Reproduction in Bacteria

Offspring are produced through reproduction. Sexual and asexual reproduction are the two most common types. When an organism reproduces sexually, it integrates genetic material from both parents to create a genetically unique organism. One parent copies itself to produce genetically identical kids in asexual reproduction. A sexually reproducing animal is a sea turtle, an asexually reproducing organism is a volvox (green algae), and a brittle star can reproduce in either way.

Bacterial Reproduction

Just like any other organism, bacteria also reproduce to continue their species. Since they are unicellular and do not have a well-organized cell, bacteria have been grouped under prokaryotes. A bacterial population grows in a geometric or exponential fashion, with each division cycle (generation) producing two cells, four cells, eight cells, sixteen cells, 32 cells, and so on. However, they do show both sexual and asexual means of reproduction. In this topic, we will have a brief overview of all types of means of reproduction in bacteria.

In asexual reproduction in bacteria, there are five following types of Asexual reproduction:

1. Binary fission

2. Reproduction through conidia

3. Budding

4. Reproduction through cyst formation

5. Reproduction through endospore formation

Asexual Reproduction in Bacteria

• Binary Fission

In binary fission, a single bacterial cell divides into two daughter cells. At first, the bacterial cell reaches critical mass in its form and cell components. The circular double-stranded DNA of the bacteria undergoes replication and new complementary strands are formed. These two strands of DNA are then moved to the different poles of the cell and a transverse septum then takes place and develops in the middle region of the cell which separates the two new daughter cells and thus binary fission I completed. It is a rapid process and takes minutes to complete.

Binary fission is used by most bacteria, including Salmonella and E.coli. The single DNA molecule duplicates in this sort of asexual reproduction, and both copies bind to the cell membrane at distinct places. The distance between the two DNA molecules grows as the cell grows and elongates. The cell membrane pinches inward toward the middle of the bacterium once it has nearly doubled in size.

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• Conidia Formation

The formation of conidia takes place in filamentous bacteria such as Streptomyces through the formation of a transverse septum at the apex of the filament. The part bearing the conidia is called the conidiophore and after it is detached from the mother cell, in a suitable substratum it germinates giving rise to new mycelium. This type of asexual reproduction is also called fragmentation. Conidia production can be seen in filamentous bacteria such as Streptomyces. Conidia are small, chain-like, spherical, spore-like entities created by a transverse wall at the terminals of filaments. Conidiophore is the portion of the filament that contains conidia. Each conidium detaches from the mother and germinates in an appropriate substratum, producing a new mycelium.

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• Budding

In this method of reproduction, the bacterial cell develops a small swelling at one side which continuously increases in size. At the same time, the nucleus also undergoes division where one part with some cytoplasm enters the swelling and the other part remains with the mother cell. The outgrowth is called the bud and it eventually gets separated from the mother cell by a partition wall. This method of reproduction also comes under vegetative reproduction in bacteria. Example: Rhodomicrobium vannielii

Budding is an asexual reproduction method in which a bacterial cell generates a tiny protrusion or bud as a result of cell division at a single location. These buds grow into little individuals, and the nucleus divides at the same time. The bud is entered by one nucleus with some cytoplasm. When a bud reaches full maturity, a partition wall separates it from the parent cell.

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• Cysts

Cysts are formed by the deposition of additional layers around the mother cell and are the resting structure during unfavorable conditions. When conditions are favorable again, the mother cell behaves like its normal self again. Cysts are the mother bacterium cell's inactive or resting stage. Cysts are generated when an extra layer is deposited around the mother wall. The cell's metabolic process has slowed at this point. When bacteria find a favourable environment, they use a process called excystation to break down the cyst's wall and germinate to generate a new bacterium. Cysts' primary job is to protect the body from harmful environmental changes.

Example: Azotobacter.

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Reproduction Through Endospore Formation

Endospores in a bacterial cell are formed during stressful conditions such as desiccation and starvation. They contain a central protoplast, and a core consisting of DNA, ribosomes, enzymes, and the t-RNA, everything necessary for the formation of a new cell. Only one endospore is formed in one bacterial cell and on germination, it gives rise to a new bacterial cell. 

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Sexual Reproduction in Bacteria

However, in asexual reproduction, genetic recombination is not observed and that is why sexual reproduction has high significance in the continuation of a bacterial species. This is because, in sexual reproduction, genetic material is exchanged between two cells which facilitates genetic recombination and creates a genetic drift in the species of bacteria. There are 3 ways bacteria reproduce sexually, these are:

1. Transformation

2. Transduction

3. Conjugation

• Transformation

In transformation, a bacterium takes up DNA from its environment and often DNA that’s been shed by another bacteria. The phenomenon was first discovered by Griffith in 1928 and the mechanism was worked out by Avery in 1944. In this process, the DNA of a capsulated bacteria is transferred into a non-capsulated bacteria. If the DNA is circular it is called a plasmid. 

The plasmid can be copied in the receiving cell and passed on to its descendants. For gene transfer via transformation, which does not require the presence of a living donor cell, all that is necessary is the presence of persistent DNA in the environment. Bacteria must be able to take up free, extracellular genetic material in order to change. Bacteria that fulfill this characteristic are known as competent cells.

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• Transduction

In this type of sexual reproduction of bacteria, foreign genes are transferred into a bacterial cell with the help of a virus. These viruses are called bacteriophages and they are not virulent. The virus acts as a carrier vehicle and passes over genes from one host to another. Transducing bacteriophages may carry the same genes in which the reproduction method would be known as restricted transduction. They can also carry different genes at different times which the reproduction process would be known as generalized transduction.

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• Conjugation

This process was first discovered in Escherichia coli by Tatum and Lederberg in 1946. They found that two different types of nutritional mutants grown together on a minimal medium produced an occasional wild type.

Bacteria that show conjugation are dimorphic, meaning that they have two types of cells, one male (F+) or donor cell and a female (F-) or recipient cell.

The male or donor cell possesses 1 to 4 sex pili on the surface and fertility factor (transfer factor, sex factor) in its plasmid. It contains genes for producing sex pili and other characteristics needed for gene transfer. Sex pili are 1to 4 narrow protoplasmic outgrowths. The sex pili and fertility factors are absent from the female or recipient cells. 

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If these two types of cells happen to come nearer, a pilus of a male cell establishes a protoplasmic bridge or conjugation tube with the female cell. It takes 6-8 minutes for the process to complete.

These were the three types of sexual reproduction in bacteria and it introduces genetic variation in a bacterial species which is important for the survival of any species and allows groups to adapt to environmental changes.

Reproduction in bacteria can be extremely quick, with some species taking only a few minutes to produce a generation. Bacteria (and other prokaryotes) may change very quickly thanks to the short generation period.