Final answer:
Modifiers are substances that alter drug properties without changing the mode of action. A semisynthetic drug is a modified drug with enhanced efficacy. The mode of action refers to a drug's ability to target harmful microbes without affecting the host.
Step-by-step explanation:
The substances that do not change the mode of action but alter properties such as half-life, acid sensitivity, charge, and ability to cross the outer membrane are B. Modifiers.
In the scenario presented, the scientist has modified the antimicrobial compound from a soil bacterium, changing its chemical structure to enhance its effectiveness against a broader spectrum of bacteria. This modified antimicrobial drug which now can kill both gram-negative and gram-positive bacteria is considered B. semisynthetic, because it is derived from a natural source but has been chemically altered.
Such modifications to drugs are generally designed to increase the range of bacteria targeted, increase stability, decrease toxicity, or confer other properties that are beneficial for treating infections. The term that refers to the ability of an antimicrobial drug to harm the target microbe without harming the host is A. mode of action.