Two different types of nucleus are found to occur in bacteria. The first is a spherical, central or eccentric body occurring in some types of cocci and in Corynebacteria. By analogy with a similar structure in Azotobacter, it is considered that the nucleus may be of vesicular form, surrounded by chromatinic granules, and resembling the nuclear vacuole of certain yeasts.

The second type of nucleus consists of paired chromosomes, usually either one or two pairs. These chromosomes form fusion nuclei within which chromosome divisions take place; they are then redistributed to a number of daughter cells.

The majority of rod-shaped Eubacteria, including the lanceolate-celled streptococci, have this type of nucleus, which is also found in Myxobacteria. The nuclear cycle is quite distinct from the cytological processes which accompany spore and microcyst formation and germination.