Response of Desulfovibrio desulfuricans colonies to oxygen stress
Oxygen tolerance of the strictly anaerobic sulfate-reducing bacteria is well documented and poorly understood. This capacity for surviving brief exposures to oxygen must be a major factor in the diversity of environmental niches observed for these bacteria. We observed that viable cells of Desulfovibrio desulfuricans (ATCC 27774) could be found in colonies on the surface of solidified medium exposed to air for periods as long as 1 month. During exposure to air, the originally black colonies became greyish white, presumably as a result of the air oxidation of the metal sulfide deposits. A black, brittle deposit formed at the bottom of the colony and, simultaneously, the colony descended into a dimple that developed into a well in the agar. Eventually the colony reached the bottom of the Petri dish. These changes did not take place when the colonies were maintained in an anaerobic chamber. The morphological changes took place with all strains tested: three strains of D. desulfuricans and one strain of Desulfovibrio gigas and Desulfovibrio multispirans. Continued sulfate reduction appeared to be essential. Cyclic sulfate (thiosulfate or sulfite) reduction to sulfide and reoxidation of sulfide by the oxygen in air are proposed to maintain the viability of the bacteria by providing substrates for energy production and by reducing oxygen tension. Scanning and transmission electron microscopy of colony and cellular changes are shown. Key words: Desulfovibrio, sulfate-reducing bacteria, oxygen tolerance, sulfate cycling, scanning electron microscopy.