Chromatin-derived histone proteins from Schmidtea mediterranea function as innate antimicrobial effectors

Planarians display remarkable resistance to bacterial infection, including infection by Staphylococcus aureus, yet the molecular mechanisms underlying this antibacterial capacity remain poorly defined. Histones are highly conserved chromatin-associated proteins traditionally known for their structural role in nucleosome organization; however, studies in diverse organisms have revealed that histones can function as antimicrobial effectors when released extracellularly. Whether histone proteins contribute to antibacterial defense in planarians has not been investigated. Here, we identified core histone proteins (H1/H5, H2A, H2B, H3, and H4) from the transcriptome of Schmidtea mediterranea through comparative sequence analysis using reference histones from Homo sapiens, Drosophila melanogaster, and Caenorhabditis elegans. Domain architecture and phylogenetic analyses confirmed their evolutionary conservation. Core histones were isolated from planarian nuclei by acid extraction and validated by SDS-PAGE and immunoblotting. Functional assays revealed that histone-enriched extracts isolated from planarian nuclei exhibit selective and dose-dependent antibacterial activity against Gram-positive bacteria, with the strongest inhibition observed for S. aureus, whereas minimal effects were detected against Gram-negative species such as Escherichia coli. Histone treatment induced membrane permeability and bacterial death, as demonstrated by SYTO9/propidium iodide staining and confocal microscopy. Importantly, protease treatment and heat denaturation abolished antibacterial activity, confirming that the effect depends on intact protein structure. Collectively, our findings demonstrate that histone-enriched chromatin extracts from S. mediterranea possess intrinsic antibacterial activity with preferential efficacy against Gram-positive bacteria, supporting a model in which chromatin-derived histone proteins represent conserved components of innate immune defense in planarians.