In antimicrobial synergy testing, what defines synergism?

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

Answer: (C)

Synergism in antimicrobial interaction means the two drugs together produce a stronger effect than the sum of their separate effects. In lab testing, this appears as lower inhibitory concentrations when the drugs are used in combination, often quantified by a fractional inhibitory concentration index of 0.5 or less. This indicates that each drug helps the other achieve inhibition at lower doses than when used alone. Time-kill and checkerboard assays are common ways to assess this. For example, a beta-lactam that damages the cell wall can enhance uptake of another agent, boosting overall activity. Indifference means the combination has no notable interaction; additive means the combined effect equals the sum of individual effects; antagonism means the combination is worse than expected. Therefore, the defining feature is a greater-than-additive effect when drugs are used together.