Organization of the Nucleoid and Differences from the Eukaryotic Nucleus
The bacterial cell lacks a true nucleus. Its function is performed by the nucleoid, located in the central zone of the cytoplasm. Normally, the genetic apparatus is represented by a single chromosome. Physically, this is a double-stranded DNA molecule, closed in a ring and coiled like a dense tangle. An exception is the Lyme disease pathogen, the spirochete Borrelia burgdorferi, which features a linear DNA molecule.
There are strict differential diagnostic features distinguishing the nucleoid from a fully formed eukaryotic nucleus:
- Absence of a nuclear envelope: the genetic material is not separated from the cytoplasm by a membrane.
- Absence of a nucleolus: there are no additional dense structures inside.
- Absence of histones: DNA is not bound to these core proteins characteristic of eukaryotes.
During disturbances and errors in cell division processes, the normal amount of genetic material may change: a single bacterial cell may temporarily contain four or more chromosomes.
Methods for Visualizing the Genetic Apparatus
Studying the bacterial genetic apparatus requires special techniques, as the nucleoid is not visible under a light microscope using simple staining methods. Specific DNA stains—such as the Feulgen method or Romanowsky-Giemsa stain—are used for its detection.
Electron microscopy of ultrathin sections provides the most detailed picture:
- On electron micrographs, the nucleoid appears as a light (electron-lucent) zone.
- Within this zone, fibrillar (thread-like) structures representing the DNA molecule itself are clearly distinguishable.
The structure varies among different microorganisms. In Gram-positive bacteria (e.g., Listeria monocytogenes), a thick cell wall, a division septum, and a mesosome contacting the nucleoid are visualized. In Gram-negative bacteria (e.g., Brucella melitensis), the cell wall is thinner, the cytoplasmic membrane is visible, and the nucleoid occupies a significant volume of the cytoplasm as an extensive fibrillar network.
Connection of the Nucleoid with Membrane Structures
The nucleoid DNA does not float freely in the cytoplasm; it has defined attachment points. The chromosome is anchored at specific sites to the cytoplasmic membrane or to a mesosome (a specific inward invagination of the membrane).
These attachment sites are of critical functional importance. They act as molecular anchors actively participating in bacterial chromosome replication, as well as ensuring the correct segregation of daughter DNA molecules into new cells during division.
Extrachromosomal Factors: Bacterial Plasmids
In addition to the main nucleoid, bacterial cells may contain plasmids—autonomous genetic elements. Structurally, these are covalently closed circular DNA molecules. Their primary biological role is to encode additional traits that provide the bacterium with a selective (evolutionary) advantage under specific conditions.
Classification of plasmids based on their functions:
- R-plasmids: confer antimicrobial resistance.
- F-plasmids: control the cell's ability to undergo conjugation, i.e., transfer genetic material to other bacteria.
- Col-plasmids: are responsible for the production of specific substances called bacteriocins (specifically, colicins), which are necessary to suppress the growth of competing microflora.