Accordingly, the process of Se(IV) reduction appears to be an NADPH- or NADH-dependent pathway and indicates two possible pathways. One possibility is that Se(IV) did not enter the cytoplasm of strain S44 or only trace levels

of Se(IV) were present in the cytoplasm. The Se(IV)-reducing determinant might have initially been assembled SB203580 mouse in the cytoplasm and then transferred across cytoplasmic and outer membrane. The Se(IV)-reducing determinant would then be only active outside of cells in vivo [21]. Another possibility, and more likely at that, is that Se(IV) was reduced to Se(0) in the cytoplasm and then Se(0) was pumped out of the cells where small SeNPs aggregated into bigger particles. In many cases, the big

and smooth-surface nanoparticles occurred outside of cells [20,21,32]. Here, a large number of SeNPs ranging from 100–200 nm were observed by SEM (Figure 1) and further confirmed by EDX (Figure 3A). In our experiment it was obvious that small selenium particles aggregated into bigger particles as observed by TEM (Figure 3 and Additional file 1: Figure S1). This was different from previous TEM images of a homogeneous density of SeNPs [20,21,32]. In addition, this was not impacted by sample preparation because other strains produced big and homogeneous nanoparticles outside of cells using the same sample preparation and TEM observation technique (Data not shown). Previous

studies confirmed small particles having low negative charges to have a propensity to come together and form aggregates [12]. R788 datasheet In addition, proteins and/or other biomolecules such as polysaccharides and fatty acid may play a key role in controlling selenium nanoparticle size Tyrosine-protein kinase BLK and the morphology of the resultant SeNPs [30]. The bulk of the Se(VI) and Se(IV) reduction to Se(0) was reported to occur on or outside the envelope [21]. This is very different from the reported mechanism where selenium was bound to the assembling protein SefA and then formed nanoparticles which were exported from cells [35]. In most reported cases, Se(VI) reduction occurred under anaerobic condition [36-38]. C. testosteroni S44 has a weak ability to reduce Se(VI) into red-colored selenium under aerobic condition (Figure 5B). The Se(VI) reductase complex was identified as a periplasmic Mo-containing enzyme in T. selenatis [38,39] and B. selenatarsenatis [40]. The Se(VI)-reducing determinant of C. testosteroni S44 also is most likely a Mo-enzyme because tungstate inhibited Se(VI) reduction (Figure 5B). In contrast, the Se(IV)-reducing determinant did not appear to contain Mo because tungstate did not inhibit Se(IV) reduction. Accordingly, Se(VI) reduction is a distinct activity different from Se(IV) reduction. Iron-sulfur (Fe-S) clusters are cofactors for many proteins across all three domains of life.