Ultrastructure and Organization of Bacterial Chromosomes

Abstract

Ever since the early observations by the Dutch microscopist Antonie van Leeuwenhoek in the late seventeenth century (communicated in a series of letters published in the Philosophical Transactions of the Royal Society) researchers have been fascinated by the ability to magnify and visualize cells and microorganisms microscopically. In eukaryotic cells due to their relatively large size and the separation from the cytoplasm by a membrane, cellular organelles such as the nucleus or mitochondria can be readily visualized in a simple light microscope. The situation is more complex in organisms that are several orders of magnitude smaller and in which the genetic material is not membrane-enclosed, such as bacteria and archaea. While by the end of the nineteenth century the nucleus and its mitotic dynamics had been resolved and the terms ‘chromatin’ and ‘chromosome’ had been coined, knowledge of a possible bacterial equivalent was still lacking. This was likely due to the fact that the chromosomal DNA of bacteria is translucent and featureless in the light microscope when not stained, and that the histological stains of that time (successfully applied to the nuclei of eukaryotic cells) were not successful in revealing the morphology of the genomic material of bacteria (Robinow and Kellenberger 1994). Despite the fact that a consistent morphology of the folded bacterial genome could not be described, bacterial cytologists during the first decades of the twentieth century became convinced that bacteria indeed contain ‘chromatin bodies’ (Delaporte 1939-1940). Particularly important for this development was the introduction of the Feulgen procedure and the Giemsa stain that specifically stain DNA and yielded ‘nucleoids’ of reproducible, regular morphology (Neumann 1941; Piekarski 1937). The next big step forward in terms of resolving the nucleoid in much more detail was expected when electron microscopes became widely available in the late 1940s. Structures similar to the nucleoids observed in light microscopy studies (Piekarski 1937; Stempen 1950) were found, but, unlike the nuclei in eukaryotic cells, these had low electron density and did not resolve much additional detail (Hillier et al. 1949).