Obligate anaerobes are organisms that cannot survive in the presence of oxygen. Some anaerobes are capable of surviving both in the presence and in the absence of oxygen, but the term obligate implies that the metabolism of the organism is such that survival in the presence of oxygen is not possible under any circumstances.
Therefore, for the bacteria to survive there should be no oxygen present. For cellular respiration obligate anaerobes can use substances like sulfate, nitrate, iron, manganese, mercury, and carbon monoxide. Some places where obligate anaerobes are found are near vents that release compounds of sulfur and in deep water where there is no oxygen.