One of the greatest challenges in contemporary society is to reduce and treat household solid waste. The choice of inoculum to be used for start-up in reactors that degrade organic waste is critical to the success of organic waste treatment. In this study, the functional diversity, phylogenetic identification, and biogas production of bacterial communities from six inoculum sources were investigated. We used BIOLOG EcoPlates to evaluate the metabolic abilities of the bacterial communities, followed 16S rRNA gene sequence analysis to determine the phylogenetic affiliation of the bacteria responsible for carbon consumption. We observed great diversity in the physiological profiles. Of the six inocula tested, the sludge from an upflow anaerobic sludge blanket reactor (SRU) contained the most diverse, metabolically versatile microbiota and was characterized by the highest level of biogas production. By contrast, the sludge of the anaerobic lagoon (SAL) showed the worst performance in BIOLOG EcoPlates assays, but it exhibited the most diversity and generated the second largest amount of biogas. The bacterial isolates retrieved from BIOLOG EcoPlates were characterized as aerobic and/or facultative anaerobic, and were mainly Gram-negative. Phylogenetic analysis revealed that the isolates belonged to three major phyla: Proteobacteria, Firmicutes and Actinobacteria, represented by 33 genera. Proteobacteria exhibited the most diversity. The distribution of the bacterial genera differed considerably among the six inocula. Pseudomonas and Bacillus, which are able to degrade a wide range of proteins and carbohydrates, predominated in five of the six inocula. Analysis of the bacterial communities in this study indicates that both SRU and SAL microbiota are candidates for start-up inocula in anaerobic reactors. These start-up inocula must be studied further in order to identify their practical applications in degrading organic waste.