Lecture 16 - Antimicrobial Mechanisms and Resistance II Flashcards

1
Q

When are anti-membrane antimicrobials not used as a last resort?

A

In antifungal cases

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2
Q

Bonds linking rows of peptidoglycan

A

L-Lysine - Penta-glycine - D-Alanine

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3
Q

Polysaccharides making up peptidoglycan

A

N-acetylmuramic acid

N-acetylglucosamine

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4
Q

How are peptidoglycans cross-linked?

A

Penta-glycine displaces 2nd D-Ala on L/D polypeptide.

This is catalysed by transpeptidase

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5
Q

How is peptidoglycan assembled?
1)
2)

A

1) Building blocks are immobilised on the inside of the cell membrane
2) Assembled building blocks are flipped onto cell exterior

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6
Q

Other names for transpeptidases

A

Endopeptidases

Carboxypeptidases

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7
Q

When was vancomycin first used?

A

In the 1950s, when Staph aureus began to be resistant to beta-lactams.

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8
Q

What is MRSA?

A

Methicillin-resistant staph aureus

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9
Q

What does methicillin resistance signify?

A

Resistance to almost every other beta lactam

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10
Q

Which type of antimicrobial is vancomycin?

A

Glycopeptide

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11
Q

How does vancomycin work?

A

Binds directly to D-ala - D-ala on peptidoglycan precursor

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12
Q

Shape of vancomycin molecule

A

Very large, very charged

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13
Q

Why can’t vancomycin be used against G- bacteria?

A

Because it is very large and highly-charged

Can’t pass through thicker cell wall.

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14
Q

Which bacteria is vancomycin useful against?

A

Staph aureus

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15
Q

Bacteria innately resistant to vancomycin

A

Gram-, enterococci

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16
Q

Why are enterococci resistant to vancomycin?

A

D-ala - D-ala is replaced by D-ala - D-lac

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17
Q

Danger of VRE

A

Enterococci aren’t pathogenic, but are promiscuous in sharing plasmids, on which vancomycin resistance could be encoded

18
Q

VISA

A

Vancomycin intermediate-resistant staph aureus

19
Q

Mechanism of staph aureus partial vancomycin resistance

A

Produce more peptidoglycan.
As vancomycin inhibits peptidoglycan directly, if there is enough peptidoglycan, there can remain enough to perform function after vancomycin has been used up.

20
Q

Issue with treating VISA

A

Greater amount of peptidoglycan can be overwhelmed with more vancomycin, though vancomycin is toxic in higher doses, so this can be dangerous

21
Q

How do beta-lactams work?

A

1) Bind to transpeptidases
2) Transpeptidases can’t hydrolyse the beta-lactam ring
3) Peptidoglycan can’t be assembled

22
Q

What do transpeptidases hydrolyse?

A

CONH bond between D-ala - D-ala

23
Q

Effect on bacteria of beta-lactams

A

Cell wall is weakened

Bacterial cytoplasm is hypertonic to surrounding tissue fluid, so bacteria swell

24
Q

Action of beta-lactamases

A

Hydrolyse a bond in beta-lactam ring

From O=C-N to O=C-OH NH

25
Resistance to beta lactams 1) 2)
1) Beta-lactamase | 2) Altered penicillin-binding proteins
26
Why does the bacterial spectrum of beta-lactam antibiotics vary? 1) 2) 3)
1) Different bacteria have different penicillin-binding proteins 2) Accessibility of antibiotic to penicillin-binding protein varies 3) Antibiotics have different susceptibilities to beta-lactamases
27
Clavulanic acid 1) 2) 3)
1) A beta-lactam antibiotic 2) Binds to beta-lactamases, beta-lactamases can't hydrolyse clavulanic acid 3) A suicide inhibitor of beta-lactamases
28
Clavulanic acid uses
Used to inhibit beta-lactamases | Ineffective if used by itself, but effective if combined with another beta-lactam, EG: amoxycillin
29
How is clavulanic acid administered?
1) Equal parts clavulanic acid, amoxycillin. 2) As clavulanic acid and amoxycillin are processed in very similar ways, they are both delivered to the site of infection.
30
Co-Amoxyclav
Clavulanic acid and amoxycillin treatment
31
What is Pseudomonas aeruginosa intrinsically resistant to?
Beta-lactams
32
Why is Pseudomonas resistant to beta-lactams?
Has a beta-lactamase gene in its chromosome | Probably used to help form cell wall
33
Beta-lactams that Pseudomonas is susceptible to
Carbenicillin | Ticarcillin
34
Effect of clavulanic acid on Pseudomonas
Nothing. | Clavulanic acid can be hydrolysed by Pseudomonas chromosomally-encoded beta-lactamase
35
How can Pseudomonas become resistant to carbenicillin and ticarcillin?
If it acquires a beta-lactamase plasmid
36
How can a Pseudomonas with a beta-lactamase plasmid be treated?
With ticarcillin and clavulanic acid. Pseudomonas is susceptible to ticarcillin, but beta-lactamase plasmid degrades it. Clavulanic acid inhibits plasmid beta-lactamase.
37
Number of beta-lactamases
Used to be under 10. | With widespread use of beta-lactams, now over 500
38
Gene encoding altered penicillin-binding proteins
MecA
39
What does MecA encode?
Altered penicillin-binding protein | Is a transpeptidiase that doesn't bind beta-lactams
40
Why is ampicillin more orally available than penicillin G?
Ampicillin is more lipophilic
41
New Delhi beta-lactamase
Destroys most modern beta-lactams | Quickly transferred between bacteria on plasmid
42
New beta-lactamase that can destroy most beta lactams
New-Delhi beta-lactamase