In another study, Chen et al. [21] reported the antibacterial effects of 69% deacetylated shrimp chitosan, 0.63% sulphonated chitosan (SC1), 13.03% sulphonated chitosan (SC2) and sulphobenzoyl chitosan on oyster preservation. They observed that, except in the case of B.cereus, bacterial growth was effectively inhibited by at least one of the above four compounds tested at 200 ppm. Even though the sulphonation increased the solubility of chitosan, totally different antibacterial capabilities were observed for SC1 and SC2. For most of the bacterial cultures SC1 had a very pronounced minimal inhibitory concentration (MIC) effect even at 200 ppm level, with SC2 exhibiting no antibacterial effect at concentrations below 2000 ppm. Chen et al. [21] sug-
gested that since SC2 has more sulphonyl groups, it carries a higher negative charge than SC1, thus there would be a greater repulsive force between negatively charged SC2 molecules and bacterial cell walls (Table 2).