Effect of thiol-functionalisation on chitosan antibacterial activity: Interaction with a bacterial membrane model

The antibacterial activity of chitosan modified with the thiol-containing 2-iminothiolane HCl (TC-IMI) and N-acetyl-l-cysteine (TC-NAC) was studied by Langmuir film balance technique using a dipalmitoylphosphatidylglycerol (DPPG) monolayer bacterial membrane model. The interactions of the biopolymer with the membrane model were assessed by monitoring the differences in the shape of the compression isotherms recorded in the absence and presence of chitosan and thiolated conjugates in the subphase. A low molecular weight chitosan (15 kDa) shifted the compression isotherms of DPPG monolayers towards larger areas (A0,CS = 145 Å2), confirming its membrane disturbance capacity. Further thiolation induced higher yield of expansion, more pronounced in the case of TC-IMI. The expansion of the monolayer increased significantly (A0,TC-NAC = 150 Å2 vs A0,TC-IMI = 175 Å2) and the elasticity at a surface pressure of 30 mN/m, typical for bio-membranes decreased to a greater extent Cs,30TC-NAC-1 = 120 mN/m vs Cs,30TC-IMI-1 = 87 mN/m) in presence of TC-IMI. Antibacterial tests against a Gram-negative Escherichia coli and a Gram-positive Staphylococcus aureus were in good agreement with these findings, suggesting that chitosan thiolated with 2-iminothiolane HCl acts as a bactericide disrupting the integrity of the bacterial cell membrane.