Pseudomonas stutzeri isolate rapidly reduced both selenite and selenate ions to elemental selenium at initial concentrations of both anions of up to 48.1 mM. Optimal selenium reduction occurred under aerobic conditions between pH 7.0 and 9.0 and at temperatures of 25 to 35°C. Reduction of both selenite and selenate was unaffected by a number of anions except for sulfite, chromate, and tungstate ions, which inhibited both growth and reduction.

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When SabwzoncETu h d e l b e r g i s grown in 0.1% w/v Pu'a&Os and examined microscopi~dly during growth, two morpholo~ical changes can be seen. Red intracellular granules arc seen in most of the populatiori within 10 to 12 hours, and organisms containing granules elongate without cell d~vision. The intracellular granuI(:s produced by S. keiddblbcr~ in selenite troth have been identified b!. X-ray analysis as amorphous red r;clenium. The intermediate in the conversion of selenite to elemental selcniun~ has been trapped and identified as divalent selenium ion. Growth stndies have shown that selenite toxicity is prinrarilv associated with the lap phase of growth, and also that the divalent incerrnediate is more toxic than the tetravalent precursor.SeIenite (SeOa-) is metabolized by microorganisms in at least two different ways: for example, by the enzymatic utilization of SeOa" in lieu of sulfur, presumably by converting SeOa' to the dimlent selenide (Se-), and forming therefrom such. products as selenornethionine and selenocysteine (1, 7) ; competitive inhibition by these seieno-amino acids could account for the toxicity of SeOa" to many species. In addition some species reduce SeOs-to an insoIuble product which is deposited intracellularly. The intracellular deposition of such precipitates from SeOa-has been shown in Coryzebacte~.izrrn difihtheriae (51, Nertrospora mussa (8), and Candzda albicaas (2).The use of SeOs= as a selective agent in enrichment media for the isolation of Salmonella from natural sources was first recommended by Guth (3) in 1916, although general acceptance was not forthcoming until the publication of the later work of Leifson (4).The mechanism of .%On-reduction by Saiwonella. IzFzdelberg was studied as part of an investigation of biological isotopic enrichrrtent in group 6A elements (S, Se, Te). The tolerance of Salmonellae to SeOl-is shown to involve an active process wherein selenite is converted to an insoluble form (SeO) and the medium thus detoxified.The following abbreviations are used throughout this paper. Se+4, tetravalent selenium (as in selenite); Se+2, divalent selenium; SeO, elemental selenium; Se-2, divalent selenium (as in selenide). Experimental Organisms and Growth Conditions(a) Salmoaellu heidelberg and Esckerdchia coli No. 7438 (both obtained from t h e Alberta Provincial Laboratory of Public Health) were grown in trypticase soy broth (Baltimore BioIogical Co. Ltd.) to which 0.1 % w/v sterile NaaSeOZ in MI15 phosphate buffer (pH 7.2) was added aseptically after the medium was autoclaved and cooled, Ctiltures were incubated at 3 7 ' .