MICR2000 MOD 3 BACTERIOLOGY

Conjugation

Transfer of DNA via direct contact between cells

Plasmid in the donor cell is either cut & copied or copied & pasted

Transposable Elements

DNA sequence that can jump from 1 position to another or from 1 DNA molecule to another

Insertion sequences

• have a gene encoding a transposase protein
  - endonuclease: cut and copy function
  - intergrase: copy and past function

• has an inverted repeated sequence at its end that the transposase recognises

Transposons

• contain genes unrelated to transposition that can be mobilised along with the transposable element

• can be inserted into chromosome or plasmid

• often contains multiple antibiotic resistant genes

Transposition mechanisms

Conservative: transposable element is excised from 1 location and reinserted into another → copy number is 1

• transposase cuts target DNA → IS intergrates → DNA pol fills gaps and direct repeats flanking intergrated IS are formed

Replicative: A new copy of a transposon is produced during transposition and inserted at another location → copy number is 2

• transposable element binds to inverted repeats → initiates transposition

• Tn3 is ligated to target ends

• 3’ ends prime replication through Tn3

• cointergrate between transposon and target DNA formed

• resolvase binds to ‘res’ sequence of duplicated transposon and resolves cointergrate by site specific recombination

Mobile Antibiotic Resistant Genes

DNA containing resistant genes move from cell to cell via conjugative plasmids

Conjugative plasmids acquire resistant genes via transposons

Transposons acquire multiple resistant genes via integrons

Integrons

Capture and express genes contained in mobile elements called gene cassettes

• has a promoter → cloning expression vector for gene cassettes

• found within transposons

• features
  - attachment site
  → recognised by intergrase
  → acceptor sites for cassettes
  - intergrase gene encoding a site specific recombinase
  - promoter → drives expression of incorporated sequence
  - cassettes must have an intergrase specific recombination site “59 base element” → can be excised as circles and move from integron to integron

Reduce Antibiotic Resistant Spread

• reduce selective pressures → stop inappropriate use of antibiotics

• remove ineffective antibiotics from use

• monitoring, isolation and treatment programs to prevent resistant pathogens from establishing and spreading

Measure for Microbial Virulence

Can be measured using Lethal Dose 50 which is the dose of an agent that kills 50% of a test group
A lower LD50 is associated with higher virulence or pathogenicity as a smaller amount is needed to kill

Virulence Factors

Virulence factors aid in:

Colonisation

• Adhesins resist physical removal and adhere to host

• Invasins invade host

• Motility allows movement through mucous

Evasion of host immune system

• Biofilms and capsules resist phagocytosis

• Phase and antigenic variation of surface proteins trick host immune system into thinking it is not foreign

Damage to host

• Competes for nutrients (sequests iron using siderophores)

• Endotoxins from lipid A damage host

• Exotoxins inhibit host metabolic function

Staphylococcus aureus
 

• G+ cocci

• MRSA and VRSA

• Adhesins to attach to host and resist removal

• Capsule to resist phagocytosis

• Protein A used as an immunological disguise

• Coagulase secreted to create small blood clots in host to impair immune system

• Secreted enzymes break down DNA, fibrin and hyaluronic acid

• Secretes exotoxins, particularly TSST a toxin

• Produces catalase to neutralise host immune system hydrogen peroxide defence