Which characteristic is true of all bacteria?

• A. They lack mitochondria in their cytoplasm.
• B. They have a well-defined nucleus.
• C. They can only live inside a host's cells.
• D. They do not require oxygen to survive.

Answer: (A)

Bacteria are prokaryotes, which means they do not have membrane-bound organelles. Mitochondria, the energy-producing organelles found in many eukaryotic cells, are not present in bacteria. Instead, bacteria generate energy across their cell membrane and in the cytoplasm using their own enzymes. That makes it true that they lack mitochondria.

Other statements don’t fit because bacteria do not have a true, well-defined nucleus; their DNA is organized in a nucleoid region rather than a membrane-bound nucleus. Bacteria are also not restricted to living inside host cells; many are free-living in soil, water, and on surfaces. And oxygen needs vary widely among bacteria—some require oxygen, some cannot tolerate it, and others can grow with or without it.