In situ electron microscopic observation of negatively stained tissue culture cells contaminated with mycoplasmas.

A simple, fast, and in situ method of detecting the inapparent infection of cultured cells with mycoplasmas is reported. Animal cells grown on Formvar-coated electron microscopic grids were directly fixed with glutaraldehyde, negatively stained with phosphotungstic acid and examined by transmission electron microscopy. Cells contaminated with mycoplasmas could be discriminated from uncontaminated cells. The micro-organisms in the negatively stained preparations corresponded with those revealed by thin sectioning, and the distribution of mycoplasmas in cultured cells coincided with those revealed by the Hoechst staining method. Most of the highly resolved mycoplasmas were polymorphic, and closely associated with host cells; often more than 500 organisms per host cell were seen.