Bactericidal activity of mammalian histones is caused by large membrane pore formation.

Histones have an important role in eukaryotic innate immunity, wherein histones co-localize with antimicrobial peptides (AMPs). The mechanism of histone cooperation with AMPs and the extent to which histones form pores both remain a mystery. Here, we show that histones form large pores in bacterial membranes that lack lipopolysaccharide (LPS) and that their antimicrobial effect is significantly stronger than that of the clinical AMP polymyxin B. We find that histones and AMPs together produce potent antimicrobial synergy through the formation of 26 nm pores, whereby the pore-forming activity of AMPs on LPS-containing membranes enables histones to enter the periplasmic space and subsequently attack unprotected membranes to create pores. We provide a mechanistic explanation for the long-standing observations of histone antimicrobial activity and demonstrate how antimicrobial synergy arises. The ubiquity of histones and AMPs in innate immunity has significant implications for organismal defense and can be leveraged for novel antibiotic strategies.