Ultrastructural evidences for chromium(III) immobilization by Escherichia coli K-12 depending on metal concentration and exposure time

Microorganisms can mediate in heavy metal sequestration through several cellular strategies and pathways. This offers an efficient way to remediate heavy metal polluted environments. This paper describes the ability of Escherichia coli K-12 to capture chromium(III) (Cr(III)) and the ultrastructural effects of this metal on cells, as well as the cellular metal localization and the possible sequestration strategy uses for it. The study was mainly performed by using several electron microscopy techniques and is based on the chromium trivalent concentration and the related exposure time. Transmission electron microscopy (TEM) assay was performed along with field emission scanning electron microscopy (FESEM) for morphological responses. Furthermore, TEM was coupled with an energy dispersive X-ray (TEM-EDX) and TEM with selected area electron diffraction (TEM-SAED) to conduct analytical assays. The exposed cultures to 10 and 12 mM Cr(III) at 12 h and to 5, 7, 10, 12, 13, and 15 mM of Cr(III) at 24 h indicated the presence of multiple electrodense granules that were significantly enriched in chromium and phosphorus content via EDX analysis. Moreover, these granules were observed to be attached to external membrane and/or surrounding cells in the respective ultrathin sections analyzed under TEM. According to these results, E. coli K-12 possesses the ability to immobilize Cr(III) in external polyphosphate granules through a strategy of accumulation, where cell response to Cr(III) toxicity seems to have a dose-dependent and time-dependent relation, thereby offering significant potential for bioremediation in Cr(III)-contaminated areas.