Antimicrobial peptides (AMPs) have the ability to penetrate the cell membrane, form pores which eventually lead to cell death. Immobilization of AMP on nanoparticles can play a major role in antimicrobial materials, biosensors for pathogen detection and in food safety. The minimum inhibitory concentration (MIC) of free Cecropin P1 (CP1, sequence SWLSTAKKLENSAKKRLSEGIAIAIQGGPR) and adsorbed on silica nanoparticle against E. coli O157:H7 EDL933 were 0.78μg/ml. This was found to be consistent with preservation of α-helical secondary structure of CP1 upon adsorption as indicated by circular dichroism (CD). Cysteine-terminus modified Cecropin P1 (CP1C, sequence SWLSTAKKLENSAKKRLSEGIAIAIQGGPRC) was chemically immobilized onto silica nanoparticles with maleimide-PEG-NHS ester cross-linkers of different PEG chain lengths. The antimicrobial activity of CP1C in solution and adsorbed on silica nanoparticles against E. coli O157:H7 EDL933 were found to be the same as those for CP1. However, tethered CP1C exhibited much higher MIC of 24.38, 37.55 and 109.82μg/ml for (PEG)20, (PEG)6 and (PEG)2 linkers respectively. The antimicrobial activity of CP1C tethered to silica nanoparticles with (PEG)20 linker was found to be lower for lower surface coverage with MIC values being 86.06, 36.89, 24.38 and 17.84μg/ml for surface coverage of 12.3%, 24.4%, 52.8% and 83.8% respectively. All atom MD simulation of 1:3 DOPG/DOPC mixed membrane interacting with free and PEGlyated CP1C indicated that presence of PEG linker prevented CP1C from interacting with the bilayer which may explain the loss of antimicrobial activity of tethered CP1C.