| | | | | | The Medical Algorithms Project |
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| Chapter : | ch25. Antibiotics, Vaccines & Prophylaxis | Section : | Interpretation of Antibiotic Susceptibility Testing | | Patterns of Antibiotic Interaction (Additive, Synergistic, Antagonistic) During Disk Diffusion Testing | | Copyright (c) 2006-2007, Institute for Algorithmic Medicine, Houston, TX, USA. All rights reserved. |
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Overview :
The effect of antibiotics may interact when they are used in
combination. In disk diffusion testing the in vitro effect is evident based on
the zone of inhibition around the disks.
|
Term |
Effect of Both
Antibiotics Together |
|
additive (indifference, no interaction,
independent) |
sum of the individual effects; no change when two
overlap |
|
synergistic |
greater than the sum of the parts |
|
antagonistic |
decreased; one antibiotic interferes with the action of
the other |
When 2 antibiotics are tested in combination, standard
concentration disks should be placed at a standard distance from each other.
Placement may require some trial and error, with the goal to have some overlap
in diffusion without overwhelming the other's effect.
|
Interaction |
Zone Away from
the Other Disk |
Zone Between
the Two Disks |
|
additive |
none, or uniform and circular |
bacterial growth where the two zones do not touch
(indifferent to each other) |
|
synergistic |
none, or uniform and circular |
no bacterial growth in an area beyond the individual
zones |
|
antagonistic |
none, or uniform and circular |
flattened, noncircular zone of growth inhibition facing
other disk |
| | References: | |
Pillai SK,
Moellering RC Jr, Eliopoulos GM. Chapter 9: Antimicrobial combinations. pages
365-440 (pages 376-377). IN: Lorian V (editor). Antibiotics in Laboratory
Medicine, Fifth Edition. Lippincott Williams & Wilkins. 2005.
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