V. Oxygen - Bacteria vary in their oxygen requirement and tolerance. Bacteria which are like us, require the presence of oxygen gas to respire are called aerobes . We could call them obligate aerobes since oxygen is not optional. Microaerophiles are organisms than require a little oxygen but not as high an oxygen tension as found in the atmosphere. Being able to live in the presence of oxygen has to do with a cells ability to detoxify it.
Oxygen is unstable.
Superoxide is poison to all cells unless the cell has enzymes to change it into something else. The pathways most bacteria use to detoxify oxygen is a 2 step process. If they lack this set of enzymes, they are called obligate anaerobes and they only live where oxygen is not. Deep in the sea, earth or deep in the human body in concert with aerobes that use up what little oxygen is available. Some can and some can't.