Bacteria ( <i>E. coli</i> ) take up ultrasmall gold nanoparticles (2 nm) as shown by different optical microscopic techniques (CLSM, SIM, STORM)

The uptake of fluorescently labeled ultrasmall gold nanoparticles (2 nm) by Gram-negative Escherichia coli bacteria occurs within 1–3 hours. This was demonstrated confocal laser scanning microscopy (CLSM), structured illumination (SIM), stochastic optical reconstruction (STORM), and flow cytometry. For imaging, eGFP-expressing DsRed2-expressing E. strains were used in addition to non-fluorescing strains. Gold with fluoresceine (FITC), Cy3, AF647, respectively. Importantly, showed no toxicity the bacteria, indicating a non-lethal nature uptake, that is, not related cell injury.