There isn't a single type of bacteria that can turn biomass straight into methane. The process of AD occurs in several steps and requires a community of micro-organisms:
Hydrolysis
Large, complex polymers like carbohydrates, cellulose, proteins and fats are broken down by hydrolytic enzymes into simple sugars, amino acids and fatty acids.
Acidogenesis
Simple monomers are broken down into volatile fatty acids
Acetogenesis
The products of acidogenesis are broken down into acetic acid, releasing hydrogen and carbon dioxide.
Methanogenesis
Bacteria called methane formers produce methane either by cleaving two acetic acid molecules to form carbon dioxide and methane, or by reduction of carbon hydroxide with hydrogen.