Fig. 2
Plaque formations of DENV-1 and DENV-2 of a focus forming assay, PRNT in 24-well plates. a Morphology of DENV-1 and DENV-2 in virus control and negative serum control wells of a-24 well plate (magnified 2 ×). Patient sera with PRNT70 titer of >625 against both DENV-1 and DENV-2 were used as the positive serum controls, whilst healthy human serum was used as a negative control. b A proportion of the 24 tested patient sera against DENV-1 (i) and DENV-2 (ii). Patient sera (diluted 1:25) were incubated with 20–40 PFUs of DENV-1 or DENV-2 for 1-h at 37 °C, followed by adsorption for 1-h on Vero cells monolayer, overlaid with CMC medium and further incubated for four days. The infected cells or plaques were immunostained with a mouse monoclonal dengue virus type 1, 2, 3, and 4 antibody [D1–11(3)] (GeneTex, Inc.) followed by alkaline phosphatase-conjugated anti-mouse IgG, and viral plaques were visualised using NBT-BCIP substrate