Interactions between dendrimer biocides and bacterial membranes

Dendrimer biocides have been shown to be more potent than their small molecule counterparts. In this study, several techniques were utilized to investigate the interactions between quaternary ammonium functionalized poly(propylene imine) dendrimers and bacterial membranes. Both Gram-positive and Gram-negative bacteria were tested. The techniques employed include UV–Vis spectroscopy, differential scanning calorimetry, and bioluminescence. When treated with dendrimer biocides, release of 260 nm adsorbing materials from Escherichia coli strains quickly increased and reached a plateau afterwards, while release of 260 nm absorbing materials from Staphylococcus aureus increased monotonically with the concentration due to the difference in cell structures. The different release behavior also correlates with the antimicrobial properties against these two types of bacteria. Bioluminescence experiments using bacteria containing stress-responsive bioluminescent reporter gene fusions provided information suggesting that damage to the cell membranes is the primary mechanism of the antimicrobial action for dendrimer biocides. High concentrations of calcium ions can limit the efficacy of the dendrimer biocides, although the tested concentration range is much higher than most practical applications. Differential scanning calorimetry studies showed at high concentrations that dendrimer biocides formed precipitates with phospholipid vesicles, suggesting a strong interaction with this model of bacterial membrane. These results provide insight about the antibacterial action of dendrimer biocides and establish the basis for their mode of action.