3. What is Bacterial Transformation?
• Transformation is the direct uptake, incorporation
and expression of exogenous genetic material
from its surroundings.
• Bacterial transformation is a process of horizontal
gene transfer by which some bacteria take up
foreign genetic material (naked DNA) from the
• It was first reported in Streptococcus pneumoniae
by Griffith in 1928.
• DNA as the transforming principle was
demonstrated by Avery et al in 1944.
4. Griffith Experiment and Its Results
• In the 1920s, while studying two different strains of a bacterium,
called R (rough) strain and S (smooth) strain, Frederick Griffith
made an important discovery.
– He injected the two strains into mice.
– The S strain killed (virulent) the mice, but the R strain did not (non-
– Griffith also injected mice with heat killed S-strain bacteria. As
expected, the killed bacteria did not harm the mice.
– However, when the dead S-strain bacteria were mixed with live R-
strain bacteria and injected, the mice died.
– Based on his observations, Griffith deduced that something in the
killed S strain was transferred to the previously harmless R strain,
making the R strain deadly.
– He called this process transformation, as something was
"transforming" the bacteria from one strain into another strain.
7. Bacterial Gene Transformation
• Transformation is a form of genetic
recombination in which a DNA fragment from a
dead, degraded bacterium enters a competent
recipient bacterium and is exchanged for a piece
of DNA of the recipient.
• Transformation usually involves only homologous
recombination, a recombination of homologous
DNA regions having nearly the same nucleotide
• Typically this involves similar bacterial strains or
strains of the same bacterial species.
8. Bacterial Gene Transformation
• Once the transforming factor (DNA) enters the
cytoplasm, it may be degraded by nucleases if
it is different from the bacterial DNA.
• If the exogenous genetic material is similar to
bacterial DNA, it may integrate into the
• Sometimes the exogenous genetic material
may co-exist as a plasmid with chromosomal
9. Bacterial Gene Transformation
• The prerequisite for bacteria
to undergo transformation is
its ability to take up free,
extracellular genetic material.
Such bacteria are termed as
• The process of gene transfer
by transformation does not
require a living donor cell but
only requires the presence of
persistent DNA in the
• The factors that regulate
natural competence vary
between various genera.
Step 1: A donor bacterium dies
and is degraded
Step 2: DNA fragments, from the
dead donor bacterium bind
to transformasomes on the cell wall
of a competent, living recipient
Step 3: Then, a nuclease degrades
one strand of the donor fragment
and the remaining DNA strand enters
DNA-binding proteins bind to the
donor DNA strand to prevent it from
being degraded in the cytoplasm.
Step 4: RecA proteins promotes genetic
exchange between a fragment of the
donor's DNA and the recipient's DNA This
involves breakage and reunion of paired
12. Significance of Transformation
• The phenomenon of natural transformation
enable the bacterial populations to overcome
great fluctuations in population dynamics and
overcome the challenge of maintaining the
population numbers during harsh and extreme
• This is achieved when some bacterial genera
spontaneously release DNA from the cells into the
environment, free to be taken up by the
competent cells. The competent cells also respond
to the changes in the environment and control the
level of gene acquisition through natural
13. Applications of Transformation
• The phenomenon of transformation has been
widely used in molecular biology. As they are
easily grown in large numbers, transformed
bacteria may be used as host cells for the
– to make multiple copies of the DNA
– in cloning procedures
– to express large amounts of proteins and enzymes
– in the generation of cDNA libraries
– in DNA linkage studies
14. Biology of Homologous Recombination
in Bacteria ????
• This will be dealt with in ‘Presentation 3’
of ‘Block 2’.