Introduction of 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.

 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.

 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.

 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.

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. 

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.

 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.

 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. 

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.