Microbiology of the 'G-bacteria' in activated sludge

This review discusses a group of bacteria, the 'G-bacteria', which have a d istinctive morphology of cocci in tetrads, sheets or clusters, that are see n in large numbers in many activated sludge biomass samples. Isolates of 'G -bacteria' that have been grown axenically are phylogenetically diverse, Th e Gram-negative members include several alpha- and beta -proteobacteria, am ong which is the genus Amaricoccus, while the Gram-positive 'G-bacteria' co ntain several members of the actinobacteria, It is probable that other, as yet uncharacterized, 'G-bacteria' exist in activated sludge. The hypothesis that these 'G-bacteria' are detrimental to the process of enhanced biologi cal phosphate removal by competing for substrates anaerobically with the ph osphate-accumulating bacteria in such systems, based as it is largely on mi xed-culture studies, receives little support from studies using those avail able in pure culture. The evidence on which these conclusions are founded i s discussed, as are the arguments used to explain why these 'G-bacteria' al l appear to thrive under conditions found in certain activated sludge syste ms.