Q.Penicillin is an -

Imagine you are a gardener. Weeds keep sprouting in your flower bed, choking your plants. You could pull them out by hand, but that is slow and you might miss the roots. A better way is to use a selective weed-killer — a chemical that attacks the weed's biology without harming your flowers. Antibiotics work on the same principle, but inside your body.

The "weeds" are harmful bacteria. The "flowers" are your own cells. An antibiotic is a chemical that is designed to disrupt something essential in the bacterial cell — its cell wall, its protein-making machinery, or its DNA replication — while leaving your human cells untouched. This selective toxicity is the entire point.

Now, the precise statement. An antibiotic is a chemical substance, originally produced by one microorganism (like a fungus or a bacterium) to inhibit the growth of or destroy another microorganism. Today, many are also made synthetically in labs.

Antibiotics are classified by their effect on bacteria:

• Bactericidal antibiotics kill the bacteria outright. They are like a bomb. Penicillin is a classic example — it prevents bacteria from building a proper cell wall, so they burst open.
• Bacteriostatic antibiotics stop the bacteria from growing and multiplying. They are like a prison. Tetracycline works this way — it blocks the bacterial ribosome, so the bacteria cannot make new proteins. Once the bacteria stop growing, your own immune system can mop them up. …