Antibacterial activity and mechanism of bifidocin A against Listeria monocytogenes

Abstract

Bifidocin A, produced by Bifidobacterium animalis BB04, is a novel bacteriocin with antimicrobial activity against a wide range of gram-positive and gram-negative foodborne bacteria. The objective of this study was to investigate the antibacterial activity and mechanism of action of bifidocin A against Listeria monocytogenes, one of the most susceptible bacteria to this bacteriocin. The minimum inhibitory concentration (MIC) of bifidocin A for L. monocytogenes 35152 was 0.029 mg/mL. Time-kill assays showed that bifidocin A effectively inhibited the growth of L. monocytogenes in a time-and concentration-dependent manner. The mechanism of action of bifidocin A was studied by analyzing its effects at a MIC on the cell morphology, intracellular organization, membrane permeability, membrane integrity, and membrane proton motive force (PMF) of L. monocytogenes. Scanning and transmission electron microscopy analyses showed that bifidocin A induced alterations in the morphology and intracellular organization of L. monocytogenes cells. Confocal laser scanning microscopy images showed that L. monocytogenes cells treated with bifidocin A took up propidium iodide. Bifidocin A treatment also induced the leakage of K+ and inorganic phosphate, the hydrolysis and release of ATP, and a collapse of the transmembrane electrical potential and pH gradient in L. monocytogenes cells. These results suggested that bifidocin A exerted its anti-Listeria monocytogenes effect through the dissipation of the cytoplasmic membrane PMF, increased membrane permeability, cell membrane pore formation, destruction of membrane integrity, and ultimately complete disintegration of the cells.