HOW DO BACTERIA REPRODUCE? ASEXUAL AND SEXUAL REPRODUCTION - BIOLOGY - 2025

Bacteria reproduce mainly 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.

Schematic of a prokaryon cell (Source: This vector image is completely made by Ali Zifan via Wikimedia Commons)

Before delving into the different reproduction mechanisms in bacteria, it is pertinent to remember key information to understand some aspects of these biological processes.

General characteristics of bacteria

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 mainly single-celled organisms, although some can form colonies and filaments. They are found in virtually all ecosystems over 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 it 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 made up of 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.

As not all bacteria can be grown in vitro in a laboratory, the information that the scientific community manages regarding their metabolism, reproduction, morphology and general characteristics depends, to a great extent, on the data obtained from studies carried out with model species.

Asexual reproduction of bacteria

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.

- Binary fission

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 gone through a prior "preparatory" process, through which it doubled its internal contents (chromosome, proteins, ribosomes, etc.) and increased its cytosolic content to just under double.

Binary fission (Source: Binary_fission.svg: Drawn by w: User: JWSchmidt (w: Image: Binary fission.png); vectorized by w: User: JTojnarderivative work: Ortisa via Wikimedia Commons)

The two copies of the bacterial chromosome are secreted toward 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.

- "Unusual" asexual reproduction

The asexual reproduction process is not the same in all groups of bacteria. As mentioned at the beginning, some species are characterized by budding, others by multiple division, by the formation of intracellular offspring, etc.

Formation of multiple intracellular offspring

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.

Gemmation

Budding in bacteria has been observed in cyanobacteria, firmicutes, and planctomycetes, as well as in some proteobacteria.

The 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” of 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.

Baeocyte production in some cyanobacteria

Some cyanobacteria, such as those of the genus Stanieria, 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 large numbers of new tiny baeocytes, which are released by breaking the outer covering of the "mother" baeocyte.

This process is also known in other species as fragmentation.

"Sexual" reproduction of bacteria

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 about the direct transfer of genetic material from one cell to another through a specialized structure known as "sexual pili".

Bacterial conjugation (Source: derivative work: Franciscosp2 (talk) Bacterial_Conjugation_en.png: Mike Jones via Wikimedia Commons)

This last process requires physical contact between two cells and, according to the 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 useful genetic information.

References

1. Angert, ER (2005). Alternatives to binary fission in bacteria. Nature Reviews Microbiology, 3 (3), 214.
2. Helmstetter, CE (1969). Sequence of bacterial reproduction. Annual Review of Microbiology, 23 (1), 223-238.
3. Lutkenhaus, J., & Addinall, SG (1997). Bacterial cell division and the Z ring. Annual review of biochemistry, 66 (1), 93-116.
4. Prescott, H., & Harley, JP Klein 2002. Microbiology.
5. Scheffers, DJ (2001). Bacterial Reproduction and Growth. eLS.
6. Solomon, EP, Berg, LR, & Martin, DW (2011). Biology (9th edn). Brooks / Cole, Cengage Learning: USA.