A Syntrophic Propionate-Oxidizing, Sulfate-Reducing Bacterium from a Fluidized Bed Reactor

A mixed-culture HP1 was enriched with propionate/sulfate from a methanogenic fluidized bed reactor fed with propionate as the sole organic carbon source in a medium low in sulfate. The mixed-culture contained strain HP 1.1 as the only propionate-oxidizer, which was Gram-negative, lemon- or peanut-shaped, 0.95-1.15 x 1.4-2.4 µm in size, occurred singly or in pairs, and did not form endospores. Strain HP1.1 oxidized propionate to acetate and carbon dioxide with concomitant reduction of sulfate. Scanning electron microscopy revealed that strain HP1.1 was morphologically distinct from Syntrophobacter wolinii and Desulfobulbus propionicus. We separated strain HP1.1 from other strains by Percoll gradient centrifugation, and from these cells the total sequence of 16S rDNA from the PCR-amplified 16S rRNAgene from DNA was obtained. The phylogenetic analysis revealed that strain HP1.1 was less closely related to Syntrophobacter wolinii than to two recently described, propionate-oxidizing strains MPOB and KoPropl within the 8-subclass of the Proteobacteria (Ddrner, 1992; Harmsen et al., 1995), while a more distant relationship was found to Desulfobulbus propionicus.