Bacteria reproduce primarily by asexual pathways such as binary fission, which is a process by which a cell divides into two identical smaller cells. However, these microorganisms also exhibit a type of sexual reproduction through a process called "conjugation.".
Regarding asexual reproduction, binary fission is predominant, but there are reports of "unusual" or "alternative" patterns of division in this group of unicellular organisms, such as budding, multiple division and the formation of "intracellular" offspring. to name a few.
Before delving into the different reproduction mechanisms in bacteria, it is pertinent to remember key information to understand some aspects of these biological processes..
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Bacteria are prokaryotic organisms, that is, their genetic material is not enclosed in the membranous structure known as the "nucleus", which does exist in eukaryotes..
It is an extremely successful and diverse group, composed of primarily single-celled organisms, although some can form colonies and filaments. They are found in virtually all ecosystems on the biosphere and are of utmost importance to many biological processes inherent in multicellular living things..
Bacterial cells are small cells (no more than a couple of microns in diameter) especially compared to eukaryotic cells. Its cytosol is considerably "simpler" than that of an animal or plant cell, for example, since there is no membranous organelle..
Inside there are a large number of ribosomes, storage granules of different substances, proteins of different types and a large, circular and supercoiled chromosome, as well as small extra-chromosomal DNA molecules known as "plasmids".
The cell coat of bacteria is quite particular, as it is formed by a complex polymer called "peptidoglycan", which consists of amino sugars linked with polypeptides. The characteristics of the wall and the plasma membrane of these organisms are used for their classification..
Some bacteria are also covered by a "capsule" that surrounds their cell wall and that has protective functions. Other bacteria are mobile and have specialized structures that protrude from their surface, such as cilia and flagella..
How not all bacteria can be grown in vitro In a laboratory, the information that the scientific community handles regarding its metabolism, reproduction, morphology and general characteristics depends, to a great extent, on the data obtained from studies carried out with model species.
Asexual reproduction consists of the formation of clones from an individual that functions as a “mother”. Unlike sexual reproduction, this process requires only one cell that duplicates its internal components and divides into two equal daughter cells, for example.
Also known as "bipartition," binary fission is the main form of reproduction for most bacteria in nature. In this process, the dividing cell does so in such a way that it produces two smaller cells that are genetically identical, and the initial cell "disappears.".
A dividing cell must necessarily have undergone a previous “preparatory” process, through which it doubled its internal contents (chromosome, proteins, ribosomes, etc.) and increased its cytosolic content to just under double..
The two copies of the bacterial chromosome are secreted towards both poles of the dividing cell, as well as some of the internal material that has duplicated. After this, a "septum" forms in a specific place in the cell called the "division site"..
The septum consists of a “centripetal” invagination of the cytoplasmic membrane, which precedes the synthesis of a new cell wall and the separation of the two daughter cells.
The asexual reproduction process is not the same in all groups of bacteria. As commented at the beginning, some species are characterized by budding, others by multiple division, by the formation of intracellular offspring, etc..
Many lineages of bacteria belonging to the group of gram-positive bacteria with low GC content (Firmicutes) reproduce asexually through a process that implies that multiple endospores (also dormant) or a large number of active descendants.
For many lineages, the formation of this “multiple intracellular offspring” is the main means of reproduction, so binary fission may rarely or may not occur at all..
Budding in bacteria has been observed in cyanobacteria, firmicutes, and planctomycetes, as well as in some proteobacteria..
Schematic representations of this process (little studied and known among bacteria) illustrate how bacteria that reproduce asexually in this way shed a “fraction” or “yolk” from their cell body, a fraction that is considerably smaller than the “mother” cell. that originates them and that probably does not contain all the cytosolic components of the latter.
Some cyanobacteria, such as those of the genus Stanieria, they never reproduce by binary fission; instead, they divide by a process that begins with a small cell known as a baeocyte..
The baeocyte progressively increases in size and, as it does so, the chromosomal DNA replicates many consecutive times. When the baeocyte enters the cytoplasmic division phase, it produces a large number of new tiny baeocytes, which are released when the outer covering of the "mother" baeocyte breaks..
This process is also known in other species as fragmentation.
Sexual reproduction, as we know it, consists of the fusion of two specialized sex cells called "gametes", through which two individuals exchange genetic information and produce offspring with genes that result from the combination of both..
Although neither gametes nor specialized sex cells are produced in bacteria, sometimes two different cells exchange genetic material with each other, which can occur through three different processes:
- The transformation
- Transduction
- The conjugation
Although it does not involve the production of new cells or different cells, transformation is an event by which a bacterium can acquire DNA fragments from another bacterium that has released them into the environment in which they live or that has died and disintegrated..
Transduction, on the other hand, has to do with the combination of bacterial DNA (from different bacteria) thanks to the participation of phages, which are viruses that attack bacteria.
Finally, conjugation, which is not characterized by the production of new individuals, is the direct transfer of genetic material from one cell to another through a specialized structure known as "sexual pili".
This last process requires physical contact between two cells and, according to studies carried out in the model organism E. coli, there are “male” donor cells and “female” receptor cells.
In general, what is donated and received are plasmid DNA molecules that contain very useful genetic information.
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