L20 Flashcards
Five main bacterial antibiotic resistance mechanisms
- Altered antibiotic target
- Inactivate antibiotic
a. Degrading the antibiotic
b. Chemically modify antibiotic - Reduced antibiotic accumulation
- Alternate pathway to bypass the inhibited one
- Altering the target of the antibiotic
- Target Protection - block access
- Target Modification - mod target
- Insensitive Functional Target - alt enzyme
Target Protection
- Resistance protein blocks access of antibiotic target
Target Modification
–Resistance enzyme modifies antibiotic target
–e.g. vancomycin R - arises when bacteria change the terminal D-ala in the pentapeptide of peptidoglycan into D-serine or D-Lactate, so vancomycin cannot bind
Insensitive Functional Target
–Resistance protein is an alternative enzyme that is insensitive to antibiotic
–e.g. β-lactam R - MRSA produces an alternative PBP MecA which has low binding affinity for all β-lactams
2a. Degrading the antibiotic
–e.g. hydrolysis of the β-lactam ring of penicillins and cephalosporins by penicillinase (β-lactamase) enzymes
2b. Modifying the antibiotic
- Chemically altering antibiotic
- e.g. acetyltransferase adds an acetyl group to aminoglycosides or chloramphenicol, inactivating them
- Reduced antibiotic accumulation
* Antibiotic efflux
–Rapid extrusion of antibiotic via efflux pump –e.g. multidrug exporter RND system of Gram negs can pump out quinolones, tetracycline and chloramphenicol
–extrusion is energy-dependent
–Note: bacteria can also reduce uptake
- Alternate pathway to bypass the inhibited one
* Target bypass
–e.g. Enterococci are intrinsically resistant to sulphonamides because they are able to use pre-formed folic acid from their environment (unlike other bacteria)
–Group A Streptococcus reduced folate transporter
- Vertical gene transfer
transfer of genes from parent to progeny
–bacteria have this, but as they don’t reproduce sexually so only limited genetic variation (via mutation)
- Horizontal gene transfer (HGT)
transfer of genes from one independent, mature organism to another
–HGT is of major importance in bacterial evolution, including the gain of antibiotic-resistance genes
Horizontal gene transfer is a one way process [t/f]
True
How is HGT a one way process?
- Donor DNA enters a recipient cell
- If donor DNA contains genes already present in recipient, recipient
becomes temporarily a partial diploid for those genes - The donor DNA then has four possible fates in recipient cell
–Integration via recombination with the recipient’s genome
–Self-replication of the donor DNA (if plasmid)
–No replication and loss of donor DNA
–Degradation of donor DNA
The donor DNA then has four possible fates in recipient cell if genes are present in recipient already
–Integration via recombination with the recipient’s genome
–Self-replication of the donor DNA (if plasmid)
–No replication and 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. Includes
1. Plasmids
2. Transposable element
3. Integron
Mobile genetic elements: Plasmid
double-stranded DNA molecule, usually circular, that can exist and replicate independently of the chromosome
- R plasmid/ Resistance plasmid:
a plasmid bearing one or more antibiotic-resistance genes
The first plasmid discovered was the
F plasmid
(or F factor)
Mobile genetic elements) 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
Mobile genetic elements: Integron
very small linear DNA molecule which can capture even smaller (~1kb) “gene cassettes” encoding antibiotic resistance; is found as part of a transposon or a plasmid, cannot self-replicate
R plasmids encode antibiotic resistance;
- dis japan
- Some R have 1 res gene some have more
- Built up from smaller mobile genetic elements (MGEs)
- gene cassette encoding antibiotic resistance inserts into integron which inserts into transposon which in turn inserts into a plasmid
- R plasmids can spread throughout a bacterial population (by conjugation, transduction or transformation) and some can also spread between species
gene cassette encoding antibiotic resistance
gene cassette encoding antibiotic resistance inserts into integron which inserts into transposon which in turn inserts into a plasmid
R plasmids can spread throughout a bacterial population (by _____) and some can also spread between species
conjugation, transduction or transformation
There are three main mechanisms of horizontal gene transfer (HGT) in bacteria
- Conjugation
- Transduction
- Transformation
*Using these mechanisms, antibiotic resistance genes can spread rapidly through bacterial populations
Conjugation (HGT)
gene transfer mediated by plasmids that requires contact between cells
Transduction (HGT)
gene transfer mediated by bacteriophages (viruses of bacteria)
Transformation (HGT)
the uptake of a piece of free DNA by a cell
What is an example of a conjugative plasmid
- F plasmids
- Some R plasmids are conjugative plasmids, thus conjugation is a major route for the spread of antibiotic resistance genes among bacteria
Conjugation with F plasmids
- F plasmid encodes proteins to build sex pilus and protein systems (relaxosome, transferosome/Type 4 Secretion System) 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 plasmids, synth second strands to form double-stranded plasmids, and form pili; both cells now viable donors in conjugation.
Transduction with bacteriophages (viruses of bacteria) includes two groups
- Virulent phages: only have a lytic cycle
- Temperate phages have a lytic cycle, or a lysogenic cycle
Virulent phages:
only have a lytic cycle
Temperate phages
have a lytic cycle, or a lysogenic cycle
Virulent Phages lytic cycle:
- Phage injects DNA into cytoplasm
- Phage DNA cause synth of new phages
- Phage heads and tails made and copies of DNA genome packaged into them
- cell lyse and release new phages that bind to bacterial cells
Temperate Phages
* Lysogenic cycle
where 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.
Phage DNA is inactive
prophage
prophage induction occurs when
Under environmental stress
once prophage induction happens:
prophage DNA is activated and excised from chromosome, replicates, and lytic cycle resumes – killing the cell
Prophage induction:
exposure to stress such as UV light triggers activation of the prophage and its excision from the host chromosome – lytic cycle resumes
Lysogenic cycle
- When phage inject DNA into cytoplasm
- Phage DNA integrates into host chromosome
- prophage DNA copied when cell divides
- Prophage induction - activate prophage and excision from host to continue cycle
Generalized transduction def
transfer of any genes from donor
–Specialized transduction def
transfer of only genes adjacent to prophage
Generalised transduction
- 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
Specialized transduction
- Occurs during the lysogenic cycle of temperate phages only
- When a prophage is induced to excise from the host chromosome, excision
is sometimes done incorrectly - resulting p. genome contains portions of the bacterial chromosome next to the prophage’s integration site in host chromosome
- thus transducing phage carries 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 & transduced by the temperate phage (i.e. tnext to the prophage)
Specialized transduction is an important mechanism for ≈___ in bacterial evolution, as prophages can often carry virulence genes. Probably less significant in antibiotic resistance gene transfer.
Specialized transduction
Transformation def
- Uptake of a piece of free DNA by a cell
Transformation (HGT)
- Both chromosomal DNA and plasmid DNA transformed
- Bacteria use complex DNA uptake systems to move DNA through b. cell envelopes
- Natural transformation occurs when bacteria lyse and release DNA into environment
- in soill, aquatic environments, in vivo
during infections. Biofilms - Important for HGT
- Natural transformation best studied in Streptococcus, Bacillus and Neisseria –artificial transformation carried out in molecular biology labs to induce other bacteria like ____ to take up DNA
E. coli
____ (model conjugative plasmid), _____are built up from smaller Mobile Genetic Elements (MGEs): gene cassettes, integrons, transposons, and may be conjugative
F plasmid
R plasmids
