Black swimming dots in cell culture: the identity, detection method and judging criteria

Black swimming dots (BSDs) are extremely tiny dot-like contaminants found in dishes of cultured cells. They are harmful to cells and very hard to be removed. BSDs contamination has always been a worldwide unsolved conundrum. Now almost nothing is known about BSDs. It is generally accepted that BSDs come from serum, so speculations that BSDs are serum precipitates or some kind of unknown protozoa parasitizing in bovine serum are popular. And there are also some other proposed viewpoints to explain the identity of BSDs: cell debris, mycoplasma, Achromobacter, nanobacteria, nonliving calcified nanoparticles such as hydroxyapatite or CaCO3. Here we show the above speculations are incorrect or incomplete with firm empirical evidence. We demonstrate BSDs possess biotic-abiotic duality. Namely, BSDs per se are nonliving inorganic nanoparticles yet should derive from an unidentified airborne infectious organism. Therefore, we suggest that future investigations should focus on both the cells and the dots for the final identification of BSDs pathogen. We also present a new method of observing the perivitelline space of MII oocytes to screen out BSDs infected animals. This method can accurately tell whether the donor cattle for FBS production or the animals for primary cells isolation carry BSDs. Thus it is of great scientific and economic significance for the credibility and reproducibility of basic research and serum production enterprise. Moreover, we propose some criteria for judging whether tiny-black-dots are definitely BSDs or other BSDs-like-but-not-be contaminants when they are observed in cell or animal samples.