What should be done if flocculent precipitate always appears in serum-free mesenchymal stem cell culture medium?

If precipitation occurs after P3, it is recommended to check both the basal medium and the supplement factor lot numbers. Contact the manufacturer's technical support, provide photos of the precipitate, time of occurrence, supplementation protocol, and storage conditions, and request re-examination of appearance, turbidity, and osmolality. If multiple bottles from the same lot show precipitation, even if the cells appear fine temporarily, it is not recommended to continue using them for critical experiments, especially for subsequent induction or cryopreservation. A different lot or brand in a smaller package can be used as a control to see whether precipitation still occurs. For each 2 L preparation, be sure to record the lot numbers of the basal solution, factors, and consumables so that it can be determined whether the precipitate comes from the basal solution or the supplement.

Don't rush to conclude contamination. Take the precipitate suspension for Gram staining and DNA fluorescence staining, and simultaneously inoculate the supernatant onto blood agar or perform sterility culture; if sterile and cell condition has not significantly declined, it is more likely protein or lipoprotein precipitation. After supplementing factors in serum-free systems, fine flocculent precipitates are inherently prone to form, especially after P3 when cell density is high and metabolites change; pH and temperature fluctuations promote aggregation. It is recommended to centrifuge at low speed to collect the precipitate and check whether it is loose and can be dispersed by pipetting; if it decreases after rewarming at 37°C, it is mostly reversible precipitation. If the precipitate has an odor, is turbid, or bacteria are seen on staining, immediately discontinue use of that batch and investigate the procedure.

For each 2 L preparation, after adding the supplements, the solution must be passed through a 0.22 μm filter again; do not rely solely on the base solution already being sterile. The supplements must be fully dissolved in a suitable solvent, then slowly added along the wall with stirring to avoid localized high concentrations. The water bath temperature should not exceed 37°C, and the time should not be too long. After preparation, aliquot into single-use portions to avoid repeatedly warming the entire bottle. If precipitate appears only after standing at 4°C and disappears after rewarming at 37°C, it may be reversible precipitation; if it still persists after rewarming, it is recommended to discontinue that lot and perform a small-scale recovery test to see whether the cells can attach and proliferate normally.

First rule out contamination, then check the batch of serum replacement; switching to a different batch may eliminate the flocculent precipitate.

We encountered precipitation at 4°C, which can be dissolved by shaking in a 37°C water bath, but it is best to filter before use.