What term describes when the combined effect of two antimicrobials is greater than the sum of their individual effects?

What term describes when the combined effect of two antimicrobials is greater than the sum of their individual effects?

• A. Additivism
• B. Synergism
• C. Antagonism
• D. Sensitivity

Answer: (B)

When two antimicrobials work together to produce an effect greater than the sum of their separate effects, that relationship is called synergy. This stronger killing often happens because the drugs hit different targets in a complementary way. For example, one drug may damage the cell wall, making it easier for the other to enter the bacterial cell and exert its effect, leading to more rapid or complete bacterial killing than either drug could achieve alone.

In the lab, synergy can be shown with methods like checkerboard assays or time‑kill curves, often summarized by metrics that indicate the combined effect is greater than expected from the individual effects. This concept is distinct from additivity (the combined effect equals the sum of the two effects), antagonism (one drug reduces the other's effect), and sensitivity (how susceptible the organism is to a drug in general, not how two drugs interact).