Unicellular microorganisms, called obligate anaerobes, strictly use anaerobic respiration for energy production. Common examples of obligate anaerobes are some species of bacteria, such as Clostridium tetani, Clostridium botulinum, Clostridium perfringens, Clostridium sporogenes and Clostridium difficile.
Anaerobic respiration is the process of producing the high-energy molecule adenosine triphosphate, or ATP, in the absence of oxygen. In terms of tolerance to oxygen, microbes are classified into five: obligate anaerobes, obligate aerobes, microaerophiles, aerotolerant and facultative.
Obligate anaerobes are unable to survive in the presence of oxygen due to the lack of catalase and superoxide dismutase, which are enzymes that catalyze the breakdown of harmful substances. These organisms utilize the anaerobic respiration pathways to create ATP for various cellular functions. Compared to aerobic respiration, which is primarily driven by oxygen, anaerobic pathways are considerably less productive.