Degradation of extracellular Aβ by mycoplasma contaminants. (A) PCR amplification of mycoplasma genomic DNA was performed with an internal negative control template (Control -), a positive control template of M. orale genomic DNA (Control +), or with medium from mycoplasma-positive HEK293 cell cultures (Mycoplasma+ Medium). Assay was carried out according to manufacturer's instructions (MycoSensor PCR Assay Kit, Stratagene). The changes in intensity of the internal control amplification are due to competition with the mycoplasma-specific PCR. (B) Medium from mycoplasma-positive cells was centrifuged at 1000 × g for 10 min to pellet the cell debris. Conditioned medium from APP-transfected HEK293 cells was diluted 1:1 with either fresh medium (Control DMEM), or with unfiltered (Mycoplasma+ Medium) or 0.2 μm filtered [Mycoplasma+ Medium (Filtered)] medium from mycoplasma-positive cells. The resulting mixtures were incubated at 37°C for the indicated periods of time. sAPPα and Aβ were analyzed by WB, as described in Figure 1B.