Bacteria in the bottom sediment of a drinking water reservoir

In addition to the total number of bacteria (direct count) and enzymatic ac tivity (fluorescein diacetate hydrolysis), the proportions between the phyl ogenetic groups of bacteria have been examined in samples from the bottom s ediment of the Saidenbach drinking water reservoir. The chemical properties of the interstitial water likewise have been investigated. Microbial biofi lms grown on glass slides vertically exposed in the sediment were accessibl e to hybridization with rRNA-directed probes and subsequent analysis by mea ns of the fluorescence microscope. This "in situ cultivation" method yielde d better results than the direct examination of bacteria from sediment samp les. The alpha- and the beta Proteobacteria in all of the samples were the most abundant. Members of the Cytophaga-Flavobacterium group, sulfate reduc ers and Archaebacteria have occasionally been found also at high densities. Gram-positive bacteria with a high G+C content, gamma-Proteobacteria and N itrosomonas were present with low cell numbers. The depth down to which a c omparatively dense colonization by bacteria became visible did in no case e xceed 20 mm. The conclusion may be drawn that only bacteria which were a) a ctive and b) able to grow, to migrate, and to colonize glass surfaces could be detected in the sediment biofilms. probably this depended on the substr ate supply from above (deposited phytoplankton cells).