FIG. 5.

The 1918 influenza virus replicates efficiently in MDCK but not Huh-7 cells in the absence of trypsin. MDCK and Huh-7 cells were infected at an MOI of 0.001 and then cultivated in the absence or presence of trypsin. Supernatants were collected at 24, 48, and 72 h p.i. from MDCK cultures and at 48 h p.i. from Huh-7 cultures, and the viral titers were determined by a 50% tissue culture infective dose (TCID₅₀) assay on fresh MDCK cells. A representative experiment performed in duplicate is shown; error bars indicate standard deviations.