ELECTRON AND LIGHT MICROSCOPIC STUDIES OF BACTERIAL NUCLEI I

Abstract

A preparative procedure is described which permits parallel electron and light cytological observations to be made to complement one another to learn the effects of successive steps of cytological processing and in analyzing the nature of the structures under observation. Osmic acid vapor is thus shown to be an excellent fixative in that it preserves the pre-existing pattern of nuclear sites and enveloping cytoplasm, and accentuates contrast both in the electron and light micro-scopic pictures. Treatment with HC1 completely reverses, in the electron microscope image, the pattern of density contrast between nuclear sites and cytoplasm. The HC1 treatment removes the cytoplasmic basophilia, permitting clear visualizing in the light microscope of the stained nuclear chromatin. The unfixed nuclei of young actively growing Escherichia coli cells are less dense than the enveloping cytoplasm. The contrast in density between nuclear sites and cytoplasm is accentuated both in electron and in light pictures by fixation with OsO4 vapors. These facts strongly suggest that the bacterial nuclei have a lower density (mass per volume) of solid matter to affect electrons and photons and to fix osmium than the surrounding cytoplasm. The nuclei also have characteristic shapes and do not merge with the cytoplasm. In the nuclear sites after appropriate processing Feulgen-positive bodies are found that stain with chromatin dyes and behave characteristically toward nucleic-acid-splitting enzymes. Expressed in the idiom of the cytologist, bacteria possess vesicular nuclei containing chro-matin.