Mechanisms of Genetic Exchange and Modification

Original Postulates Proposed by Koch

1. The microbe must be associated with symptoms of the disease and must be preesnt at the site of infection

2. The microbe must be isolated from the lessons of disease and grown as a pure culture

3. A pure culture of the microbe, when incubated into a susceptible host, must reproduce the disease in the experimental host

4. The microbe must be reisolated in pure culture from the experimentally infected host

Slow Processes of Genetic Change and Diversification

• Point Mutation (nt change, ng insertion, nt deletion)

• Gene duplication

• Gene deletion

• Chromosomal Rearrangement (inversion and intragenic recombination)

Rapid Processes of Genetic Change and Diversification

• Phase variation (promoter inversion, transcriptional blockage, slipped-strand synthesis)

• Antigenic Variation (gene shuffling and conversion)

• HGT

C. jejuni motility variants

Variable motility partly due to slip strand mispairing in multiple phase variable genes

• Depends on genes that’re on and off due to multi-G

Whether or not the microbe can get through mucus

Slipped-Strand Mispairing in More Detail

Insertion or Removal of nt will activation or inactivate certain genes and thus proteins

Mechanisms that create genetic diversity in H. pylori

• Reassortment

  • Natural Transformation that replaces alleles (allelic change) or adds new properties (new gene introduced)

  • Diversification through a point mutation (allelic change), intragenomic recombination (gene conversion or mosaic gene), or slipped-strand mispairing (phase variation)

Up to 50% of genome can be replaced during 40 years of chronic infection!

Transformation

chromosomal genes, can lead to intraspecies transfer

• Donor cell: phage lysis, autolysis or active secretion of DNA (T4SS → N. gonorrhoeae, C. jejuni, H. pylori)

• Recipient cell: DNA uptake by naturally competent bacteria (ex. N. gonorrhoeae, C. jejuni, and H. pylori)

Transduction

Primarily intraspecies transfer by bacteriophage

Generalized or Specialized Transducing Phage

Conjugation

Intra- and inter-species transfer of plasmid, conjugative transposons, or chromosomal DNA. Bacteria can pick up new resistances or viluence factors from mobile elements

Generalized Transduction

• Lytic Phage infects bacterial cell

• Host DNA is hydrolyzed into pieces, and phage DNA and proteins are made

• Occasionally a bacterial DNA fragment is pachaged by a phage capsid

• Transducing phages infect new host cells, where recombination can occur

Specialized Transduction

• Bacterial cell has prophage integrated (lysogenic phage) between genes A and B

• Occasionally, prophage DNA exits incorrectly, taking adjoining bacterial DNA with it

• Phage particles carry bacterial DNA (here, gene A) along with phage DNA

• Transducing phages infect new host cells, where recombination can occur

BoNTX/C1 Phages

Of Clostridium botulinum encode botulinum toxin

Cornynephage beta

Of Corynebacterium diphtheria encodes diphtheria toxin (AB toxin - ADP-ribosylates eEF-2)

T12 Siphoviridae (lambda-like)

Of Streptococcus pyogenes encodes pyrogenic toxins (superantigens)

Stx-encoding Lambda phages

of Enterohemorrhagic E. coli (EHEC); the stx genes encode Shiga-like toxins (AB5 toxin-cleaves 28S rRNA of 60S ribosomal subunit)

CTX Phage

Of Vibrio cholera encodes the cholera toxin (AB5 toxin - ADP-ribosylates Galpha subunit of adenylate cyclase)

Bacterial Conjugation

1. Donor cell constructs pilus with F plasmid

2. Donor cell attaches pilus to Recipient cell

3. Plasmid transfer from donor to recipient (donor maintains original and makes copy in recipient)

4. Now have and old donor and new donor where both can donate plasmid

Transposon

Can move from conjugative plasmid and transfer to recipient cell

• Cuts itself out with sticky repeat ends

  • transposase gene found in inverted repeats

• Fills in the gaps and jumps

ex. vancomycin resistance genes

Genomic Islands can Move Between Bacteria by Conjugation

Integrative and conjugative elements (ICEs) or Conjugative Transposons

ICE on chromosome and excise into a plasmid where the plasmid can transport via a pore and then insert back into both chromosomes

Pathogenicity Islands (PAI)

Virulence genes acquired by HGT

• Typical structure of PAI: tRNA → promoter → int → virulence genes

• LEE: Locus of Enterocyte Effacement (EPEC and EHEC)

Acquisition of VIrulence Factors by E. coli

HGT can lead to several pathotypes

• PAIs such as LEE

• Plasmids such as pEAF (enteropathogenic E. coli adhesion-factor plasmid) and pENT (enterotoxin-encoding plasmids)

  • Bacteriophages like Stx (Shiga-toxin-encoding bacteriophage)

Mutation

DNA sequence changes that are usually non-reversible

Reversible DNA Sequence Changes

via DNA recombination or mutation within repeated sequences

Antigenic Variation

alternate expression of multiple variants of a surface antigen, such as a protein or carbohydrate

Phase Variation

On/Off expression or switching between only two variants of a surface antigen (like Campy)

Pilin Antigenic Variation

• Gene conversion: can lead to super piliated cells or medium piliated cells

• Gene conversion, pilC variation or pilE deletion can lead to no pili

Streptococcus pneumoniae Unique Mechanism

Secretes AMPs (bacteriocins) targeting organisms for lysis and uptake their DNA