cover-slip by adding a minimum of 2 ml of freshly prepared Carnoy’s fi xative (1:3 glacial acetic acid: absolute methanol) to each dish and leave for 3 to 5 minutes. Then decant to toxic waste bottle. Repeat once more. Add a minimum of 2ml Hoechst stain 0.4μg/nl. Leave for 5 minutes shielded from direct light e.g. by covering with aluminium foil.
- Decant used and unused stain to toxic waste.
- Add 1 drop of mountant to a pre-labelled microscope slide and place coverslip (cell side down) onto slide.
- Keep slide covered with aluminium foil, allowing it to set for at least 15 minutes at 37°C or for 30 minutes at room temperature.
- Observe slide under UV Epi-Fluorescence at x1000.
Criteria for a Valid Result
Negative controls show no evidence of mycoplasma infection Positive controls show evidence of mycoplasma infection Vero cells clearly seen as fluorescing nuclei.
Criteria for a Positive Result
Samples infected with mycoplasma are seen as fluorescing nuclei plus extra-nuclear fluorescence of mycoplasma DNA (small cocci or filaments).
Criteria for a Negative Result
Uninfected samples are seen as fluorescing nuclei against a dark background. There should be no evidence of mycoplasma i.e. extra-nuclear fluorescence of mycoplasma DNA.
- DNA stains such as Hoechst stain bind specifi cally to DNA. In all cultures cell nuclei will fl uoresce. In theory uncontaminated cultures will show only fl uorescent nuclei whereas mycoplasma positive cultures contain small cocci or fi laments which may or may not be adsorbed onto the cells (see fi gure 16). However, please note that any extraneous DNA will also fl uoresce e.g. cell debris - which is sometimes mistaken for mycoplasma contimation.
- Hoechst stain is toxic and should be handled and discarded with care.
- Culture dishes should be placed in a sealed box or cultured in large petri dishes to reduce evaporation.
- In some instances results may be diffi cult to interpret for the following reasons:
- Bacterial/yeast/fungal contamination
- Too much debris in the background (as in the case of hybridoma cell lines)
- Broken nuclei as cells are all dead
- Too few or no live cells
- Although this procedure recommends the use of positive controls, this may not necessarily be feasible or desirable in a cell culture facility with limited resources. If positive controls are to be used they should be done so in a separate laboratory from the main tissue culture facility. If this is not possible then positive control slides can be purchased from ECACC.
If positive controls are not being used then it is strongly recommended that you get an independent testing laboratory to periodically test your cell lines.
- ECACC recommends that samples are tested for mycoplasma using at least two detection methods (e.g. indirect DNA stain and culture isolation) for a more reliable result. This is due to the varying detection sensitivities of the methods for different species of mycoplasma.
ECACC offers a mycoplasma testing service where three different methods of detection are available i.e. PCR, indirect DNA stain and culture isolation.
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