You’ve probably heard of bacteria. The small organisms are regularly in the headlines because they can cause serious infections in humans and animals. In addition, there are more and more bacteria that cannot be treated or can only be treated with difficulty with the drugs available to us. But what exactly is a bacterium?
Structure of a bacterial cell
are bacteria unicellular prokaryotic organismsthat is, like all prokaryotes, they have none nucleus. In addition to the bacteria, the prokaryotes also include the archaea counted. A prokaryotic cell is also called Per- or protocyte designated.
The term prokaryote derives from the Greek words Per for «before» and karyon for «nut» or «core».
In contrast to eukaryotic cells, the DNA in prokaryotes swims freely in the cytoplasm. In Figure 1 you can see the schematic structure of a bacterial cell.
Figure 1: Structure of a bacterial cell Source: wikipedia.de
bacterial forms
Bacterial cells come in different forms. Their diameter is between 0.1 and 700 µM. That is, the largest bacteria, such as the sulfur bacterium Thiomargarita namibiensis, you can even see with the naked eye. However, most bacterial cells have a diameter of up to a micrometer and can only be observed with a microscope.
You can divide bacterial cells into different groups based on their shape. You can find an overview of this in Figure 2.
Figure 2: Forms of bacteria Source: wikipedia.de
The cell wall of the bacterial cell
The bacterial cell is from a cell wall and one cell membrane encased. It is a mechanical barrier to unwanted invaders such as bacteriophages. In addition to the cell wall and cell membrane, many bacteria are still one capsule or mucous membrane, also glycocalyx called, which additionally protects the procytes.
There are two major types of bacteria: den gram positive and the gram negative. The classification is based on the behavior of the respective bacterial type in the Gram stain. Gram-positive bacteria can be stained violet with the dye used, gram-negative bacteria cannot.
See the Gram stain article for more information on this important microbiological method.
Gram positive bacteria have an inner cell membrane (also called the cytoplasmic membrane) overlaid by a thick cell wall. The cell wall consists mainly of peptidoglycan. This stuff, too murein is a branched polymer of sugars and peptides. In the cell wall one often also finds other molecules, such as teichoic acids and proteinsassociated with the cell wall. Some gram-positive bacteria also have a sugar-based one capsulewhich gives them additional protection against invaders and the immune system of other organisms.
Gram-negative bacteria have a cytoplasmic membrane and a cell wall as well as a outer cell membrane, which spans the cell wall. The peptidoglycan layer is much thinner here than in gram-positive bacteria and does not completely fill the space between the membranes. Therefore, the gram-negative bacteria have a so-called periplasmic space.
The cell membrane of a bacterial cell
The cell membrane of a bacterial cell, like that of a eukaryotic cell, consists of a bilayer phospholipids. However, these are structurally different from those of eukaryotes. The cell membrane of the bacterial cell acts as a fabric barrier, since it is only permeable to a few molecules. It also contains important proteins. The skill vans for certain substances or needle-like structures that bacteria use to inject toxins into their host cells (secretion systems).
The transporters mentioned above also include so-called efflux pumps respectively ABC transporter (ABC = ATP binding cassette). You can imagine these as small pumps that «throw» substances that are harmful to the bacteria out of the cell. These transporters are often a reason why bacteria are resistant to antibiotics.
The cytoplasm and the periplasm
As you might imagine, a bacterial cell isn’t just empty inside. The cell components, such as DNA or ribosomes, «swim» in a liquid that Zytosol names. The cytosol consists mainly of water and serves as a reaction space for all metabolic processes that take place in the cell. Together with the solid cell components, the interior of a bacterial cell is called a cytoplasm designated.
Gram-negative bacteria also have an additional area between the cell wall and the outer membrane, which periplasmic space names. This one is with the periplasm filled. The periplasm has the same function as the cytoplasm.
membrane invaginations
Maybe you have the term in connection with bacteria membrane invagination or mesosome heard. However, this is a so-called artifact.
An artefact in science is a supposed result, which, however, came about solely through the method of investigation.
This means that mesosomes, i.e. conspicuous indentations in the membrane, as you may know them from chloroplasts or mitochondria, occur in bacteria not natural on. They are presumably the result of a membrane damage through chemicals.
flagella
Some bacterial cells can actively move. To do this, they use so-called flagella. These are long strands of protein that are attached to the cell wall and cytoplasmic membrane. A bacterial cell can have one flagella (monotrich), several in a tuft (lophotrich) or distributed over the entire surface (peritrich).
The flagella can rotate around their own axis and thus enable the locomotion of the cell.
You often hear the term here scourge. However, since there are fundamental differences between eukaryotic and bacterial flagella, the term has evolved flagellum enforced for the bacterial «scourge».
pili and fimbriae
pili or fimbriae are long protein threads that are also found on the surface of some bacteria. They make it easier for bacterial cells to adhere to certain surfaces. Certain Pili, such as the F- or Sexpilusare used by cells to exchange genetic material between themselves.
You can find out more about Pili in the article on this topic on !
cell organelles of the bacterial cell
Unlike eukaryotic cells, bacterial cells possess no cell organelles. These functional units within the cell, delimited by a membrane, as the endoplasmic reticulum or the glogi apparatusonly found in eukaryotic cells. However, some bacteria that carry out photosynthesis, for example, have intracellular membranes (thylakoids) that provide space for certain metabolic processes.
ribosomes
The ribosomes are the protein factories the bacterial cell. The protein blueprints are attached to the ribosomes in the form of what are known as messenger RNA (mRNA) and are assembled piece by piece from amino acids.
This process is called protein biosynthesis. You can find more information about this in the related articles.
The bacterial ribosomes perform the same tasks as the eukaryotic ribosomes. However, they are smaller and constructed a little differently. You will also as 70S ribosomes labeled and each consist of one 50S and 30S subunit.
The «S» in the 70S ribosomes stands for unity Svedberg, which was coined by the Swedish chemist Theodor Svedberg (1884–1971). It is a measure of the sedimentation coefficient. This describes how quickly a particle settles (sediments) on the ground in a solution. The coefficient depends on the size and shape of the particle.
DNA in the bacterial cell
As in all other organisms, the DNA is also the carrier of genetic information in bacteria. Two types of DNA are found in bacterial cells: chromosomes and plasmids.
The bacterial chromosome
The largest DNA molecule in a bacterial cell is also known as the bacterial chromosome. The DNA is usually found in the cytosol of bacterial cells as a double-stranded, ring-shaped molecule. The area around the chromosome is also called nucleoid or core equivalent designated. Bacterial DNA is usually over a millimeter long. Bacteria only have one chromosome.
plasmids
as plasmid is called small circular DNA moleculesthat do not belong to the bacterial chromosome. Plasmids often contain blueprints for additional proteins and can even be transferred between bacteria. In genetic engineering, plasmids are often used to allow bacteria to produce specific proteins in a targeted manner.
The transfer of plasmids between bacteria is also called horizontal gene transfer. Horizontal because genes are not exchanged from one generation to the next (vertical), but within a generation. Horizontal gene transfer is relevant primarily for the treatment of bacterial infections because often resistance genes be transmitted to antibiotics.
Reproduction and lifestyle of bacterial cells
You already know that bacterial cells are different from animal or plant cells. This is also particularly evident in their way of reproducing and their diverse lifestyles.
propagation
Bacteria multiply through cell division. This means that two identical clones emerge from one cell. So it is one asexual form of reproduction. The fastest bacteria need for a complete division about 20 minutes. Since one bacterial cell becomes two during cell division, the growth of bacterial populations is referred to as exponentially. This means that the number of bacteria in the culture doubles every 20 minutes for fast-growing bacteria.
Such fast-growing bacteria usually include clinically relevant pathogens such as staphylococci, streptococci or enterobacteria (for example Escherichia coli). However, some pathogens take a lot longer, like something like this Mycobacterium tuberculosis, the cause of tuberculosis. Bacteria of this species need up to 18 hours for a division – one reason, incidentally, why their discovery was so difficult.
The cell division can take place in the middle (equal) or at one end of the bacteria (unequal). Some bacteria also multiply by the so-called budding, others form spores.
way of life
The ways of life are as varied as the bacterial habitats. Some bacteria cannot live without it oxygen grow – they are aerobic.
For others, oxygen is harmful. These bacteria are called obligatory anaerobic.
In between there is facultative anaerobic Bacteria that tolerate oxygen, but do not necessarily need it. In the following table you will find a few examples of the respective lifestyles.
way of lifebacteriumgram behaviorshapediseaseaerobicNeisseria meningitidesgram-positive diplococcimeningitis obligate…