Sulfur assimilation peculiarities

We (Vorobjeva and Charakhchjan, 1983) showed that propionic acid bacteria can utilize any source of sulfur, fi om the most oxidized (sulfate) to the most reduced (sulfide) (Table 4.3). Good growth was observed in the presence of 6 mM sulfide, a concentration ten times higher than the purple nonsulfur bacterium Rhodopseudomonas globiformis can tolerate. 

Good growth of P. shermanii was supported by thiosulfate and elemental sulfur. In the latter case the culture released sulfide. The reduction of sulfur to H2S can proceed by enzymatic and nonenzymatic pathways, for example, via the formation of sulfide as in E. coli (Okada et al., 1982). The ability of P, shermanii to utilize thiosulfate, sulfite, elemental sulfur and sulfide shows that these compounds serve as intermediates of the assimilatory sulfate reduction in propionic acid bacteria. 

An unusual pattern of sulfate utilization was observed in growing cultures of P. shermanii. Uptake of S04 by the cells had an oscillatory pattern, alternating between the periods of sulfate utilization and release (Fig. 4.1). The accumulation of S04 in the medium was maximal in the exponential phase of growth (24-28 h). 

It was also found that the direction of sulfate fluxes in or out of the cell depended on its intracellular pool. When sulfate-depleted cells were transferred to a new medium, they actively accumulated sulfate first, but then started to release it (Fig. 4.2). Cells grown first in the medium high in sulfate, when transferred to a fresh medium, started to release sulfate at once. Interestingly, such an oscillatory pattern of sulfate utilization was not a common property of all propionic acid bacteria. For instance, P. petersonii steadily took up sulfate from the medium, while the two closely related strains, P. shermanii and P. freudenreichii, showed an oscillatory pattern of sulfate consumption, like the distantly related E. coli (Fig. 4.3). 

In resting cells of P. shermanii a sharp increase in radioactivity ( S) was observed in the first 2 min of incubation, followed by a decrease due to the release of sulfate into the medium, and a new increase in radioactivity 6 min 

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