2.1. Chemicals and bacterial cultures
AgNO3, HCl, NaOH, and sodium alginate were purchased from Sigma Aldrich, USA, dihydroethidium (DHE) from Yeasen Biotechnology, China and both PBS buffer solution (135 mM NaCl, 4.7 mM KCl, 10 mM Na2HPO4, 2 mM NaH2PO4, pH=7.3±0.1) and polydimethylsiloxane (PDMS) were from Beyotime Biotechnology, China. All these reagents were analytical-grade and used as received. All solutions were prepared with Milli-Q ultrapure water (18 MΩ·cm, Thermo Fisher).
Deinococcus radiodurans Strain R1 (ATCC 13939) was grown from a single bacterium aerobically (under atmospheric conditions) at 30°C in peptone yeast glucose (PYG) broth (containing 2% peptone, 0.5% glucose, 1% yeast extract, pH 7.2) with agitation at 300 rpm. The pH of the broth was adjusted to 7 by adding hydrochloric acid or sodium hydroxide. The bacterial growth was assessed by measuring optical density (OD) via a microplate reader (see below Section 2.2.3) at the wavelength of 600 nm of the strain. The OD value stayed stable since reaching 1.0, and the D.r. strain at this stage was considered to have a stable population and morphology, and therefore selected to be used throughout the experiment. For obtaining the bacteria containing silver particles, the grown culture above was cultivated in a fresh PYG broth initially containing 2.0 mM AgNO3 with a pH of 7 and at 30 ℃ for another 9 hours. The bacteria after incubation in AgNO3 solutions were centrifuged at 8000xg to collect the microbes and washed with PBS for further analysis.