Anaerobic, methane producing bacteria
Methanogens generate biogas, when act on cellulose rich biowaste (anaerobically).
Detailed Answer :
Micro-organisms that produce methane along with CO2 and H2 under anaerobic conditions are called methanogens. Example : Methanobacterium. These are the bacteria found in cattle dung (gobar) and in anaerobic sludge during sewage treatment. They grow anaerobically on cellulosic material and produce a large amount of methane (main constituent of biogas) along with CO2 and H2. Thus, methanogens are used in biogas production.