Which characteristic defines bacteria?

Bacteria are fundamentally defined as single-celled microorganisms. This characteristic is key to their classification in the biological sciences. They belong to a group of organisms that are prokaryotic, meaning that they do not have a nucleus or other membrane-bound organelles. Most bacteria reproduce asexually through a process called binary fission and are capable of thriving in a variety of environments, ranging from extreme heat to cold, and even in extreme acidity or alkalinity.

The statement about bacteria being multicellular is incorrect, as bacteria are typically unicellular, which is one of their defining traits. While some bacteria can form clusters or chains, they remain single-celled organisms. The notion that bacteria can only live as parasites is misleading; many bacteria are free-living and play essential roles in ecosystems, such as decomposing organic matter or aiding in the digestive processes of animals. Furthermore, the idea that all bacteria are harmful to humans is an oversimplification. While some bacteria do cause diseases, many are beneficial and necessary for health, such as those found in the human gut microbiome that aid in digestion and help protect against harmful pathogens.