CPA1 and infection: In MDCK cells, the C-PA titer was always lower than that of WSN and plateaued (4.7 ± 0.2 × 104 PFU/mL) 16 hr post-infection (pi), while the WSN titer was 2.2 ± 0.03 × 105 PFU/mL 16 hr pi and plateaued (5.6 ± 1.6 × 106 PFU/mL) 36 hr pi (Figure 2A).