Effect of hydroxypropyl-beta-cyclodextrin on the antimicrobial action of preservatives

The interaction between hydroxypropyl-beta-cyclodextrin (HP-beta-CyD) and several preservatives with different chemical structures was investigated in aqueous solution. Complex stability constants of the 1:1 complexes were calculated from differential spectra. Using the serial dilution test the antimicrobial activities of the preservatives and their complexes against Staphylococcus aureus, Pseudomonas aeruginosa, Escherichia coli and Candida albicans were tested and MIC values determined. For highly water-soluble substances like thimerosal and bronopol, low or no inactivation was found; the more lipophilic substances, such as the phenolic compounds, showed strong inactivation when used in combination with HP-beta-CyD. The loss in activity by complex formation correlated with the bound fraction, thus suggesting that the appropriate antimicrobial substance for the preservation of cyclodextrin solutions can be selected according to the results of this study.