Figure 2

Freezing is insufficient for inactivation of CMV infectivity: detection of plaque-forming virus after freeze-storing of spiked milk. Native seronegative milk was spiked with 10⁵ TCID₅₀/mL CMV laboratory strain AD196 and stored for 10 d at −20°C. After thawing, milk whey was prepared and inoculated on microculture-monolayers of HFF. After incubation for 5 d, CMV IEA staining was performed (13). Intranuclear staining of CMV IEA (brown) revealed replication competent virus with plaque formation. Control: spiked milk without freezing, 150 plaques/microculture; freeze-thawed milk: 29 plaques/microculture. Counterstaining of noninfected fibroblast nuclei (blue) was performed using Mayer's hemalum/hematoxylin solution (13).