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.


Effect of Both Antibiotics Together

additive (indifference, no interaction, independent)

sum of the individual effects; no change when two overlap


greater than the sum of the parts


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.



Zone Away from the Other Disk

Zone Between the Two Disks


none, or uniform and circular

bacterial growth where the two zones do not touch (indifferent to each other)


none, or uniform and circular

no bacterial growth in an area beyond the individual zones


none, or uniform and circular

flattened, noncircular zone of growth inhibition facing other disk


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