Horizontal Gene Transfer – Bacterial Transformation

Bacterial Transformation
“The Measure of Intelligence is the ability to change” Perhaps this is the reason why bacteria are able to attain resistance to variety of drugs in very short span of time. They are able to adapt the change in the environment by exchanging their genetic material by Horizontal Gene Transfer method. One such fascinating method is Bacterial Transformation.

What is Horizontal gene transfer?

Horizontal gene transfer enables bacteria to respond and adapt to their environment much more rapidly by acquiring large DNA sequences from another bacterium in a single transfer. It is a process in which an organism transfers genetic material to another organism that is not to its offspring or daughter cells. Horizontal gene transfer is the primary mechanism 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; transmission and maintenance of virulence.

Now let’s see how the microscopic organism carries out this essential process.

   Genetic Exchange –

 Three mechanisms of genetic exchange are known in bacteria –

  1. Transformation – The free DNA released from one bacterial cell is taken up by another cell.
  2. Transduction in which DNA transfer is mediated by virus
  3. Conjugation – in which DNA is transferred from donor to recipient in the form of F plasmid via Pilli. It requires cell-to-cell contact.
Horizontal Gene transfer
Horizontal gene transfer between bacteria (from: Furuya and Lowy, 2006)

Fate of Transferred DNA –

Before discussing mechanisms of transfer, we should consider the fate of transferred DNA first. Regardless of how it was transferred, DNA that enters the cell by horizontal gene transfer faces three possible fates –

  • It may be degraded by the recipient cell’s restriction enzyme or other DNA destruction systems.
  • It may replicate by itself (but only if it possesses its own origin of replication, such as a plasmid or phage genome).
  • It may recombine with the recipient cell’s chromosome.

This article is dedicated to understand the Bacterial Transformation.

Bacterial Transformation

       Bacterial transformation is a process of horizontal gene transfer by which some bacteria can take up foreign genetic material (naked DNA) from the environment. In 1928, Griffith was the first to report transformation in Streptococcus pneumoniae. By then, we were unaware about the biomolecule, which is being transformed. The Scientist of those times believed that either of DNA, RNA or Protein was the transforming agent. In 1944, Mc. Avery and his team proved that DNA is the transforming agent.

The process of gene transfer by transformation does not require a living donor cell but only requires the presence of persistent DNA in the environment. The prerequisite for bacteria to undergo transformation is its ability to take up free, extracellular genetic material. Such bacteria are termed as competent cells. Examples of naturally competent bacteria are Bacillus subtilis, Streptococcus pneumonia, Neisseria gonorrhoeae and Haemophilus influenza.

The factors that regulate natural competence vary between various genera. Once the transforming factor (DNA) enters the cytoplasm, it may be degraded by nucleases if it is considered as foreign. If the exogenous genetic material is similar to bacterial DNA, it may integrate into the chromosome. Sometimes the exogenous genetic material may co-exist as a plasmid with chromosomal DNA.

Artificial Competence of Bacteria

      Not all bacteria are capable of taking up exogenous DNA from their environment. The transforming ability of bacteria is exploited for producing new products or for research purposes. The practical approach to acquire competent cells in-vitro is to make the bacterial cells artificially competent using chemicals or electrical pulses.

Chemical induction of competence involves the following steps:

  • Using Calcium Chloride – DNA are large molecule, they are negatively charged and hydrophilic in nature and hence it cannot pass through hydrophobic cell membrane. The Calcium chloride catalyzes the binding of DNA to outer membrane or Lipopolysaccharide (LPS). The positively charged Calcium Chloride is able to bind to Negative groups of DNA and lipopolysaccharide. The heat shock treatment makes the cell membrane permeable temporarily. This makes the cell competent and allows the exogenous DNA to enter the cells. The calcium chloride and heat shock treatment is suggested to conduct in bacterial log phase.
  • Electroporation – Alternatively, the bacterial cells are made permeable by subjecting them to electrical pulses, a process known as electroporation.  The electric pulse create temporary pore in the cell membrane through which the exogenous DNA can enter the bacterial cell. It is is more efficient and reliable than the calcium chloride method.

Mechanism of Bacterial Transformation –

  1. Binding of a double stranded DNA to a membrane bound DNA binding proteins or surface receptors.
  2. Passage of one of the two strands into the cell while other strand is degraded by nuclease.
  3. The entered single strand of DNA in the cell is received and escorted by cytoplasmic specific proteins to bacterial chromosome.
  4. If the exogenous DNA and bacterial DNA has homologous sequence, then the exogenous DNA get integrated with bacterial chromosome by recombination.
  5. Then the recipient cell gets transformed.

Reasons for Transformation

     The phenomenon of natural transformation has enabled bacterial populations to overcome great fluctuations in population dynamics and to overcome the challenge of maintaining the population numbers during harsh and extreme environmental changes. During such conditions some bacterial genera spontaneously release DNA from the cells into the environment and make freely available for the competent cells. The competent cells also respond to the changes in the environment and control the level of gene acquisition through natural transformation process.

Let’s take a look back in history!
Griffith experiment

Griffith experiment was a stepping stone for the discovery of genetic material. He conducted his experiments on Streptococcus pneumoniae. For his experiment, Griffith cultured Streptococcus pneumoniae bacteria and he observed two patterns of growth on solid media plate. One culture showed smooth shiny colonies (S) on the surface of agar plate while other showed rough colonies (R). The difference was due to the presence of mucous coat in S strain bacteria, whereas it was absent in R strain.

He performed his experiment in following steps –

  1. He injected R and S strain in two difference mice.
  2. The one which was infected with the S strain developed pneumonia and died while that infected with the R strain stayed alive.
  3. In the second stage, Griffith heat-killed the S strain bacteria and injected into mice, but the mice stayed alive.
  4. Then, he mixed the heat-killed S and live R strains and the mixture was injected into mice and they died.
  5. Surprisingly, he found living S strain bacteria in dead mice.
  6. Conclusion: Based on the observation, Griffith concluded that R strain bacteria had been transformed by S strain bacteria. The R strain inherited some ‘transforming principle’ from the heat-killed S strain bacteria which made them virulent.
Grifith Experiment Bacterial Transformation
Khanacademy.org

References:

https://www.nature.com/scitable/definition/conjugation-prokaryoteshttps://www.genome.gov/genetics-glossary/Translationhttps:

//www.ncbi.nlm.nih.gov/books/NBK21760/

https://en.m.wikipedia.org/wiki/Horizontal_gene_transfer

https://en.m.wikipedia.org/wiki/Lysogenic_cycle

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