376
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
B. BIOCHEMISTRY OF SULFUR OXIDATION BY MICROORGANISMS
1. Sulfide
The elementary sulfur which accumulates in cells of Beggiatoaceae
and Thiorhodaceae or extracellularly in cultures of Chlorobacteriaceae
obviously is an intermediate in the oxidation of sulfide by these organisms. A transitory accumulation of sulfur is also found in the intracellular vacuoles of T. thiooxidans and in cultures of T. thioparus on sulfidecontaining media. In thiobacilli, however, elementary sulfur may perhaps
not be an intermediate in sulfide oxidation but may be produced in a
secondary reaction. In fact, the formation of sulfur from sulfide is
variable and Vishniac and Santer (38) found, moreover, that when cells
of T. thioparus are incubated in the presence of radioactive sulfide, the
label is found after a minute in polythionate and thiosulfate, without
evidence of the formation of elementary sulfur.
2. Sulfur
T. thiooxidans grows rapidly on media containing a suspension of
powdered sulfur as the sole energy source. That other species of thiobacilli, particularly T. thioparus and T. denitrificans, also utilize elementary sulfur is demonstrated by the fact that the often quite abundant
precipitates of sulfur present in cultures during the early period of incubation have disappeared at the end of the growth phase and are thus
oxidized secondarily. Several hypotheses have been advanced to explain
how bacteria can metabolize an exogenous substrate as insoluble as
elementary sulfur. Vogler and Umbreit (9S) showed that the oxidation
of S° by T. thiooxidans requires direct contact of the bacterial cells with
sulfur. They postulated that sulfur enters the cells by being dissolved
in a lipide globule attached to one end of the bacterium. However, electron micrographs of T. thiooxidans (99) failed to confirm the existence
of such fat globules. According to another hypothesis, sulfur is solubilized
by an exocellular reduction process or by contact with the bacterial wall.
Starkey (100) does not consider an extracellular reduction of sulfur as
likely but, in contrast, Vishniac and Santer (38) think it possible that
sulfur is reduced by glutathione or another sulfhydryl compound and is
then transported across the cell wall as free or combined sulfide.
Whatever its mechanism of entry into the cell, it is certain that sulfur is then oxidized successively to thiosulfate, polythionates, and Sulfate. These intermediates were demonstrated by Guittoneau and Keilling
(101) in soils experimentally enriched with elementary sulfur. According
to Vishniac and Santer (38), thiosulfate is formed by the condensation
CLAUDE FROMAGEOT AND JACQUES C. SENEZ
B. BIOCHEMISTRY OF SULFUR OXIDATION BY MICROORGANISMS
1. Sulfide
The elementary sulfur which accumulates in cells of Beggiatoaceae
and Thiorhodaceae or extracellularly in cultures of Chlorobacteriaceae
obviously is an intermediate in the oxidation of sulfide by these organisms. A transitory accumulation of sulfur is also found in the intracellular vacuoles of T. thiooxidans and in cultures of T. thioparus on sulfidecontaining media. In thiobacilli, however, elementary sulfur may perhaps
not be an intermediate in sulfide oxidation but may be produced in a
secondary reaction. In fact, the formation of sulfur from sulfide is
variable and Vishniac and Santer (38) found, moreover, that when cells
of T. thioparus are incubated in the presence of radioactive sulfide, the
label is found after a minute in polythionate and thiosulfate, without
evidence of the formation of elementary sulfur.
2. Sulfur
T. thiooxidans grows rapidly on media containing a suspension of
powdered sulfur as the sole energy source. That other species of thiobacilli, particularly T. thioparus and T. denitrificans, also utilize elementary sulfur is demonstrated by the fact that the often quite abundant
precipitates of sulfur present in cultures during the early period of incubation have disappeared at the end of the growth phase and are thus
oxidized secondarily. Several hypotheses have been advanced to explain
how bacteria can metabolize an exogenous substrate as insoluble as
elementary sulfur. Vogler and Umbreit (9S) showed that the oxidation
of S° by T. thiooxidans requires direct contact of the bacterial cells with
sulfur. They postulated that sulfur enters the cells by being dissolved
in a lipide globule attached to one end of the bacterium. However, electron micrographs of T. thiooxidans (99) failed to confirm the existence
of such fat globules. According to another hypothesis, sulfur is solubilized
by an exocellular reduction process or by contact with the bacterial wall.
Starkey (100) does not consider an extracellular reduction of sulfur as
likely but, in contrast, Vishniac and Santer (38) think it possible that
sulfur is reduced by glutathione or another sulfhydryl compound and is
then transported across the cell wall as free or combined sulfide.
Whatever its mechanism of entry into the cell, it is certain that sulfur is then oxidized successively to thiosulfate, polythionates, and Sulfate. These intermediates were demonstrated by Guittoneau and Keilling
(101) in soils experimentally enriched with elementary sulfur. According
to Vishniac and Santer (38), thiosulfate is formed by the condensation