Bacterial Genetics

Overview

• the uptake of naked DNA from the environment
  • any DNA can be used
  • only bacteria that are “competent” are able to undergo transformation
    • competence factor released by stressed bacteria induces this unique ability
  • a feature of many bacteria
    • especially S. pneumoniae, H. influenzae type B, Neisseria (“SHiN”)
  • “having naked SHiNs was a transforming experience”
• Conjugation  
  • “bacterial sex”
  • the major mechanism by which bacteria exchange plasmids
    • allows the transmission of resistance to drugs
  • F plasmid contains the genes required for conjugation
    • F⁺ bacteria have the plasmid
    • F^– bacteria do not
    • Hfr (“high frequency”) bacteria have incorporated the F plasmid into their chromosomal DNA
  • F⁺ x F^–
    • plasmid is replicated inside F⁺ cell
    • plasmid is transferred through pilus from F⁺ cell to F^– cell
      • only plasmid itself is transferred
      • no transfer of chromosomal genes
  • Hfr x F^–
    • plasmid and some flanking chromosomal DNA is replicated inside Hfr cell
    • plasmid and some chromsomal DNA is transferred through pilus from F⁺ cell to F^– cell
      • in this case, both plasmid itself and chromosomal genes are transferred
  • “a conjugal visit”
• Transduction 
  • transfer of bacterial DNA using virus (phage) as a carrier
  • generalized transduction (“packaging”)
    • lytic phage
    • phage cleaves bacterial DNA
    • parts of bacterial chromosomal DNA may become packaged in viral capsid
    • DNA then travels in phage to infect another bacterium, transferring genes
  • specialized (“excision”)
    • lysogenic phage
    • phage incorporates viral DNA into bacterial chromosome
    • when phage DNA is excised from the bacterial chromosome, flanking bacterial genes may be excised with it
    • DNA then travels in phage to infect another bacterium, transferring genes
    • toxin genes encoded in a lysogenic phage 
      • shiga-like
      • botulinum
      • cholera
      • diphtheria
      • erythrogenic toxin of S. pyogenes
• Transposition 
  • transposons (also known as insertion elements or transposable elements) are segments of DNA that can move from one location to another
  • allows transfer of genes from plasmid to chromosome and vice-versa
    • removal of a plasmid from the chromosomal DNA may include some flanking chromosomal DNA that may be transferred to another bacterium along with the plasmid