The microbial community composition in a full-scale biogas plant fed with sewage sludge and fat, oil and grease (FOG) was investigated over a 15-month period, including two foam formation events. Addition of FOG as a substrate in the biogas plant together with high abundances of Microthrix parvicella were found to promote foam formation in the downstream digester of a cascade of two biogas digesters. Genetic fingerprinting and quantitative PCR (qPCR) indicated a higher abundance of M. parvicella in the digester, when the digestion process was accompanied by excessive foaming relative to the reference digesters without disturbance. The creation of foam depended on the introduced proportion of FOG and the abundance of M. parvicella. Furthermore, shifts in the abundance of M. parvicella in the biogas plant were observed within the 15-month monitoring period corresponding to its seasonal abundance in the sludge of the wastewater treatment plant (WWTP). (C) 2013 Elsevier Ltd. All rights reserved.