Antibiotic Resistance: Gene Exchange Flashcards
Describe the clinical relevance of bacterial gene exchange.
Transformation - change appearance of surface antigens to evade host immune system; mechanisms differ between gram (+) and (-)
Describe DNA transformation in gram (+) streptococcus pneumoniae
- donor cell releases naked DNA due to lysis, trauma, or death
- naked DNA is susceptible to environmental degradation and is broken into small pieces by donor enzymes
- pneumococcal cells secrete a protein competence factor to induce competence
- surface protein binds to any naked dsDNA in the environment
- as DNA passes across recipient membrane, one strand is degraded
- 3 fates of the exogenate: degradation (no change), if independently replicative, circularizes, or recombines into host genome
Describe conjugation.
- transfer of a conjugal plasmid via cell-cell contact (mating)
- requires conjugal plasmid (replicates independently, can transfer independently)
- circular, dsDNA
State why the medical community fears conjugal plasmids.
- conjugal R plasmids (ABX resistance) can conjugate between cells of the same family AND can conjugate multiple resistance to antibiotics
- most frightening scenario is that a multiple-resistance gene R plasmid is residing in benign commensal bacteria => conjugates to extremely pathogenic infection that is typically treated with ABX => resistant pathogen
Describe transduction.
- gene transfer mediated by bacteriophage
- does not require cell-cell contact
- lytic or lysogenic infection
- phage is independent and can travel long distances
- resistant to nucleases and detergent because phage capsid protects DNA
- limited cross-species gene exchange, unlike conjugation
Compare and contrast virulent and temperate phage. List their respective infections.
LYTIC (virulent)
- hijacks cell machinery
- creates phage progeny
- lysis + release of progeny
LYSOGENIC (avirulent; temperate)
- can enter lytic phase
- becomes a prophage (quiescent)
=> host is now called a lysogen
=> prophage can be circularized plasmid or recombined into host genome
- passed onto host progeny
=> ex: cholera toxin is encoded from phage DNA, not cholera bacterial DNA
List and describe the steps leading up to lysogeny.
- infection
- lysogenic response
- phage DNA circularizes into plasmid or recombines into host genome
- host becomes a lysogen - carrying latent prophage and passing it on
List the possible fates of the exogenate.
- degradation: no change in recipient
- circularization: becomes a plasmid
- recombination into host genome
Distinguish between exogenate and endogenate.
endogenate - recipient cell’s own genome
exogenate - donor cell’s genome
Define competence.
ability of a bacterium to uptake naked DNA from its environment
- can be encoded in chromosomal DNA and activated under specific environmental conditions
Describe the steps involved in conjugation by gram (-) E. coli.
- donor cell containing R plasmid encodes for sex pilus
- sex pilus connects to R- cell and pulls them closer (creating a mating pair)
- formation of a conjugation bridge
- transfer replication occurs at the origin of transfer (oriT) region on the R plasmid
- rolling circle mechanism disconnects the dsDNA of the R plasmid
- one strand is pulled off and is transferred to the recipient through the conjugation bridge
- in the donor cell, remaining ssDNA plasmid is replicated to reform the dsDNA R plasmid
- in the recipient cell, ssDNA transferred is replicated to form dsDNA R plasmid
- detachment
- results in 2 R+ cells that can mate with other R- cells
NOTES
- conjugation bridge must remain intact during the entire process
- DNA is never naked and exposed to nucleases
Define phage.
- aka bacteriophage, virion
- obligate parasite (must replicate inside bacterial cells)
- contains protein capsid enclosing nucleic acid genome and a tail
- can carry genes not required for phage propagation
- infects a sensitive cell by adsorbing (binding) to the cell surface receptor and injecting its DNA
