What is the key characteristic of anaerobic bacteria?

The key characteristic of anaerobic bacteria is that they grow only in the absence of oxygen. These bacteria thrive in environments where oxygen is not present, which is essential for their survival and replication. Instead of utilizing oxygen for energy production, anaerobic bacteria use alternative metabolic pathways that do not rely on oxygen, allowing them to colonize environments such as deep soils, underwater sediments, and the gastrointestinal tracts of animals.

The other options relate to different characteristics of bacteria in general or specific types of bacteria but do not accurately define anaerobic bacteria. For instance, while some bacteria may tolerate high temperatures or extreme cold, these traits are not exclusive to anaerobic bacteria and can apply to various organisms, including thermophilic or psychrophilic bacteria. Additionally, while moisture levels can impact bacterial growth, anaerobic bacteria specifically do not require limited moisture to thrive, since their primary limitation is the absence of oxygen, not moisture levels.