In situ environmental cell-transmission electron microscopy study of microbial reduction of chromium(VI) using electron energy loss spectroscopy

Reduction of Cr(VI) by the bacterium, Shewanella oneidensis (previously cla ssified Shawanella putrefaciens strain MR-1), was studied by absorption spe ctrophotometry and in situ, environmental cell-transmission electron micros copy (EC-TEM) coupled with electron energy loss spectroscopy (EELS). Bacter ia from rinsed cultures were placed directly in the environmental cell of t he transmission electron microscope and examined under 100 Torr pressure. B right field EC-TEM images show two distinct populations of S. oneidensis in incubated cultures containing Cr(VI)O-4(2-): those that exhibit low image contrast and heavily precipitate-encrusted cells exhibiting high image cont rast. Several EELS techniques were applied to determine the oxidation state of Cr associated with. encrusted cells. The encrusted cells are shown to c ontain a reduced form of Cr in oxidation state +3 or lower. These results d emonstrate the capability to determine the chemistry and valence state of r eduction products associated with unfixed, hydrated bacteria in an environm ental cell transmission electron microscope.