What characterizes anaerobic bacteria?

Anaerobic bacteria are characterized by their ability to thrive in environments devoid of oxygen. These microorganisms utilize fermentation or anaerobic respiration to generate energy, allowing them to flourish in various habitats such as deep soils, sediments, and the gastrointestinal tract of animals, where oxygen levels are minimal or nonexistent. This capability is crucial for their survival and metabolism, as oxygen can be toxic to many anaerobic species.

In contrast, other options don't accurately describe anaerobic bacteria. Some require oxygen for growth, while others do not specifically relate to anaerobic conditions. The reference to saltwater is not pertinent, as many anaerobic bacteria can live in environments other than saline ones, and being spore-forming is not a requirement for anaerobic characteristics, as some anaerobes do not form spores at all. Consequently, the defining trait of anaerobic bacteria is indeed their adaptability to oxygen-free environments.