text: Since facultative anaerobes are able to grow in both the presence and absence of oxygen, they can survive in many different environments, adapt easily to changing conditions, and thus have a selective advantage over other bacteria. As a result, most life-threatening pathogens are facultative anaerobes.
  The ability of facultative anaerobic pathogens to survive without oxygen is important since their infection is shown to reduce oxygen levels in their host's gut tissue. Moreover, the ability of facultative anaerobes to limit oxygen levels at infection sites is beneficial to them and other bacteria, as dioxygen can form reactive oxygen species (ROS). These species are toxic to bacteria and can damage their DNA, among other constituents.