Bacterial Genetics: Gene Transfer

Bacterial Genetics: Gene Transfer

• Three methods of gene transfer:
• Transduction
• Conjugation
• Transformation

1. Transformation

• Random free DNA is picked up from the environment.

• Involves the transfer of free DNA.

• Uptake of soluble DNA fragments by bacteria.

• Occurs through the cell wall.

• Happens during the log phase (division).

• Requires a competence factor.

• Example: Streptococcus pneumoniae.

2. Transduction

• Most common (MC) mode of drug resistance in S. aureus.

• Occurs through a Bacteriophage.
• A bacteriophage is a DNA virus.

• Bacteriophages can be:
• Tadpole shape
• Circular
• Filamentous

• Two types of transduction:
• Generalized Transduction
• Specialized/Restricted Transduction

Generalized Transduction involves:

• Bacteriophage with free DNA form donor

• Transfer/deliver to tissue

• Multiplication

• Virus capsid synthesis.
• Replication of viral DNA.
• Assembly.
• Destruction of bacterial DNA.

• Lysis of the bacterial cell.
• Phage release.
• New bacteriophage deliver nucleic acid
• Infection of new bacterial cells.

• Nucleic acid + Recipient bacteria = New bacteria

• Crossover and stable gene transfer into the recipient cell chromosome.

Specialized/Restricted Transduction involves:

• Phage DNA integrates into the chromosome.

• Prophage de-integrates.
• Picks up a piece of the bacterial chromosome.

Bacteriophage Cycle

• Two cycles:
• Lysogenic cycle:
• Bacteriophage DNA combines with donor DNA.
• Leads to vertical transfer to daughter cells.
• Lytic cycle:
• Leads to lysis of infected bacteria.
• Breakdown of the bacterial cell wall.

• Bacterial Toxins Coded by Lysogenic Conversion:
• Mnemonic: A-B-C-D-E
• A - GABS: A and C toxins of S. pyogenes
• AC- pus
• B - C and D toxins of Botulinum
• CD - Boat
• C - Cholera toxin
• D - Diphtheria toxin
• E - Enterohemorrhagic E. coli (EHEC) - Shiga-like toxin/verocytotoxin

3. Conjugation

• It is the MC mode of resistance.

• Involves a bridge between bacteria.

• A conjugation tube is formed by Pili (Fimbriae).

• To Remember:
• Exceptions:
• S. aureus: Transduction is more common.
• Strep. pneumoniae: Transformation is the primary mode.
• New transformation audi

Plasmids

• Plasmid: Extrachromosomal circular dsDNA.

• Fertility (F) plasmid: Codes for the fertility factor.

• Resistance (R) plasmid: Codes for resistance.

Summary of Plasmid Transfers:

• a. F-Plasmid
• F: Fertility factor.
• Male: Present (F+).
• Female: Absent (F-).

• b. F- with F-
• Conjugation between male (with fertility factor) and female (without fertility factor).
• A conjugation tube is formed by sex pili.
• The fertility factor is copied.

• c. Both have fertility factors (both males - Hfr)
• F+ combined with F+ forms Hfr (High Frequency Recombinant) cells.
• Cell DNA is combined with F+.
• Hfr cells combine to transfer into a female cell.
• A conjugation tube is formed.
• Tube is broken mid-way, leading to incomplete transfer.
• The F- recipient receives:
• Small amounts of donor DNA.
• A part of the F-.
• Hfr with F- results in F- DNA \& F- Hfr Cells.

• d. F' (SexDuction)
• Hfr splits into a donor plasmid and donor DNA.
• Donor plasmid carries some donor DNA with it.
• Called F prime (F').
• F' conjugates with F-.
• F- cell gets a copy of the F' plasmid.
• F' x F- = F- and F'.

Transferable Drug Resistance

• R plasmid consists of:
• Resistance Transfer Factor (RTF)
• Resistance determinants

• Considered more dangerous.

• Difficult to treat.

Mutational Drug Resistance

• Due to a single mutation.
• Results in single drug resistance.

• Easier to treat.

• Example: Rifampicin resistance in TB is due to the rpoB gene.

CRISPR-Cas9

• A genome editing tool.

• Trick: Cas9 acts as scissors.

• Mnemonic for CRISPR:
• C - Clustered
• R - Regularly
• I - Interspaced
• S - Short
• P - Palindromic
• R - Repeats

• Process:
• Uses Non Homologous → Gene knock-out
• (1) Virus invades the bacterial cell.
• (2) A new spacer is derived from the virus.
• Integrated into the CRISPR sequence (Adaptation).
• (3) Production of CRISPR RNA is formed.
• (4) CRISPR RNA guides molecular machinery.
• Targets and destroys the viral genome.
• Its memory is saved as interspersed spaces (Viral elemental memory).

• Important Information:
• CRISPR-Cas9 is a bacterial defence system against viruses.
• The FELUDA test, based on CRISPR-Cas9, is used for COVID.