Size and pH effects of chitooligomers on antibacterial activity against Staphylococcus aureus

The antimicrobial activity of chitooligomers (COSs) has attracted considerable interest but there are few reports on antibacterial activity of each individual COS. This study focuses on investigating the size and pH effects of COS on its antibacterial activity against Staphylococcus aureus. Minimum inhibitory concentrations (MICs) of five single COSs and five COS fractions with narrow degrees of polymerization (DP) were determined at different pH mediums. The results revealed that the antibacterial activity of COS required structural essential with a DP of at least 5 and the inhibitory effect increased with increasing DP. Lower pH value could enhance the antibacterial activity of COS. The COS with DP > 12 showed a MIC value of 62.5 mg/mL at pH 6.0, while the MIC value increased to 500 and 1000 mg/mL at pH 6.5 and 7.0, respectively. The cell membrane integrity assay and SEM suggested that the COS with high DP could cause bacteria clustering and further lysis of cell.