Before negative staining, the purified grouper iridovirus particles were treated with enzyme and detergents according to the methods of Heppell and Berthiaume (1992) with modification. Briefly, for enzymatic digestion, the purified virus particles
suspended in TN buffer were adsorbed on formvar-coated copper grids. The grids were immersed in an enzymatic solution containing 20 U ml−1 of proteinase K (Sigma, St Louis, USA) in
TN buffer and incubated at 37 °C, then the grids were removed after incubation for 10 min, rinsed
with TN buffer three times and stained with 2% phosphotungstic acid (PTA, pH 6.8). For detergent
treatments, the purified virus particles in TN buffer were first applied onto formvar-coated
grids. After removing the excess fluid by touching the edge of grids with a filter paper, the grids were
covered with 1% sodium dodecyl sulfate (SDS) or 1% alkyl phenoxy polyethoxy ethanol (Triton X-
100) at different incubation periods. The grids were then washed with distilled water and stained
with 2% PTA (pH 6.8). The purified virus without any treatments was stained negatively with 2%
PTA served as control. Both ultrathin sectioned and negativly stained specimens were observed under an electron microscope (JEOL JEM-1000CX II) at 100 kV.