IN-SITU DETECTION OF BACTERIA IN CONTINUOUS-FLOW CULTURES OF SEAWATERSEDIMENT SUSPENSIONS WITH FLUORESCENTLY LABELED RIBOSOMAL-RNA-DIRECTED OLIGONUCLEOTIDE PROBES

rRNA-targeted and fluorescently labelled oligonucleotide probes were u sed to study the composition of natural bacterial populations in conti nuous-flow cultures of seawater sediment suspensions. The cultures wer e run as enrichment cultures with increasing dilution rates, and hexad ecane as the sole carbon source. Total cell numbers were analysed by c ounting DAPI (4',6-diamidino-2-phenylindole)-stained cells. To differe ntiate the population composition, oligonucleotide probes for eubacter ia, for Cytophaga/Flavobacteria, and for four subclasses of the Proteo bacteria (alpha, beta, gamma and delta) were used. About 40-80% of the DAPI-stained cells could be detected with the EUB338 probe. Moreover, it was possible to detect a shift in the composition of the natural b acterial population with increasing dilution rate of the continuous cu lture, from large amounts of Cytophaga/Flavobacteria to large numbers of members of the gamma-Proteobacteria. The cell recovery rate for bac teria labelled with specific oligonucleotide probes was analysed with defined cell numbers of Rhodospirillum rubrum, Comamonas testosteroni and Desulfovibrio vulgaris subsp, vulgaris introduced into the seawate r sediment suspension, and was determined to be 13.9-33.5%. The standa rd deviation determined for this method applied to sediment suspension s was +/-8.3%. The results suggest that the application of the in situ hybridization technique allows a good insight into the structure of p opulations growing in sediment suspensions.