Abstract: Chitosans with different molecular weights (3, 300－400 kD and 700 kD) were labeled with fluorescein isothiocyanate (FITC-CT) and tested for their antibacterial activity in vitro. Besides, confocal laser scanning microscope was used to study the antibacterial action sites of fluorescently labeled chitosans in E.coli. The results indicated that all 3, 300－400 kD and 700 kD fluorescently labeled chitosans had antibacterial activity, although their inhibitory rates were decreased by 4.22%, 19.56% and 14.54%, respectively compared to their corresponding non-labeled counterparts. The antibacterial action site of 3 kD chitosan, which could enter bacterial cells, may be a given substance in bacterial cells, while 300－400 kD and 700 kD chitosans could not enter bacterial cells so that their antibacterial action site was presumably the surface of bacterial cell membrane.

Key words: chitosan, action sites, E.coli