60+61 - Antimicrobial Resistance III and IV

Question 1

How is metronidazole targeted to pathogens?

Answer 1

Must be reduced to be activated (nitroreductase).

Anaerobic microbes have a lot of reductases to remove oxygen from environment.

Question 2

How can disc susceptibility break points be determined?

Answer 2

Have to use MIC from dilution tests to construct a curve.

Curve has diameters of inhibition on one axis, MICs from dilutions on the other.

Question 3

Worst thing to do when prescribing antimicrobials

Answer 3

Prescribe one when one isn’t needed

Question 4

Considerations when prescribing antimicrobials empirically (best-guess prescription)
1)
2)
3)
4)
5)
6)
7)

Answer 4

1) Is treatment with an antimicrobial needed?
2) Is it safe and reasonable to wait before treating?
3) Are diagnostic samples needed?
4) If so, have they been collected?
5) What is the likely aetiological agent?
6) What is its antimicrobial susceptibility?
7) Is there evidence that treatment will benefit the patient?

Question 5

Definition of extensively drug-resistant TB (XDR-TB)

Answer 5

Resistant to all four front-line antimicrobials

Question 6

What are hypervirulent C. difficile strains?

Answer 6

Increased toxin secreiton, antimicrobial resistance

Question 7

Antimicrobial that all G- bacteria are resistant to

Answer 7

Vancomycin

Question 8

Method of horizontal gene transfer that can occur between unrelated bacteria

Answer 8

Conjugation

Question 9

Do bacteria need to be closely-related for transduction to occur?

Answer 9

Yes

Question 10

Genome of most bacteriophages

Answer 10

dsDNA

Question 11

Examples of phage-encoded toxins

Answer 11

DIphtheriatoxin, shigatoxin, choleratoxin

Question 12

Stages of bacteriophage lifecycle
1)
2)

Answer 12

1) Temperate (lysogenic) - Infected with phage, express new phenotype, but bacterium is unharmed
2) Lytic - Phage begins replicating, lyses host

Question 13

Most-resistant bacteria known

Answer 13

Commensals (EG: S. epidermitis)

Question 14

Why is most TB resistance from mutation?

Answer 14

Thick cell wall is an impediment to transduction, conjugation

Question 15

Example of antagonistic antimicrobial combination

Answer 15

Tetracycline and penicillin.

Penicillin only affects actively-growing bacteria. Tetracycline prevents bacterial growth.

Question 16

Examples of synergistic antimicrobial combination

Answer 16

Beta lactams and aminoglycosides.
Amoxycillin and clavulanic acid
Sulphonamides and trimethoprim

Question 17

Mechanisms of synergy
1)
2)
3)

Answer 17

1) Inhibit sequential steps in a metabolic pathway (sulphonamides and trimethoprim)
2) Inhibit enzymatic degradation (amoxycillin and clavulanic acid)
3) Enhance antimicrobial uptake by target cell (beta lactams and aminoglycosides)

Question 18

Mechanisms of antagonism
1)
2)
3)
4)

Answer 18

1) Inhibition of bactericidal activity by bacteriostatic agent
2) Induction of enzymatic degradation
3) Competition for binding of the same target
4) Inhibition of target

Question 19

Example of antimicrobial antagonistic induction of enzymatic degradation

Answer 19

Ampicillin and piperacillin.
Ampicillin is a strong inducer of beta lactamases, and piperacillin only weakly induces beta lactamases. If administered together, ampicillin induces beta-lactamases which degrade both antimicrobials.

Question 20

Example of antimicrobials that are theoretically antagonistic, but are synergistic in practise.

Answer 20

Polyenes and imidazole