ABSTRACT
Microbial Groups and Their Interactions. The hydrolytic and fermentative bacteria comprise a rather diverse group of facultative and obligately anaerobic Bacteria. Although facultative bacteria were originally thought to be dominant, evidence now indicates that the opposite is true,33 at least in sewage sludge digesters where the numbers of obligate anaerobes have been found to be over 100 times greater. This does not mean that facultative bacteria are unimportant, because their relative numbers can increase when the influent contains large numbers of them,29 or when the bioreactor is subjected to shock loads of easily fermentable substrates.38 Nevertheless, it does appear that most important hydrolytic and fermentative reactions are performed by strict anaerobes, such as Bacteroides, Clostridia, and Bifidobacteria,66 although the nature of the substrate will determine the species present.
