Final answer:
The effect of two antimicrobials being more effective together than individually is called synergism. This is different from antagonism, which is harmful, and symbiosis, which encompasses a broader range of interactions between organisms.
Step-by-step explanation:
The situation in which the effect of two antimicrobials given together is more effective than the effect of either medication given individually is referred to as synergism. This occurs when two antibacterial drugs are administered together to provide a benefit greater than the efficacy of either drug alone. An example of synergistic combinations is trimethoprim and sulfamethoxazole. In contrast, antagonism refers to drug interactions that produce harmful effects, such as loss of drug activity or increased potential for toxicity. It's important to differentiate this from other types of interactions such as symbiosis, which is a broad term covering relationships between two organisms where at least one benefits.