151 lec 9 bacte gmr 2

• Bacterial Chromosomes

  • Structures that contain DNA and physically carries the hereditary information
  • Dictate the shape of the bacteria (cocci, rod-shaped, etc.)
  • Determines if it is virulent or not
  • Prokaryotes possess 1 chromosome with a circular DNA molecule
  • 1,000 microns long
  • Bacterial DNA: 4,000 kilobases (1 Kb = 1,000 base pairs, A-T and G-C)

  
• Plasmids

  • Extrachromosomal genetic elements that are not essential for the survival of bacteria but aids in it
  • Enables them to have resistance to antimicrobials
  • Important in the production of toxins
  • Circular DNA molecules that are located in the bacterial cytoplasm
  • Able to replicate autonomously or independently from chromosomal DNA
  • Capable of transferring genes from one cell to another
  • In Genetic engineering, it is used as vectors
• Classification of plasmids

  • based on number of copies they make
  • single copy = one copy per host cell
  • multicopy = present at 40 or more per cells
• Plasmid

  • can be used in gene knockout or DNA vaccine
  • can be integrated with chromosomal DNA
• Episomes

  • integrated form of plasmid with DNA
  • when bacterial cell replicates, the 2 daughter cells inherit the integrated plasmid
  • Genetic material that can independently exist of may be integrated into the chromosome

  
• Transferrable genetic elements

  • Genetic elements that can move from one site on a chromosome to another through transposition
  • Responsible for majority of insertion rotations
  • Examples
  • transposons
  • insertion sequences
• Transposons

  • Small DNA segments that are able to move from one location of a DNA molecule to another
  • Self-integrating DNA fragments
  • Can also insert into plasmids
  • “Jumping genes” – changes, rotates between chromosomes
  • Occurs in all organisms
• Insertion sequences

  • Small DNA segments that can be inserted in another position
  • Also known as inverted repeats
  • Also present in plasmids
  • Important in the formation of high frequency recombinant (Hfr) strains
  • Simplest type of transposable element but they cannot replicate on their own
• Role of plasmids

  • Bacterial resistance to antibiotics
  • Production of bacteriocins
  • Enhance pathogenicity of bacteria
  • help in degrading complex organic molecules
  • Production of enterotoxins
  • Reduce sensitivity of bacteria to mutagens
• Bacterial Resistance to antibiotics

  • Plasmids are transferred between bacteria by which the genetic material may induce the bacteria with the new plasmid to develop resistance to several antibiotics.
  • Virulence-influencing genes are typically found in the plasmids
• Bacteriocins

  • or colicins
  • proteins or peptide toxins that are produced by bacteria in order to inhibit the growth of bacterial strains that are similar or closely-related
• Virulence plasmid

  • plasmids that carry bacteria's virulence factor
  • usually large, low copy elements and encode genes that promote host-pathogen interactions
• Help in degrading complex organic molecules

  • The genes responsible for the degradation of organic compounds are frequently found in plasmids, and as a result, can be disseminated by introduction of horizontal genes into established, environmentally native, competitive bacterial populations
• Enterotoxin

  • Protein exotoxin released by a microorganism that targets the intestine
  • often cytotoxic, killing cells by changing the intestinal wall's mucosal epithelial cells' apical membrane permeability
• Mutagen

  • A chemical, physical, environmental, or biological agent that has the ability to change genetic code in a harmful way
• Classification of plasmids

  • according to ability to be transferred to other bacteria
  • according to function
• Plasmids according to transferrability

  • Conjugative
  • Non-conjugative
  • Mobilisable
  • Incompatible
• Conjugative plasmids

  • Used for sexual transfer of plasmids to other bacteria through pili
  • Usually found in gram-negative bacteria
  • Carry genes for sex pili
• Non-conjugative plasmids

  • Transferred with the help of conjugative plasmids
  • Do not have sex pili to transfer genes from one cell to another
  • Do not initiate conjugation
  • Found in both gram-negative and gram-positive organisms
• Mobilisable plasmids

  • Carry only a subset of genes
  • Intermediate class of plasmid that can parasitize another plasmid
  • Transfer at high frequency
  • Known as exploiters of conjugative plasmid for horizontal dissemination
• Incompatible plasmids

  • Seen among related plasmids
  • Plasmid Incompatibility – inability of two plasmids to co-exist
  • Reason: they possess a replicon with same specificity or controlling element
  • One plasmid is lost over the other (survival of the fittest)
• Plasmids according to function

  • Fertility (F) plasmids
  • Resistance (R) plasmids
  • Col or Colicin Plasmins
  • Degradative Plasmids
  • Virulence Plasmids
  • Addiction system
• Fertility (F) Plasmids


  has the ability to conjugate or to transfer one gene to another

  Containes genes for pili
• Resistance (r) Plasmids


  HAs genes that can mount resistance to antibiotics or poison
• Col or COlicin plasmids


  has genes that code for protein and a type or bacteriocin that can kill other bacteria
• Degradative plasmids


  can digest unusual substances such as toluene and salicylic acid
• Virulence plasmids


  important for pathogenicity as it has genes for infection
• Addiction system


  plasmids that can produce both antidote (short-lived) and poison (long-lived)

  daughter cells that do not inherit this system eventually die
• Horizontal Gene transfer (HGT)

  • Also known as Lateral gene transfer
  • Process where genes are transferred across special lines or from one bacterial cell to another (not to descendant cells)
  • Rare in humans and other higher organisms but can be observed in bacteria and in eukaryotes (plants, fungi, lower forms of animals)
• Conjugation mechanism

  • Process wherein DNA is transferred through a plasmid from a donor cell (male) to a recipient cell (female) during cell-to-cell contact
  • Contact is made possible through a tube known as F or sex pilus
  • Made up of proteins and produced by donor cells
  • Genetic information necessary for conjugation is carried by a plasmid
  • Direct transfer of DNA
  • Only the donor cell or male bacterium is F+ the recipient is F- cell

  
• Transduction mechanism

  • Process of transferring bacterial DNA from one bacterium to another bacterium through a virus (bacteriophage)
  • Most common mode of gene transfer in prokaryotes
  • Aids in genetic mapping

  
• Types of transduction

  • Generalized transduction
  • Complete transduction
  • Abortive Transduction
  • Specialized transduction
• Generalized transduction

  • Mediated by lytic phages
  • The lytic cycle results in the destruction of the infected cell and also its membrane
  • Bacterium phages are also known as virulent phages, in contrast to the temperate phages
  • Virus transfers DNA segment, which may not be integrated in the bacterial chromosome
  • Part of the bacterial DNA of the donor gets enclosed in a capsid accidentally
  • Can be complete or abortive

  
• Complete transduction

  • Transduced DNA fragment (exogenote) is integrated within the endogenote (recipient bacterial chromosome)
  • After receiving the transduced element, the recipient bacteria can undergo replication
  • After replicated through binary fission, it is passed on to its daughter cells
• Abortive transduction

  • Exogenote is not integrated with the endogenote but remains in the cytoplasm
  • Cannot undergo replication
• exogenote


  Transduced dna fragment
• endogenote


  recipient bacterial chromosome
• complete vs abortive transduction

  in complete, exogenote is integrated within the endogenote. in abortive, exogenote is not integrated but is in the cytoplasm
• Specialized transduction


  1. Mediated by lysogenic phages

  2. Specific DNA fragments are integrated into the host chromosome

  3. Phage DNA gets integrated with the bacterial chromosome

  4. Prophage replicates together with the bacterial genome

  5. Prophage detaches from the bacterial chromosome during induction

  6. Excised phage DNA goes through lytic cycle, infects another bacterium, and transfers bacterial genes from the donor bacterium
• Lysogenic cycle

  One of the cycles of viral reproduction that happens in the cytoplasm of the bacterium