Evaluation of viable counting procedures for the enumeration of sulfate-reducing bacteria in estuarine sediments

The recovery of known viable populations of sulfate-reducing bacteria belonging to the genera Desulfovibrio, Desulfobacter and Desulfobulbus from inoculated, sterile, anoxic estuarine sediments has been determined using Postgate's and Widdel's media. Recovery of Desulfovibrio populations was consistently higher with Postgate's medium, whilst Widdel's medium always yielded higher viable counts of Desulfobacter and Desulfobulbus. In all cases the average recovery after 5 days incubation at 4 °C was only about 50(percent)of the added viable cells. Two surface active agents, cetyl trimethylammonium bromide (CTAB, final concentration 0.00001070 w/v)and sodium tripolyphosphate (NAPE final concentration 0.0005°70 w/v), both increased recoveries of viable cells, with CTAB showing the greatest effect (16070 increase). The addition of CTAB to untreated sediment samples also significantly increased the viable counts of in situ populations of sulfate-reducing bacteria, although the effect was more variable (0-200070 increase). Viable counts of different types of sulfate-reducing bacteria from estuarine sediments, even using the most appropriate growth medium and in the presence of CTAB, grossly underestimate the in situ populations of these bacteria when compared with total direct counts and rates of sulfate reduction measured within the same sediment. The significance of viable counts of sulfate-reducing bacteria within sediments should therefore be interpreted with caution.