A novel antibacterial strategy: histone and antimicrobial peptide synergy

The rate at which antibiotics are discovered and developed has stagnated; meanwhile, antibacterial resistance continually increases and leads to a plethora of untreatable and deadly infections worldwide. Therefore, there is a critical need to develop new antimicrobial strategies to combat this alarming reality. One approach is to understand natural antimicrobial defense mechanisms that higher-level organisms employ in order to kill bacteria, potentially leading to novel antibiotic therapeutic approaches. Mammalian histones have long been reported to have antibiotic activity, with the first observation of their antibacterial properties reported in 1942. However, there have been doubts about whether histones could truly have any such role in the animal, predominantly based on two issues: they are found in the nucleus (so are not in a position to encounter bacteria), and their antibiotic activity in vitro has been relatively weak in physiological conditions. More recent studies have addressed both sets of concerns. Histones are released from cells as part of neutrophil extracellular traps (NETs) and are thus able to encounter extracellular bacteria. Histones are also present intracellularly in the cytoplasm attached to lipid droplets, positioning them to encounter cytosolic bacteria. Our recent work (Doolin et al., 2020, Nat Commun), which is discussed here, shows that histones have synergistic antimicrobial activities when they are paired with antimicrobial peptides (AMPs), which form pores in bacterial membranes and co-localize with histones in NETs. The work demonstrates that histones enhance AMP-mediated pores, impair bacterial membrane recovery, depolarize the bacterial proton gradient, and enter the bacterial cytoplasm, where they restructure the chromosome and inhibit transcription. Here, we examine potential mechanisms that are responsible for these outcomes.