The structure of bacteria is simpler than that of other organisms. Bacteria are microorganisms that lack membrane-bound organelles and a well-defined nucleus. They usually adopts one of four shapes, depending on the type of bacteria and have numerous appendages with distinct functions.
Bacteria are unicellular, which means they consist of a single cell only. Everything that the bacteria needs to survive and reproduce, except for food, is contained within this single cell. As such, the bacterial structure has developed in ways that allow the cell to go about its business efficiently.
Bacteria are prokaryotes, meaning they have no organized nucleus. The nucleus is the organelle that houses the deoxyribonucleic acid (DNA) and the genetic materials necessary for replication in more evolved cells. Despite the absence of a nucleus, bacteria still possess DNA, but it floats freely within the cytoplasm.
A cell wall surrounds the cytoplasmic region, or interior, of the bacteria and acts as the support structure for the entire organism. The underlying layer of the cell wall is formed from peptidoglycan, a molecule that is comprised of amino acids and sugars. This is a feature unique to bacteria and is what makes the cell wall resilient to pressures both internal and external.
The peptidoglycan gives the bacterium its shape. The shape of a given cell is usually distinct to that species and is an important detail in identifying unknown species. Two common shapes for bacteria are the bacillus, which is rod-shaped, and the spirillum, which is spiral-shaped. Other common forms are the coccus, or spherical, and the filamentous, in which the individual cells grow in length but do not separate.
Lining the cell wall is a membrane that selectively allows molecules into and out of the cell. Cytoplasm fills the area inside the cell membrane. Within the cytoplasm are the chromosomes, the DNA of the cell. The cytoplasm also contains the ribosomes that create the proteins used by the cell.
On the exterior of the cell wall, various extensions exist to help bacteria perform actions such as moving and attaching to other objects. Flagella, filaments of protein, are located on the exterior of the organism and move the bacteria through its environment. Bacteria may have one or many flagella depending on the species.
Another surface feature of bacteria are fimbrae. These filaments of protein are used by the bacteria to attach itself to other structures. They exist over the entire surface of the bacterium and are much smaller than flagella. A third component is a thread-like protein extension called a pilus, also found on the exterior of the cell. Pili aid in bacterial conjugation and attachment.