Tobacco mosaic virus
(TMV) makes two types of basic inclusions,
crystalline aggregates of virus particles (top left
and middle) and a vacuolate inclusion called an X
body (top right). The X body is usually present in
early stages of the infection but may be absent in an
older infection. A third type of inclusion often seen
in older infections is called a paracrystal (bottom
right). All these inclusions stain in OG and Azure A
(AA). However the AA stain must be heated or
microwaved for the AA stain to penetrate the
crystalline inclusions (compare the staining of the
top middle crystal to those in the bottom left and
bottom right). The X body will stain in AA without
heat (top middle). These differential staining
characteristics can be diagnostic for TMV infections.
