Which type of bacteria cannot exist in the presence of free oxygen?

The type of bacteria that cannot exist in the presence of free oxygen is obligate anaerobes. These organisms thrive in environments devoid of oxygen and are often harmed or killed by its presence. Obligate anaerobes rely on anaerobic respiration or fermentation for their energy production, processes that do not involve oxygen.

In contrast, facultative anaerobes can grow in both the presence and absence of oxygen, adapting their metabolic pathways according to the environment. Microaerophiles require a low level of oxygen to survive but cannot tolerate atmospheric levels, while aerobes depend on oxygen for their metabolic processes and thrive in its presence. This clear distinction highlights the unique physiology of obligate anaerobes, emphasizing their inability to survive when exposed to oxygen, which is toxic to them.