Nucleoids from Escherichia coli were isolated in the presence of spermidine at low salt concentrations. The nucleoids denature at relatively low temperatures or salt concentrations, yielding broad slowly sedimenting zones and/or macroscopic aggregates upon sucrose gradient centrifugation. Denaturation is accompanied by a loss of a characteristically compact shape as visualized by light and electron microscopy. Addition of polyethylene glycol or dextran prevents these changes, extending the range of stability of the isolated nucleoids to temperatures and ionic conditions like those which commonly occur in vivo. The effects of the polymers are consistent with stabilization by macromolecular crowding. Enzymatic digestion of the nucleoid DNA primarily releases three small proteins (H-NS, FIS, and HU) and RNA polymerase, as well as residual lysozyme from the cell lysis procedure. If isolated nucleoids are extracted with elevated salt concentrations under crowded, stabilized conditions, two of the proteins (HU and lysozyme) are efficiently removed and the compact form of the nucleoids is retained. These extracted nucleoids maintain their compact form upon reisolation into the initial uncrowded low-salt medium, indicating that HU, the most common ''histone-like'' protein off. coil, is not a necessary component for maintaining compaction in these preparations. (C) 1997 Academic Press.