Common antibiotic combinations

Question 1

What are the general principles of antimicrobial selection?

Answer 1

Apply the principles of good stewardship.
Planned use where possible
Select antimicrobials based on C&AST wherever possible or, failing that, select antibiotics based on the likely pathogens.
Consideration of case
Empirical decisions in critical cases

Question 2

Why may you choose combinations of antibiotics?

Answer 2

Treatment of mixed bacterial infections in which the organisms are not susceptible to a common agent
To achieve synergistic antimicrobial activity against particularly resistant strains e.g. Pseudomonas aeruginosa
To reduce the risk of, or overcome, bacterial resistance.

Question 3

What considerations should be made when choosing antibiotic combination therapy?

Answer 3

Combinations need to be different targets or process.
OR
Need to be different points on the same process.

Question 4

What is synergy?

Answer 4

where activity is greater than either of the two individually.

Question 5

What is an additive effect of an antimicrobial?

Answer 5

where there is a benefit for treatment but does not increase individual activity.

Question 6

What is antagonism in combination therapy?

Answer 6

one drug inhibits the action of another

Question 7

Give examples of synergy in combination therapy

Answer 7

Trimethoprim with a sulphonamide (act on different targets in the same pathway)
Clavulanic acid with amoxycillin (helps amoxycillin bypass beta-lactamase destruction of penicillin)

Question 8

Give examples of incompatible antibiotic combinations

Answer 8

Beta lactams antibiotics require growing cells so are incompatible with bacteriostatic antibiotics. The bacteria stop growing but are not killed.
Multiple drugs binding the same site/region of target - may interfere and reduce their overall binding and efficacy.

Question 9

Give examples of bacteriocidal antimicrobials

Answer 9

penicillins,
cephalosporins,
aminoglycosides,
trimethoprim/sulfonamides (“potentiated”),
metronidazole,
quinolones,
rifampin,
glycopeptides

Question 10

Give examples of bacteriostatic antimicrobials

Answer 10

tetracyclines,
phenicols (eg, chloramphenicol, florfenicol),
macrolides,
lincosamides,
spectinomycin,
sulfonamides (non-potentiated)