Agent which on addition to a colony inhibits its growth and on removal the colony regrows is
**Core Concept**
The question is testing the concept of bacteriostatic and bactericidal agents in microbiology. Bacteriostatic agents inhibit the growth of bacteria, whereas bactericidal agents kill them. This concept is crucial in understanding the effects of antibiotics on bacterial colonies.
**Why the Correct Answer is Right**
The correct answer is a bacteriostatic agent, which means it inhibits the growth of bacteria but does not kill them. When the agent is removed from the colony, the bacteria can regrow because they are not killed. This is in contrast to bactericidal agents, which kill bacteria and prevent regrowth.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because bactericidal agents kill bacteria, not just inhibit their growth. If the agent is removed, the bacteria would not regrow because they are killed.
**Option B:** This option is incorrect because bacteriostatic agents inhibit the growth of bacteria, but the question is looking for an agent that inhibits growth and allows regrowth upon removal. Bacteriostatic agents do not allow regrowth.
**Option C:** This option is incorrect because bacteriostatic agents inhibit the growth of bacteria, not just inhibit their growth. The correct answer is looking for an agent that inhibits growth and allows regrowth upon removal.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that the mode of action of an antibiotic (bacteriostatic vs. bactericidal) determines its clinical use and potential side effects. Bacteriostatic agents are often used in combination with other antibiotics to enhance their effects.
**Correct Answer:** D.