Bacteria are defined as:

• A. Multicellular organisms
• B. Viruses
• C. Algae
• D. Single-celled or noncellular spherical or spiral or rod-shaped organisms lacking chlorophyll that reproduce by fission

Answer: (D)

Bacteria are single-celled, prokaryotic organisms that have cellular structures but no nucleus or membrane-bound organelles. They come in common shapes—spherical (cocci), rod-shaped (bacilli), or spiral. Most bacteria do not contain chlorophyll, so they don’t perform photosynthesis like plants do. Instead, they reproduce by binary fission, where one cell splits into two.

This combination of being unicellular prokaryotes, having these shapes, lacking chlorophyll, and reproducing by fission is what defines bacteria. In wastewater contexts, they’re the workhorses that break down organic matter, distinguishing them from viruses (noncellular and requiring a host to replicate) and algae (usually photosynthetic and eukaryotic).