Defining Features of Eubacterial Cells
Eubacterial cells are prokaryotic, meaning they lack a membrane-bound nucleus and other specialized membrane-enclosed organelles. Their genetic information, primarily a single, circular double-stranded DNA molecule, is located in an irregularly shaped area within the cytoplasm called the nucleoid. This allows direct access to essential cellular processes.
These microorganisms are typically unicellular and vary in size, generally falling within a range of 0.2 to over 5 micrometers. Their morphology is diverse, presenting as spherical cocci, rod-shaped bacilli, or spiral-shaped spirilla. A defining structural characteristic is their rigid cell wall, which contains peptidoglycan. This distinct cell wall composition differentiates them from other cellular life forms.
Internal Cell Components
Beneath the protective outer layers, the eubacterial cell is filled with cytoplasm, a jelly-like substance where most metabolic activities occur. This aqueous matrix contains dissolved nutrients, enzymes, and various cellular structures, providing the necessary environment for cellular growth and replication. The cytoplasm’s gel-like consistency is due to the presence of these dissolved substances.
The nucleoid region houses the bacterial chromosome, a single, long, circular DNA molecule carrying essential genetic instructions.
Many eubacteria also contain plasmids, smaller, extra-chromosomal DNA molecules. These circular DNA segments replicate independently and often carry genes that provide selective advantages, such as antibiotic resistance. Scattered throughout the cytoplasm are numerous ribosomes, responsible for protein synthesis. These prokaryotic ribosomes are of the 70S type.
Protective Outer Layers
The eubacterial cell is encased by several layers that provide protection and mediate interactions with the environment. The innermost layer is the cell membrane, a selective barrier regulating the passage of substances into and out of the cell. This membrane is composed of a bilayer of phospholipids and proteins, functioning similarly to eukaryotic cell membranes.
External to the cell membrane is the rigid cell wall. This layer is primarily composed of peptidoglycan, which provides structural integrity, maintains cell shape, and prevents osmotic lysis, the bursting of the cell due to water influx. The cell wall composition varies significantly between Gram-positive and Gram-negative bacteria.
Gram-positive bacteria possess a thick peptidoglycan layer, allowing them to retain certain laboratory stains. Gram-negative bacteria have a thinner peptidoglycan layer, covered by an additional outer membrane containing lipopolysaccharides. Some bacteria may also have an outermost capsule or slime layer, offering additional protection and facilitating adhesion.
Structures for Movement and Adhesion
Eubacteria often possess specialized external structures that enable them to move and attach to surfaces. Flagella are long, whip-like appendages that extend from the cell surface and are responsible for motility. These structures are made of a protein called flagellin and rotate like propellers, allowing the bacterium to swim through liquid environments.
Pili, also known as fimbriae, are hair-like projections found on the surface of many eubacteria. These structures are generally shorter and more numerous than flagella. Their primary function is adhesion, allowing bacteria to attach to host tissues, other cells, or environmental surfaces. Some specialized pili, like sex pili, facilitate the exchange of genetic material between bacteria through conjugation.