Antibiotics 2 Flashcards by Rosie Liddell

3 modes of AMR (antibiotic resistance)

Intrinsic resistance
Acquired resistance
Tolerance

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Intrinsic resistance

Bacteria you’re looking at can’t be killed by that drug

e. g. bacteria lack target structure or possess an impermeable cell envelope
- Gram-negatives are intrinsically resistant against many penicillins

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Acquired resistance

Renders a formally sensitive bacterium resistant
e.g. through spontaneous mutation
OR by picking up resistance determinants
(horizontal gene transfer)

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Tolerance

Cells are temporarily in a physiological state that renders them ‘resistant’
- endospores, dormant cells or cells in biofilms
= not normally susceptible to antibiotics

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4 main mechanisms of acquired AMR

Change the target

Spontaneous mutations (e.g. in enzymes)
Acquisition of alternative genes or pathways

Degrade the AB
-Via hydrolytic enzymes

Modify the AB

Modifying enzymes
(e. g. acetyltransferases)

Export the AB
- Via active transporters

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Changing the target

- 3 ways

Spontaneous point mutations

Acquisition of an alternative gene

Acquisition of an alternative biosynthetic pathway

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Changing the target
- spontaneous point mutation
> example of AB and how they work

Quinolones e.g. Ciprofloxacin

Bind to GyrA subunit of DNA gyrase near where it would normally bind to DNA
-> inhibits DNA gyrase

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Changing the target
- spontaneous point mutation
> resistance against Quinolones

Point mutation in QRDR region abolishes quinolone binding

-> gyrase is still active but now resistant

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QRDR

Quinolone resistance determinant region

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DNA gyrase

- how does it work?

2 subunits - GyrA and GyrB
slot into each other
-> slot around DNA
-> controls supercoiling

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Point mutations

- transferal

Mutations now on chromosome
-> passed onto daughter cells

NOT normally transmitted horizontally between bacteria

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Changing the target
- acquisition of an alternative gene
> example of an AB and how they work

Penicillin

Bind and irreversibly block transpeptidases (PBPs = penicillin binding proteins
- )

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Changing the target
- acquisition of an alternative gene
> some forms of PBPs

Lower substrate affinity
-> less likely to bind penicillin and become inactivated
= resistance

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Acquisition of an alternative gene
- MRSA
> what does it stand for?
> resistance

Methicillin Resistant Staphylococcus aureus

Possesses mecA gene
- encodes low-affinity PBP
= PBP2a

mecA is part of mobile genetic element - SCCmec
- can become v large + carry multiple resistance genes against different classes of AB

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Acquisition of an alternative gene
- MRSA
> spread of mecA

As part of SCCmec

> can be readily passed between different staphylococci
> spread of resistance

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Changing the target
- acquisition of an alternative biosynthetic pathway
> example of an AB and how it works

Vancomycin

Bonds to the terminal D-Ala-D-Ala of peptidoglycan precursors
-> inhibits transpeptidation

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Acquisition of an alternative biosynthetic pathway

- Vancomycin resistance

Synthesis of alternative peptidoglycan precursors
- alternative ending to D-Ala-D-Ala

e.g. D-Ala-D-Lactate
Can’t be bound by vancomycin

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Acquisition of an alternative biosynthetic pathway

- modification of D-Ala-D-Ala

Requires several genes to create an alternative biosynthesis pathway

synthesis of different peptides (vanH, vanA)
removal of regular precursors (vanX, vanY)
regulatory genes (vanR, vanS)

Acquisition of an alternative biosynthetic pathway

- Vancomycin resistance spread

Encoded on mobile genetic elements

> can be passed between bacteria
> spread of resistance

Acquisition of an alternative biosynthetic pathway
- VRE
> what is it?

= Vancomycin Resistant Enterococci

Importance of vancomycin

Beta-lactams + vancomycin both target the cell wall
BUT resistance mechanisms are so different
that vancomycin is v useful in treating beta-lactam resistant infections (like MRSA)

Degrading the antibiotic

example of AB used
process of resistance

Beta-lactam antibiotics
e.g. penicillins

Beta-lactamases

cleave the B-lactam ring between N + C
> inactivates the AB

Degrading the antibiotic
- Extended spectrum beta-lactamases (ESBL)
> what are they?
> when are they mostly a problem?

Can target a wide range of beta-lactam ABs
- including the semi-synthetic derivatives that were previously beta-lactamase resistant

In E.coli = Gram-negative

ESBL

- why are they not a problem in Gram- positives?

Most beta-lactamases are secreted into periplasm (where AB target is)
- Gram-positives have no periplasm

Modifying the antibiotic | - example of AB and how it works

Aminoglycosides e.g. Kanamycin Binds to 16S rRNA -> inhibits protein synthesis

Modifying the antibiotic | - Why no point mutations in 16S rRNA?

16S rRNA is too essential for functioning of ribosome - bacteria would just die Gene often present in several copies per genome -> would have to mutate all copies before cell is resistant

Modifying the antibiotic - resistance most commonly mediated by enzymes that modify the antibiotic by transfer of a chemical group - what does this lead to?

Acetyltransferases - add an acetate group Phosphotransferases - add a phosphate group Nucleotidyltransferase - add an adenine AB can no longer bind to its target due to steric hinderance

Modifying the antibiotic | - spread of resistance

Genes for AB modifying enzymes usually encoded on plasmids | -> can be shared by horizontal gene transfer

Exporting the antibiotic | - example of an AB and how it works

Tetracycline Binds to 16S rRNa -> inhibits protein synthesis

Exporting the antibiotic | - how does it work?

Resistance occurs via active transport | -> exports the drug faster than it can enter the cell

Exporting the antibiotic | - exporting Tetracycline

Export occurs via drug-specific pump = Tet pumps (TetA, TetL etc. depending on species) Driven by proton motive force - > pump drug out of cell into periplasm - > leaves periplasm via porins

Exporting the antibiotic | - drug exporter types

MDR = multi-drug resistance transporters - not specific 5 different families

Exporting the antibiotic | - differences between the 5 different families

Driven by proton motive force OR ATP hydrolysis Located in cytoplasmic membrane OR spanning entire cell envelope Often encoded on chromosome (intrinsic resistance) Sometimes transferred by mobile genetic elements

3 origins + sources of AMR

AB are produced by microbes Microbes naturally live in mixed communities Bacteria need their multi-drug transporters for protection

Origins + sources of AMR - AB are produced by microbes > example

Producer must be resistant themselves Fungal producers = intrinsically resistant Bacterial producers = possess immunity genes VREs carry the vancomycin resistance van genes of the producer

Origins + sources of AMR - Microbes naturally live in mixed communities > example

Had millions of yrs to develop resistance against ABs produced by their neighbours + competitors e.g. B-lactamses allow bacteria to survive in presence of a penicillin producer

Origins + sources of AMR | - Bacteria need multi drug transporters for protection

Protection against toxic compounds - NOT just ABs Need to be able to get rid of toxic metals etc.

Spread of AMR

Most resistance encoded by genes Horizontal gene transfer is v common - Transposons/insertion elements - Plasmids - SCCmec Plasmids and mobile genetic elements often collect different resistance genes = multidrug resistance Sometimes wide-spread resistance is result of massive expansion of a single resistant clone e.g. MRSA

Solutions to AMR

Use less ABs (e.g. vaccination to prevent infection) Use specific ABs (identify pathogen + selectively target that org) Use oldest effective drug (to lengthen the effective life span of the new ones) ``` Monitor treatment (use AB long enough to clear infection, but not longer than necessary) ```

Discovery of new ABs | - 2 problems

Science - drug screens today keep picking up same classes of compounds Economics - development of new ABs offers poor return on investment - long and complex approval process - ABs = cheap - short treatment times

2 new treatment strategies

New ABs Inhibitors of resistance mechanisms

New treatment strategies - New ABs > example

Cefiderocol Drug fuses to siderophore -> both actively taken up into bacterium by iron uptake system (resistant against export + bypasses intrinsic resistance) Resistant against extended spectrum beta-lactamases (ESBLs) (B-lactam ring is resistant)

New treatment strategies - Inhibitors of resistance mechanisms > example

Augmentin - treatment of some B-lactam resistant infections Has ?????