Bacteria (singular: bacterium or bacterium) are microorganisms consisting of only one single cell. They belong to the “prokaryotes”, which, unlike eukaryotes (the cells found in human, animal and plant organisms), do not have a real cell nucleus. The word “prokaryont” means nucleus replacement: instead of the typical cell nucleus of eukaryotes, which is separated from its environment by a double membrane, bacteria are referred to as having a nucleus equivalent.

The genetic material (DNA), which is located in the cell nucleus of other organisms, is freely available in the cell water (cytoplasm) of bacteria. In bacteria, this DNA is a strand-like molecule, the bacterial chromosome. However, it often does not just float around, but is attached to the cell membrane.

Cell membrane, cytoplasm, DNA and the ribosomes (smallest protein structures necessary for protein biosynthesis) are found in every bacterial cell. Other organelles, which however only occur in some bacteria, are a cell wall, an outer cell membrane, flagella (for locomotion), pili (for attachment to interfaces), plasmids (small DNA fragments that can be exchanged between bacteria and thus make an important contribution to the development of resistance and gene transfer), a mucus membrane and vesicles (vesicles) that contain gas. Besides bacteria, fungi are also important potential pathogens of diseases.


Bacteria are small microorganisms with a size of about 0.6 to 1.0 μm. They can have various external shapes such as spherical, cylindrical or helical. In their internal structure, however, they are all similar.

Bacteria consist of only a single cell. This cell contains the bacterial chromosome, which represents the genetic material of the bacterium, the DNA. This DNA is about 1.5 millimetres long and ring-shaped.

The DNA floats freely in the cell water, the cytosol. Bacteria therefore do not have a real cell nucleus and are therefore counted among the so-called prokaryotes. The cell water also contains other structures known as cell organelles.

Cell water and cell organelles together are called cytoplasm. Cell organelles are for example ribosomes and plasmids. A ribosome is a protein that the bacterium needs to produce further proteins.

A plasmid is a small DNA fragment that contains additional genetic information, for example resistance genes. Bacteria can exchange plasmids with each other and thus transfer their DNA to other bacteria. The cell water is limited by a cell wall.

The cell wall maintains the external shape of the bacteria and provides protection against external influences (other bacteria, bad environmental conditions). For further protection some bacteria are additionally surrounded by a capsule. The bacterial cell wall consists largely of murein, a multiple sugar with a net-like structure.

The net of several layers of murein envelops the entire cell. Some bacteria carry other substances in their cell wall, such as certain proteins and fatty acids. These can have a disease-causing effect on the human body and can cause fever, for example.

The cell wall is lined on the inside with a cell membrane. Invaginations of this cell membrane are called mesosomes and serve to increase the surface area. Cell extensions, the so-called pili, emanate from the cell wall.

Pili serve the bacterium to attach to other bacteria or cells. Some bacteria carry twisted protein threads, the so-called flagella, for their locomotion. These move like a propeller when consuming energy. Depending on the type of bacteria, there can be more than 12 flagella.