Abstract
Application of molecular methods to natural ecosystems has revealed the occurrence of many unexpected phylogenetic groups of prokarytoes which have not yet been isolated as cultivable strains. This is true for biological waste water treatment systems. Plate bacterial counts accounted for 1 to 18% of total (DAPI) counts in municipal sewage activated sludge. Microscopic, molecular genetic, and chemical biomarker approaches have suggested that the failure to detect many bacteria as viable counts results from the occurrence of viable but non-culturable bacteria and from the existence and diversity of bacteria undescribed so far.
