Our work here, for the first time, reported the antibacterial activity of kojic acid-grafted-chitosan oligosaccharides (COS/KA) against three gram-positive and three gram-negative bacteria. Integrity of cell membrane, outer membrane (OM) and inner membrane (IM) permeabilization assay, alkaline phosphatase (ALP) and glucose-6-phosphate dehydrogenase (G6PDH) assay, and SDS-PAGE assay techniques were used to investigate the interactions between COS/KA and bacterial membranes. The antibacterial activity of COS/KA was higher than those of unmodified COS. The electric conductivity of bacteria suspensions increased, followed by increasing of the units of average release for ALP and G6PDH. COS/KA can also rapidly increase the 1-N-phenylanphthylamine (NPN) uptake and the release of beta-galactosidase via increasing the permeability of OM and IM in Escherichia coli. SDS-PAGE indicated the content of cellular soluble proteins decreased significantly in COS/KA-treated bacteria. Hence, COS/KA has potential in food industry and biomedical sciences. (C) 2015, The Society for Biotechnology, Japan. All rights reserved.