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MICR > L20 > Flashcards

L20 Flashcards

1
Q

Five main bacterial antibiotic resistance mechanisms

A
  1. Altered antibiotic target
  2. Inactivate antibiotic
    a. Degrading the antibiotic
    b. Chemically modify antibiotic
  3. Reduced antibiotic accumulation
  4. Alternate pathway to bypass the inhibited one
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2
Q
  1. Altering the target of the antibiotic
A
  • Target Protection - block access
  • Target Modification - mod target
  • Insensitive Functional Target - alt enzyme
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3
Q

Target Protection

A
  • Resistance protein blocks access of antibiotic target
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4
Q

Target Modification

A

–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

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5
Q

Insensitive Functional Target

A

–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

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6
Q

2a. Degrading the antibiotic

A

–e.g. hydrolysis of the β-lactam ring of penicillins and cephalosporins by penicillinase (β-lactamase) enzymes

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7
Q

2b. Modifying the antibiotic

A
  • Chemically altering antibiotic
  • e.g. acetyltransferase adds an acetyl group to aminoglycosides or chloramphenicol, inactivating them
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8
Q
  1. Reduced antibiotic accumulation
    * Antibiotic efflux
A

–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

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9
Q
  1. Alternate pathway to bypass the inhibited one
    * Target bypass
A

–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

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10
Q
  • Vertical gene transfer
A

transfer of genes from parent to progeny
–bacteria have this, but as they don’t reproduce sexually so only limited genetic variation (via mutation)

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11
Q
  • Horizontal gene transfer (HGT)
A

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

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12
Q

Horizontal gene transfer is a one way process [t/f]

A

True

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13
Q

How is HGT a one way process?

A
  • 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
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14
Q

The donor DNA then has four possible fates in recipient cell if genes are present in recipient already

A

–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

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15
Q

Mobile genetic elements:

A

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

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16
Q

Mobile genetic elements: Plasmid

A

double-stranded DNA molecule, usually circular, that can exist and replicate independently of the chromosome

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17
Q
  • R plasmid/ Resistance plasmid:
A

a plasmid bearing one or more antibiotic-resistance genes

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18
Q

The first plasmid discovered was the

A

F plasmid
(or F factor)

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19
Q

Mobile genetic elements) Transposable element

A

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

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20
Q

Transposon

A

also carries other genes such as antibiotic resistance genes; is found as part of a bacterial chromosome or a plasmid, cannot self-replicate

21
Q

Mobile genetic elements: Integron

A

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

22
Q

R plasmids encode antibiotic resistance;

A
  • 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
23
Q

gene cassette encoding antibiotic resistance

A

gene cassette encoding antibiotic resistance inserts into integron which inserts into transposon which in turn inserts into a plasmid

24
Q

R plasmids can spread throughout a bacterial population (by _____) and some can also spread between species

A

conjugation, transduction or transformation

25
Q

There are three main mechanisms of horizontal gene transfer (HGT) in bacteria

A
  1. Conjugation
  2. Transduction
  3. Transformation

*Using these mechanisms, antibiotic resistance genes can spread rapidly through bacterial populations

26
Q

Conjugation (HGT)

A

gene transfer mediated by plasmids that requires contact between cells

27
Q

Transduction (HGT)

A

gene transfer mediated by bacteriophages (viruses of bacteria)

28
Q

Transformation (HGT)

A

the uptake of a piece of free DNA by a cell

29
Q

What is an example of a conjugative plasmid

A
  • F plasmids
  • Some R plasmids are conjugative plasmids, thus conjugation is a major route for the spread of antibiotic resistance genes among bacteria
30
Q

Conjugation with F plasmids

A
  1. F plasmid encodes proteins to build sex pilus and protein systems (relaxosome, transferosome/Type 4 Secretion System) that perform DNA transfer
  2. Sex pilus tip binds to recipient cell. Pilus retracts, pulling donor and recipient cells together to make contact and form a mating pore
  3. Plasmid is nicked (one strand is cut), rolling circle replication of plasmid DNA starts.
  4. Both cells recircularize plasmids, synth second strands to form double-stranded plasmids, and form pili; both cells now viable donors in conjugation.
31
Q

Transduction with bacteriophages (viruses of bacteria) includes two groups

A
  1. Virulent phages: only have a lytic cycle
  2. Temperate phages have a lytic cycle, or a lysogenic cycle
32
Q

Virulent phages:

A

only have a lytic cycle

33
Q

Temperate phages

A

have a lytic cycle, or a lysogenic cycle

34
Q

Virulent Phages lytic cycle:

A
  • 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
35
Q

Temperate Phages
* Lysogenic cycle

A

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.

36
Q

Phage DNA is inactive

A

prophage

37
Q

prophage induction occurs when

A

Under environmental stress

38
Q

once prophage induction happens:

A

prophage DNA is activated and excised from chromosome, replicates, and lytic cycle resumes – killing the cell

39
Q

Prophage induction:

A

exposure to stress such as UV light triggers activation of the prophage and its excision from the host chromosome – lytic cycle resumes

40
Q

Lysogenic cycle

A
  • 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
41
Q

Generalized transduction def

A

transfer of any genes from donor

42
Q

–Specialized transduction def

A

transfer of only genes adjacent to prophage

43
Q

Generalised transduction

A
  • 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
44
Q

Specialized transduction

A
  • 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)
45
Q

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.

A

Specialized transduction

46
Q

Transformation def

A
  • Uptake of a piece of free DNA by a cell
47
Q

Transformation (HGT)

A
  • 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
48
Q
  • 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
A

E. coli

49
Q

____ (model conjugative plasmid), _____are built up from smaller Mobile Genetic Elements (MGEs): gene cassettes, integrons, transposons, and may be conjugative

A

F plasmid
R plasmids

MICR (18 decks)

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