Title; Lethal Hydroxyl Radical Accumulation by a Lactococcal Bacteriocin, Lacticin Q Running Title; a New Antimicrobial Mechanism of Lacticin Q 3 4 Introduction

23 The antimicrobial mechanism of a lactococcal bacteriocin, lacticin Q, can be 24 described by the toroidal pore model without any receptor. However, lacticin Q 25 showed different degrees of activity (selective antimicrobial activity) against 26 gram-positive bacteria even among related species. The ability of lacticin Q to induce 27 pore formation in liposomes composed of lipids from different indicator strains 28 indicated that its selective antimicrobial activity could not be attributed only to 29 membrane lipid composition. We investigated the accumulation of deleterious 30 hydroxyl radicals after exposure to lacticin Q as a contributing factor to cell death in 31 the indicator strains. When lacticin Q of the same concentration as the minimum 32 inhibition concentration or minimum bactericidal concentration was added to the 33 indicator cultures, high levels of hydroxyl radical accumulation were detected. 34 Treatment with hydroxyl radical scavengers, thiourea and 2,2′-bipyridyl, decreased 35 the levels of hydroxyl radical accumulation and recovered cell viability. These results 36 suggest that, with or without pore formation, the final antimicrobial mechanism of 37 lacticin Q is the accumulation of hydroxyl radicals, which varies by strain, resulting in 38 the selective antimicrobial activity of lacticin Q.