Horizontal gene transfer (HGT) is the movement
of genetic material between unicellular and/or
multicellular organisms other than via vertical
transmission (the transmission of DNA from
parent to offspring.)
[1] HGT is synonymous with lateral gene transfer
(LGT) and the terms are interchangeable.
HGT has been shown to be an important factor
in the evolution of many organisms.
Horizontal gene transfer is the primary reason
for the spread of antibiotic resistance in
bacteria and plays an important role in the
evolution of bacteria that can degrade novel
compounds such as human-created pesticides
and in the evolution, maintenance, and transmission
of virulence.
This horizontal gene transfer often involves
temperate bacteriophages and plasmids.
Genes that are responsible for antibiotic
resistance in one species of bacteria can
be transferred to another species of bacteria
through various mechanisms (e.g., via F-pilus),
subsequently arming the antibiotic resistant
genes' recipient against antibiotics, which
is becoming a medical challenge to deal with.
Most thinking in genetics has focused upon
vertical transfer, but there is a growing
awareness that horizontal gene transfer is
a highly significant phenomenon and among
single-celled organisms perhaps the dominant
form of genetic transfer.
Artificial horizontal gene transfer is a form
of genetic engineering.
Transduction is the process by which DNA is
transferred from one bacterium to another
by a virus.
It also refers to the process whereby foreign
DNA is introduced into another cell via a
viral vector.
Transduction does not require physical contact
between the cell donating the DNA and the
cell receiving the DNA (which occurs in conjugation),
and it is DNase resistant (transformation
is susceptible to DNase).
Transduction is a common tool used by molecular
biologists to stably introduce a foreign gene
into a host cell's genome.
When bacteriophages (viruses that infect bacteria)
infect a bacterial cell, their normal mode
of reproduction is to harness the replicational,
transcriptional, and translation machinery
of the host bacterial cell to make numerous
virions, or complete viral particles, including
the viral DNA or RNA and the protein coat.
Bacterial conjugation is the transfer of genetic
material between bacterial cells by direct
cell-to-cell contact or by a bridge-like connection
between two cells.
Discovered in 1946 by Joshua Lederberg and
Edward Tatum, conjugation is a mechanism of
horizontal gene transfer as are transformation
and transduction although these two other
mechanisms do not involve cell-to-cell contact.
Bacterial conjugation is often regarded as
the bacterial equivalent of sexual reproduction
or mating since it involves the exchange of
genetic material.
During conjugation the donor cell provides
a conjugative or mobilizable genetic element
that is most often a plasmid or transposon.
Most conjugative plasmids have systems ensuring
that the recipient cell does not already contain
a similar element.
The genetic information transferred is often
beneficial to the recipient.
Benefits may include antibiotic resistance,
xenobiotic tolerance or the ability to use
new metabolites.
Such beneficial plasmids may be considered
bacterial endosymbionts.
Other elements, however, may be viewed as
bacterial parasites and conjugation as a mechanism
evolved by them to allow for their spread.
transformation is the genetic alteration of
a cell resulting from the direct uptake and
incorporation of exogenous genetic material
(exogenous DNA) from its surroundings through
the cell membrane(s).
Transformation occurs naturally in some species
of bacteria, but it can also be effected by
artificial means in other cells.
For transformation to happen, bacteria must
be in 
a state of competence, which might occur as
a time-limited response to environmental conditions
such as starvation and cell density.
Transformation is 
one of three processes by which exogenous
genetic material may be introduced into a
bacterial cell, the other two being conjugation
(transfer of genetic material between two
bacterial cells in direct contact) and transduction
(injection of foreign DNA by a bacteriophage
virus into the host bacterium).
"Transformation" may also be used to describe
the insertion of new genetic material into
nonbacterial cells, including animal and plant
cells; however, because "transformation" has
a special meaning in relation to animal cells,
indicating progression to a cancerous state,
the term should be avoided for animal cells
when describing introduction of exogenous
genetic material.
Introduction of foreign DNA into eukaryotic
cells is often called "transfection".
