Serum Sensitivity

The serum sensitivity assay was conducted as described previously with slight modifications (Pan et al., 2015; Lin et al., 2017). Briefly, the human blood sample from healthy donor was centrifuged at 1,000 rpm for 10 min to obtain serum and erythrocytes. K. pneumoniae 94 and 256 were used to test the serum sensitivity with Dpo42 and Dpo43, respectively. Overnight bacteria cultures were centrifuged, resuspended in PBS to give a bacteria concentration of 10⁹ cfu/mL, and treated with the depolymerase at an enzyme concentration of 10 μg/mL. Then, the treated bacteria were immediately mixed with active or inactive serum (heated at 56°C for 30 min) at a volume ratio of 1:3 to a final reaction volume of 100 μL for 1 h incubation at 37°C. The untreated bacteria were incubated with enzyme or active serum as controls. The reaction mixtures were serially diluted and plated for bacterial counting. The experiment was independently performed three times.