How are bacteria characterized?

Bacteria are characterized as single-celled organisms, typically microscopic in size. This definition reflects their basic biological structure; bacteria consist of a single cell that performs all necessary functions of life, such as metabolism, reproduction, and response to the environment. Their microscopic size often means they cannot be seen with the naked eye and require a microscope for observation. This information is crucial for understanding their role in various environmental contexts, including water systems where they can impact water quality and treatment processes.

The other characterizations do not accurately describe bacteria. For instance, bacteria are not large multicellular organisms, as that description applies to more complex life forms like plants or animals. The ability to see an organism without a microscope is also not true for bacteria, given their size. While some bacteria can indeed be beneficial for water quality, this characteristic doesn’t encompass all bacteria and doesn't define them; thus, it is not suitable as a primary classification.