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The occurrence and metabolic capacities of sulfate-reducing bacteria (SRB) were studied in 23 water samples taken from producing wells at 14 different sites. Oil fields in France, the North Sea, and the Gulf of Guinea were selected and classified according to physicochemical parameters (salinity ranging from 0.3 to 120 g.L-1 and temperature between 29 and 85 degrees C). After the distribution of SRB within oil fields was studied, several strains of SRB were isolated and characterized metabolically. Twenty of the thirty-seven strains were not related to any known species. Most of the identified strains were members of the genera Desulfovibrio and Desulfotomaculum by molecular, morphological, and physiological properties.

Two moderately halophilic, mesophilic, sulfate-reducing bacteria were isolated from production-water samples from Emeraude Oilfield, Congo. Motile, vibrioid cells of SRL4225T grew optimally at a concentration of 4 % NaCl, at pH 5·8–6·2, with a minimal pH for growth of 5·2, showing that it is a moderately acidophilic bacterium. Cells of SRL6146T were motile, curved or vibrioid, long and thin rods. Optimal growth was obtained at a concentration of 5–6 % NaCl, at pH 6·8–7·2. The nutritional requirements showed that many of the characteristics of these strains overlap with those of known Desulfovibrio species. On the basis of 16S rRNA gene sequence analysis and DNA–DNA hybridization studies, both strains are members of the genus Desulfovibrio. However, they are not closely related to any species of the genus that have validly published names. It is therefore proposed that the two strains are members of two novel species of the genus Desulfovibrio with the names Desulfovibrio bastinii sp. nov. (type strain SRL4225T=DSM 16055T=ATCC BAA-903T) and Desulfovibrio gracilis sp. nov. (type strain SRL6146T=DSM 16080T=ATCC BAA-904T).

Two moderately halophilic sulfate-reducing bacteria were isolated from an African oil pipeline and designated strains SEBR 3640 and SEBR 2840T (T = type strain). Both of these strains possessed traits that define the genus Desulfovibrio. The cells of both isolates were motile curved rods that had a single polar flagellum and contained desulfoviridin, and both isolates utilized lactate, pyruvate, malate, fumarate, succinate, and ethanol in the presence of sulfate. Sulfite, thiosulfate, and elemental sulfur were also used as electron acceptors in the presence of lactate. However, both strains tolerated higher concentrations of NaCl (up to 17%) than all other Desulfovibrio species except Desulfovibrio halophilus, which tolerated a similar level of NaCl. The results of a 16s rRNA gene sequence analysis also placed the designated type strain, strain SEBR 2840, in the genus Desulfovibrio but revealed that this organism was significantly different from D. halophilus and all other validly described Desulfovibrio species. On the basis of our results, we propose that strain SEBR 2840T is a member of a new species of the genus Desulfovibrio, Desulfovibrio gabonensis. The type strain of D. gabonensis is strain SEBR 2840 (= DSM 10636).The presence of sulfate-reducing bacteria (SRB) in oil field waters has been recognized for a long time, and the activities of these microorganisms are closely linked to the history of oil. In 1926, Bastin et al. (3) studied the SRB communities in oil fields. After this pioneering work, several microbiological studies showed that SRB communities were widely distributed in most of the oil fields investigated (1, 5, 19).In the course of a survey of different oil fields, we isolated several strains of SRB. Two strains, designated strains SEBR %40T (T = type strain) and SEBR 3640, were characterized further; these strains were isolated from water samples obtained from two oil pipelines transporting the oil produced at an offshore oil field to onshore facilities on the coast of Gabon (West Africa). Both strains were moderately halophilic, curved, rod-shaped organisms and on the basis of their morphological and physiological traits were assigned to the genus Desulfo-.vibrio. The results of a 16s rRNA gene sequence analysis of strain SEBR 2840T indicated that it was not related to any previously described Desulfovibrio species, and we propose that this strain should be assigned to a new species, Desulfovibrio gabonensis. Strain SEBR 2840 is the type strain of this species. MATERZALS AND METHODSSources of the isolates. Strain SEBR 2840T was isolated in February 1990 from a water sample taken from an oil pipeline linking offshore production platforms to onshore treatment facilities.Strain SEBR 3640 was isolated 1 year later from another pipeline in the same offshore oil field.Sampling and primary cultures of SRB were grown as described previously (4, 18). The total salinity of the culture media and the incubation temperature were