Infectivity of the components of a nuclear polyhedrosis virus of the silkworm, Bombyx mori

The polyhedra of the nuclear polyhedrosis virus (NPV) of Bombyx mori were dissolved under controlled dissolution with weak alkali. Five components were separated by centrifugation of the dissolved solution in a sucrose gradient. The assay of the separated components in B. mori pupae showed that two types of rods were almost equally, highly infectious, whereas the small spherical particles were much less infectious than the rods by an order of 4 log units. The infectious unit of this virus appeared to be the virus rod without the outer membrane, and the small spherical particle did not represent any infectious functional unit but was a degradation product of the viral internal substance and the outer membrane may not be essential for the virus infection in the pupal hemocoel.