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The feasibility of using probes directed towards ribosomal DNAs (rDNAs) as a quantitative approach to estimating cell numbers was examined and applied to study the structure of a bacterial community in humic acid-rich salt marsh sediments. Hybridizations were performed with membrane-bound nucleic acids by using seven group-specific DNA oligonucleotide probes complementary to 16S rRNA coding regions. These included a general eubacterial probe and probes encompassing most members of the gram-negative, mesophilic sulfate-reducing bacteria (SRB). DNA was extracted from sediment samples, and contaminating materials were removed by a series of steps. Efficiency of DNA extraction was 48% based on the recovery of tritiated plasmid DNA added to samples prior to extraction. Reproducibility of the extraction procedure was demonstrated by hybridizations to replicate samples. Numbers of target cells in samples were estimated by comparing the amount of hybridization to extracted DNA obtained with each probe to that obtained with a standard curve of genomic DNA for reference strains included on the same membrane. In June, numbers of SRB detected with an SRB-specific probe ranged from 6.0 x 10(7) to 2.5 x 10(9) (average, 1.1 x 10(9) +/- 5.2 x 10(8)) cells g of sediment-1. In September, numbers of SRB detected ranged from 5.4 x 10(8) to 7.3 x 10(9) (average, 2.5 x 10(9) +/- 1.5 x 10(9)) cells g of sediment-1. The capability of using rDNA probes to estimate cell numbers by hybridization to DNA extracted from complex matrices permits initiation of detailed studies on community composition and changes in communities based on cell numbers in formerly intractable environments.

Estimation of Bacterial Cell Numbers in Humic Acid-Rich Salt Marsh Sediments with Probes Directed to 16S Ribosomal DNA