The prepared Ag -ZrNT and unmodified ZrNT substrates were tested for their antimicrobial properties as proof of concept towards common pathogens that often cause biomaterial-associated infections, namely E. coli and S. aureus, which serve as model for Gram-negative and Gram-positive bacteria, respectively, to demonstrate the broad spectrum effectiveness of the modified biomaterial . Figure and Figure S 3 illustrate that Ag -ZrNT exhibited high antimicrobial activity against both S. aureus and E. coli. Against S. aureus bactericidal activity, which is defined as the killing of more than 99.9 % bacteria of the initial inoculum, meaning log reduction factor (LRF) >3 was achieved, while for E. coli an LRF>2.1 was reached, meaning less than 0.270% viable bacteria of the starting inoculum could be detected. Unmodified Zr-NT exhibited antimicrobial activity to the same extent (LRF>2.6) against E. coli but not towards S. aureus, which hints at a clear difference in the antimicrobial efficiency and presumably mode of action of Zr-NT against Gram-positive compared to Gram-negative bacteria. Figure S 4 revealed that the nanotube formation plays an essential role in the antimicrobial effect of both Zr-NT and Ag -ZrNT as the Zr-foil and the Zr-Ag alloy foil do not exhibit any antimicrobial effect as proven by control antimicrobial tests with just the corresponding foil. One has to consider that for S. aureus the incubation with PBS caused 1 LRF (Figure and ). Therefore, the antimicrobial effect caused by the tested material when comparing the log 10 CFU/ml with the control condition that was incubated for 24 h in PBS was about 1 LRF (S. aureus) or 0.5 LRF (E. coli) lower for all treatment conditions than the LRF calculated relative to the starting inoculum (0 h). These results could be all validated through independent replication of the antibacterial tests using different substrate batches and bacterial cultures (Figure ). Slight variations of antimicrobial efficiency, indicated by the LRF, were observed that could be caused by small differences in the substrates and the anodization process or the starting bacterial inoculum.