Figure 9.

Hypothesis of SeNPs formation in Bacillus mycoides SeITE01. A - Synoptic schematization of proposed biogenesis mechanisms of zero-valent selenium nanoparticles in Bacillus mycoides SeITE01. (1) Cytosolic precipitation of SeO₃²⁻ as Se⁰ nanoparticles due either to the possible activity of LMW thiols including bacillithiol (BSH) or to the Trx/TrxRed system. (2) Intracellular selenite reduction and formation of SeNPs as a consequence of presumable activity of membrane reductases. (3) Release of intracellularly generated SeNPs after cell lysis. (4) Membrane reductases may even catalyze extracellular selenite precipitation. (5) Peptides and other compounds carrying thiol groups may be released from the bacterial cell and directly react with selenite. (6) Evidence of the formation of SeNPs by Bacillus mycoides SeITE01 culture supernatant spiked with SeO₃²⁻ only after NADH addition can be due to the presence of extracellular proteins capable of mediating selenite precipitation once provided with reducing equivalents. (7) Nascent SeNPs are inherently unstable due to their high surface area and therefore tend to grow and increase their average size to attain a lower-energy state by means of an Ostwald ripening mechanism. B - Suggested mechanism of selenite detoxification in Bacillus sp. involving Brx-like proteins, according to [55].