Which description best defines bacteria?

• A. Unicellular organisms that cause disease by invading host tissues or by producing toxins.
• B. Intracellular parasites that require living host cells to replicate.
• C. Non-living infectious particles.
• D. Multicellular organisms with tissues.

Answer: (A)

Distinguishing bacteria from other infectious agents and understanding what they are like at a basic level is being tested. Bacteria are unicellular, prokaryotic organisms that can cause disease by invading host tissues or by producing toxins. This description captures two key facts: they are single-celled organisms, and they can harm hosts either by physical invasion or by releasing harmful chemicals (toxins).

The other descriptions don’t fit bacteria as well. Intracellular parasites that require living host cells to replicate describe viruses, which are not living cells in the same sense and depend on host cells to reproduce. Non-living infectious particles also describe viruses rather than bacteria. Multicellular organisms with tissues describe complex organisms like worms or certain fungi, not bacteria, which are single-celled.