UC Irvine

Antibiotic resistance is a global epidemic, becoming increasingly pressing due to its rapid spread. Thus, there is a critical need to develop new therapeutic approaches. In addition to searching for new antibiotics, gaining additional insight into existing natural host defense mechanisms may enable researchers to improve existing defenses, and to develop effective, synthetic drugs guided by natural principles. Histone proteins were originally proposed to function as antimicrobial agents, and were later recognized for their role in condensing mammalian chromosomes. More recently, the antimicrobial activity of histones has been reported in innate immune responses, including neutrophil extracellular traps and lipid droplets. However, histones exhibit weak antimicrobial activity in vitro. Whether and how histones kill bacteria in vivo has remained elusive. This weak antimicrobial activity in vitro may be attributed to the fact that histone activity has not been considered in the context of other immune mechanisms. The co-localization of histones with antimicrobial peptides (AMPs) in immune cells suggests that histones function as part of a larger antimicrobial mechanism in vivo. I have discovered that the mammalian histone H2A is taken up into the Gram-negative Escherichia coli and the Gram-positive bacteria Staphylococcus aureus through membrane pores that are formed by the human AMP LL-37 or the Xenopus laevis AMP magainin-2. Without this crucial first step, whereby AMPs form pores and enable histone entry into the bacteria, mammalian histones have little antimicrobial activity. H2A stabilizes the AMP-induced membrane pores; this depolarizes the bacterial membrane potential. Inside, H2A reorganizes bacterial chromosomal DNA, and inhibits global transcription. I show that while bacteria can recover from the pore-forming effects of AMPs alone, the effects of H2A are irrecoverable. Together, these results suggest that AMPs and histones, which are found ubiquitously in the innate immunity system, synergize as a natural host defense mechanism to kill bacteria. The membrane-permeabilizing activity of AMPs and the DNA-perturbing and proton gradient-altering effects of H2A constitute a positive feedback loop that exponentially amplifies their antimicrobial activities, thus creating a condition of antimicrobial synergy that could be incorporated into antimicrobial design.