Meta-Biol

• Pathways

Binding and disruption of microbial membranes is widely believed to be the primary mechanism of action for beta-defensins. There is no direct evidence of this, but a growing number of studies support this model (Pazgier et al. 2006). Beta-defensins have antimicrobial properties that correlate with membrane permeabilization effects (Antcheva et al. 2004, Sahl et al. 2005, Yenugu et al. 2004). The sensitivity of microbes to beta-defensins correlates with the lipid composition of the membrane; more negatively-charged lipids correlate with larger beta-defensin 103-induced changes in membrane capacitance (Bohling et al. 2006). Beta-defensin-103 was observed to give rise to ionic currents in Xenopus membranes (Garcia et al. 2001) and cell wall perforation was observed in S. aureus when treated with HBD-3 (Harder et al. 2001). Two models explain how membrane disruption takes place. The 'pore model' postulates that beta-defenisns form transmembrane pores in a similar manner to alpha-defensins, while the 'carpet model' suggests that beta-defensins act as detergents, causing a less organised disruption. Beta-defensins have a structure that is topologically distinct from that of alpha-defensins, suggesting a different mode of dimerization and an electrostatic charge-based mechanism of membrane permeabilization rather than a mechanism based on formation of bilayer-spanning pores (Hoover et al. 2000).