The antibacterial activities of chitosan and its water-soluble derivatives on E. coli were studied according to four influencing factors in vitro. The antibacterial study showed that chitosan, O-hydroxyethyl chitosan (O-HECS), and O-carboxymethyl chitosan (C-CMCS) could inhibit the growth of the microbial. To study the antibacterial mechanism, plasmid DNA pBR322 and pUC18 were selected to be the probes to find out the binding abilities of chitosans. Results showed that raw chitosan had a high binding ability with the plasmids and the influencing degrees were stable. The effects of chitosan derivatives on plasmids might be affected by space effect and static effect. With appropriate concentrations and molecular weights, the derivatives might have strong abilities to combine with DNA. The degree of influence of chitosan and its derivatives on plasmids had nothing to do with time. The experiment focusing on the relationship between chitosans and mRNA showed that O-CMCS would hinder the synthesis of mRNA, and this may give some proof to its antibacterial mechanism. (c) 2006 Wiley Periodicals, Inc.