20 - Antibiotic Resistance and Genetics Flashcards
4 main bacterial antibiotic resistance mechanisms
- Altered antibiotic target
- Inactivate antibiotic (Degrading the antibiotic or Chemically modify antibiotic)
- Reduced antibiotic accumulation
- alternate pathway to bypass the inhibited one
Altering the target of the antibiotic
- Target protection
- Target modification
- Insensitive functional target
Target protection
Resistance protein blocks access of antibiotic target
Target modification
Resistance enzyme modifies antibiotic target
Vancomycin resistance
arises when bacteria change the terminal D-ala in the pentapeptide of peptidoglycan into D-serine or D-Lac, so vancomycin cannot bind
Insensitive functional target
Resistance protein is an alternative enzyme that is insensitive to antibiotic
Antibiotic degradation
Hydrolysis of the β-lactam ring of penicillins and cephalosporins by penicillinase (β-lactamase) enzymes
Chemically altering antibiotic
Acetyltransferase adds an acetyl group to aminoglycosides or
chloramphenicol, inactivating them
Reduced antibiotic accumulation
Antibiotic efflux
Antibiotic efflux
- Rapid extrusion of antibiotic via efflux pump
- Extrusion is energy dependent
- Bacteria can also reduce uptake
Target bypass
Enterococci are intrinsically resistant to sulphonamides because they are able to use pre-formed folic acid from their environment (unlike other bacteria)
Vertical gene transfer
Transfer of genes from parent to progeny
Horizontal gene transfer
- Transfer of genes from one independent, mature organism to another
- Important in bacterial evolution, including the gain of antibiotic-resistance genes
Four possible fates of HGT donor DNA in the recipient cell
- Integration via recombination with recipient’s genome
- Self-replication of the donor DNA (if plasmid)
- No replication and eventual loss of donor DNA
- Degradation of donor DNA
Mobile genetic elements
DNA segments that can move to different locations on genomes and/or move from cell to cell.
Examples of mobile genetic elements
- Plasmid
- Transposable element
- Integron
Plasmid
Double-stranded DNA molecule, usually circular, that can exist and replicate independently of the chromosome
Resistance (R) plasmid
- A plasmid bearing one or more antibiotic-resistance genes
- Gene cassette encoding antibiotic resistance inserts into integron which inserts into transposon which in turn inserts into a plasmid
Transposable element
Small linear DNA molecule that carries genes for transposition and thus can move around the genome, or from one genome to
another (e.g. from plasmid to chromosome) via recombination
Transposon
- Also carries other genes such as antibiotic resistance genes
- Is found as part of a bacterial chromosome or a plasmid, cannot self-replicate
Integron
- Very small linear DNA molecule which can capture even smaller “gene cassettes” encoding antibiotic resistance
- Is found as part of a transposon or a plasmid, cannot self-replicate
Three main mechanisms of horizontal gene transfer in bacteria
- Conjugation
- Transduction
- Transformation
Conjugation
Gene transfer mediated by plasmids that requires contact between cells
Transduction
Gene transfer mediated by bacteriophages
Transformation
- The uptake of a piece of free DNA by a cell
- Both chromosomal DNA and plasmid DNA can be transformed
Process of conjugation
- F plasmid encodes proteins to build the sex pilus and the protein
systems that perform DNA transfer - Sex pilus tip binds to recipient cell. Pilus retracts, pulling donor and recipient cells together to make contact and form a mating pore
- Plasmid is nicked (one strand is cut), rolling circle replication of plasmid DNA starts
- Both cells recircularize their plasmids, synthesise second strands to form double-stranded plasmids, and form pili; both cells are now viable donors in conjugation
Relaxosome
Protein complex including a relaxase protein that nicks and unwinds the DNA, ready for transfer of a single DNA strand through the pore
Transferosome / T4SS
Pushes the DNA/relaxase complex into the recipient cell
Virulent phage
Only have a lytic cycle
Temperate phages
Have a lytic cycle, or a lysogenic cycle
Lytic cycle
- Phage injects its DNA into cytoplasm
- Phage DNA directs the synthesis of many new phages
- Cell lyses and releases the new phages
- New phages can bind to bacterial cells
Lysogenic cycle
- Phage DNA integrates into bacterial host’s chromosome to form a lysogen.
- Phage DNA is inactive (prophage) and replicated along with chromosome
- Host cell does not lyse and grows normally.
Prophage induction
Exposure to stress such as UV light triggers activation of the prophage and its excision from the host chromosome – lytic cycle
resumes
Generalised tranduction
- Transfer of any genes from donor
- Occurs during the lytic cycle of virulent and temperate phages
- Bacterial genes are packaged by mistake into a phage head (capsid). Phage carries only bacterial genes (i.e. no phage genes).
- Any bacterial gene can be transduced, including antibiotic resistance genes
Specialised tranduction
- Transfer of only genes adjacent to prophage
- Occurs during the lysogenic cycle of temperate phages only
- Transducing phage carries mainly phage genes plus some bacterial genes, which it will inject into its next host cell to form a transductant
- Only certain bacterial genes will be in the right location to be picked up and transduced by the temperate phage (i.e. they were right next to the
prophage)
Misuse of antibiotics
- Use as growth enhancers in animals
- Non-completion of antibiotic courses
- Unregulated antibiotic sales
- Use of wrong or suboptimal antibiotic doses
