Bacterial Genetics

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Bacterial Genetics
By
Dr. Emad AbdElhameed Morad
Lecturer of Medical Microbiology and Immunology
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Plasmids
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Characters

• Extra-chromosomal circular double stranded DNA
  molecules.
• They are dispensable as they are Not necessary
  for bacterial life.
• Capable of autonomous replication (independent
  of the bacterial chromosome).
• Multiple copies of the same plasmid may be
  present in the same bacterial cell.
• Different types of plasmids may co-exist within
  the same bacterial cell.

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Types

• According to transmissibility between the
  bacterial cells

Non-conjugative plasmids (non-transmissible) Conjugative plasmids (transmissible) Item
Usually small Large Size
10-60 (relaxed) 1-2 (stringent) Copy number
Absent Present Fertility factor (F factor)
No Yes Contain tra gene
No Yes Sex pilus formation
By the help of a conjugative plasmid By conjugation Transfer among bacteria
Common in Gram Ve cocci Common in Gram Ve bacilli Host bacteria
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Functions

• F-factor mediates gene transfer by conjugation.
• These plasmids carry genes encoding antibiotic
  resistance.
• These plasmids are usually conjugative.

F- factor plasmid
R- factor plasmid
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• Plasmids may carry genes that produce toxins
  which are virulence factors of pathogenic
  bacteria.
• Plasmids may carry genes that produce
  bacteriocins.
• Bacteriocins or colicins are bactericidal
  substances produced by certain strains of
  bacteria and kill other strains of the same or
  closely related species.
• Plasmids in some bacteria carry genes that
  produce antibiotics.
• For example antibiotic production by
  streptomyces bacteria.

Virulence
Production of bacteriocins
Production of antibiotics
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Transposons
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Characters

• Fragments of extra-chromosomal DNA.
• Move inside the bacterial cell by transposition.
• Move from one location on the bacterial
  chromosome to another location.
• Or between the chromosome and plasmid and the
  reverse.
• Thus, transposons are called jumping genes.

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Structure

Repressor gene
Inverted repeat
Inverted repeat
Transposase gene
Antibiotic resistance gene
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• Transposon is formed of 4 domains
• Inverted repeat at each end.
• Transposase gene produce the enzyme responsible
  for excision and integration of the transposon.
• Respressor gene regulates domains 2 and 4.
• Gene coding for
• Antibiotic resistance OR
• Toxin production

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• Insertion sequence
• Simple type of transposon.
• Carries only the genetic information for its
  excision and integration.
• No antibiotic resistance gene.

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