Zoogloea ramigera has long been considered the typical activated sludge bacterium responsible for the formation of activated sludge flocs. On the basis of the results of a comparative sequence analysis, we designed three oligonucleotide probes complementary to characteristic regions of the 16S rRNAs of Z. ramigera ATCC 19544T (T = type strain) and two misclassified strains, Z. ramigera ATCC 25935 and ATCC 19623. Dissociation temperatures were determined, and probe specificities, as well as the potential of probes for whole-cell hybridization, were evaluated by using numerous reference organisms. Several activated sludge samples were examined with these probes by using both the in situ and dot blot hybridization methods. Only the type strain probe hybridized to cells that accumulated in the typical branched gelatinous matrices, the so-called Zoogloea fingers. This probe revealed cells in most of the activated sludge samples studied. We found that relatively high levels of Z. ramigera cells (up to approximately 10% of the total number of cells) and typical morphology tended to be linked to overloading of sewage plants. The probe directed to rejected type strain Z. ramigera ATCC 19623 bound to only a few cells. Cells that reacted with the probe complementary to Z. ramigera ATCC 25935, which was originally isolated from a trickling filter, were not observed in activated sludge.
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