Oxidative stress–mediated formation of protein hydroperoxides can induce irreversible fragmentation of the peptide backbone and accumulation of cross-linked protein aggregates, leading to cellular toxicity, dysfunction, and death. However, how bacteria protect themselves from damages caused by protein hydroperoxidisation is unknown. Here we show that YjbI, a group II truncated haemoglobin from Bacillus subtilis, prevents oxidative aggregation of cell-surface proteins by its biologically

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... ented protein hydroperoxide peroxidase-like activity, which removes hydroperoxide groups from oxidised proteins. Disruption of the yjbI gene in B. subtilis lowered biofilm water repellence and the mechanical stiffness of the cell surface, which associated with the cross-linked aggregation of the biofilm matrix protein TasA. YjbI was localised to the cell surface, and the sensitivity of planktonically grown cells to generators of reactive oxygen species was significantly increased upon yjbI disruption, suggesting that YjbI pleiotropically protects labile cell-surface proteins from oxidative damage. YjbI removed hydroperoxide residues from a model oxidised protein substrate, bovine serum albumin, and prevented its oxidative aggregation in vitro. These findings provide new insights into the role of truncated haemoglobin and the importance of hydroperoxide removal from proteins in the survival of aerobic bacteria.