3.5 Mechanism of bactericidal action
In support of the above discussions, the plausible role of AgNP bacteria interactions and its antibacterial effect was further
analyzed using FEG-TEM and SEM analysis (Fig. 7). Here, only E. coli MTCC 443 cells treated with AgNPs (10 nm) were observed using FEG-TEM and SEM analysis. While untreated cells (Fig. 7a) appeared normal with their characteristic shape, cells treated with AgNPs (Fig. 7b) exhibited appreciable shrinkage and irregular shape. The TEM micrograph showed that AgNPs were present on the cell membrane and they appeared to be attached to the lipopolysaccharide layer present in the cell wall of Gram-negative, E. coli bacteria, as also reported in previous studies. A significant number of AgNPs were also found inside the bacterial cell and this internalization of AgNPs was further confirmed in STEM (scanning tunnelling electron microscope) mode (ESI, Fig. S8†). The presence of silver inside the bacterial cell was also analyzed using EDX (Fig. 7c) and the characteristic Ag peak (2.9 kV) was observed even afyer post-fixation procedures. Using FEG-SEM images, it was also found that the silver nanoparticles were invariably present all over the bacterial surface (Fig. 7d). Thus, AgNPs were present deep inside as well as on the bacterial surface.