Microbiology: Resistance to antibiotics

Question 1

What are the mechanisms bacteria can employ for resistance?

Answer 1

Modification/breakdown of drug 
Efflux
Altered target site 
Modify pathways to take alternative routes 
Reduce permeability

Question 2

How do bacteria change their genres to acquire these mechanisms of resistance?

Answer 2

New genes (insertions or inheritance on plasmids)
Loss of genes (deletions) 
Modification of promotors leading to altered expression (point mutations)

Question 3

What is intrinsic resistance?

Answer 3

An innate ability to resist activity of a particular antimicrobial agent through inherent structural or functional characteristics which allow tolerance of the drug.

May lack target
Or have altered target

Question 4

What is acquired resistance?

Answer 4

When a microorganism obtains the ability to resist the activity of a drug
Acquisition of new function which allows:
o Alteration drug
o Alteration of target
o Provides bypass mechanism.

Question 5

What are the major mechanisms of resistance of beta lactams?

Answer 5

Cleavage of drug by B-lactamases

Question 6

What does MDR mean?

Answer 6

Multi drug resistant - Acquired non susceptibility to at least one agent in three or more categories

Question 7

What does XDR mean?

Answer 7

Extensively Drug-resistant - Acquired non susceptibility to at least one agent in all but two of or fewer antimicrobial categories

Question 8

What does PDR mean?

Answer 8

Pan drug resistant - Acquired non susceptibility to all agents in all antimicrobial categories

Question 9

What do plasmids allow?

Answer 9

Transfer of resistance between closely related bacteria.

Transfers of complex collections of genes to express resistances

Question 10

Cassettes (region of DNA which you can add bits to) and transposable elements allow….

Answer 10

Increased mobility of complex collections of genes to express resistances
Ability to hop in chromosomes and plasmids and between

Question 11

How can we test susceptibility to drug resistance?

Answer 11

• Dilution test - used for MIC
• Disc diffusion – (Kirby-Bauer)
• E-strip diffusion – rapid simple MIC.

Question 12

How should antibiotics be used for prophylaxis?

Answer 12

Use only when animals are at risk and evidence that usage reduces morbidity and/or mortality (metaphylaxis)

The dose should be the same as therapeutic otherwise it will not work and may promote development of resistance

Question 13

Which antibiotic causes nephrotoxicity?

Answer 13

Gentamicin

Question 14

Which antibiotic are dobermans sensitive to?

Answer 14

sulphonamides

Question 15

Which antibiotic has GI effects?

Answer 15

GI side effects with erythromycin (reduced if administered with food)

Question 16

When would you use combination therapy?

Answer 16

Mixed bacterial infections
Therapy in severe conditions where aetiology is not know.
Treating life threatening cases prior to susceptibility data.
Treating unusual pathogens including Mycobacterium (or mixed types of pathogens)

Question 17

What are the risks of using combination therapy?

Answer 17

• Actions of drugs may be antagonistic
• There is increased cost
• Increased risk of toxicity
• Do not use bacteriostaic and bacteriocidal drugs in combination.