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
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