The characteristic that best indicates that bacteria are living organisms is their ability to carry out metabolism. Metabolism is the set of chemical processes that occur within an organism to maintain life. Bacteria are able to take in nutrients and convert them into energy and waste products through a series of chemical reactions, which is the essence of metabolism.
Other characteristics of living organisms, such as growth, reproduction, response to stimuli, and homeostasis, are also exhibited by bacteria. For example, bacteria can grow and divide in size, reproduce by binary fission, respond to changes in their environment, and maintain a stable internal environment through various mechanisms.
However, some non-living entities, such as viruses, can exhibit some of these characteristics, but they are not considered living organisms because they cannot carry out metabolism on their own. They require a host cell to reproduce and cannot generate energy or carry out other metabolic processes without hijacking the cellular machinery of a host cell.