Characterization of bacteriocin produced by Enterococcus faecium CN- 25 isolated from traditionally Thai fermented fish roe Noojaree Sonsa-Ard a , Sureelak Rodtong b , Michael L. Chikindas c , Jirawat Yongsawatdigul a, * a School of Food Technology, Institute of Agricultural Technology, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand b School of Microbiology, Institute of Science, Suranaree University of Technology, Nakhon Ratchasima 30000, Thailand c Rutgers, The State University of New Jersey, New Brunswick, NJ, USA article info Article history: Received 30 October 2014 Received in revised form 3 February 2015 Accepted 10 February 2015 Available online 18 February 2015 Keywords: Bacteriocin Lactic acid bacteria Enterocin Enterococcus faecium Listeria monocytogenes abstract Enterococcus faecium CN-25 was isolated from a traditionally Thai fermented fish roe, som-kai-pla, and was found to possess enterocin A and B genes. Among the 30 microorganisms tested, the produced bacteriocin exhibited antagonistic activity against Listeria monocytogenes TISTR 1327. The bacterium maximally produced bacteriocin at a concentration of 1828 AU/ml at the beginning of the stationary phase (at 18 h) in the inexpensive rice bran medium. The antilisterial activity of the compound was not affected by heating at 60e100  C for up to 30 min. High antilisterial activity remained at pH 2. Partially purified CN-25 bacteriocin derived from E. faecium CN-25 completely inhibited the growth of L. monocytogenes at the minimum concentration of 2.38 mg/ml. The compound had no effect on the efflux of ATP from L. monocytogenes but triggered the intracellular hydrolysis of ATP, as evidenced by a decrease in the luminescence of total ATP. The bacteriocin also depleted the transmembrane electrical potential (DJ) but had no effect on the transmembrane pH gradient (DpH). Therefore, the partially purified CN-25 peptide may be useful for improving the product safety of fermented fish products. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction Lactic acid bacteria (LAB) produce a great variety of bacteriocins, including nisin, enterocins, pediocins, sakacins, and lacticins (Cleveland, Montville, Nes, & Chikindas, 2001). At present, nisin produced from Lactococcus lactis is the only bacteriocin approved for use as a food preservative. Fermented food has been identified as one of important sources of bacteriocin-producing strains. L. lactis WNC 20 producing nisin Z was isolated from Thai fer- mented pork sausage, Nham (Noonpakdee, Santivarangkna, Jumriangrit, Sonomoto, & Panyim, 2003). Weissella hellenica D1501 was isolated from Chinese Dong fermented meat and found to produce weissellicin D (Chen et al., 2014). In addition, Lactoba- cillus sakei LSJ618 producing sakacin was isolated from traditional Chinese fermented radish (Jiang et al., 2012). L. lactis WNC 20 iso- lated from fermented pork sausage produced a bacteriocin that inhibited not only closely related lactic acid bacteria (LAB) but also some food-borne pathogens, including Listeria monocytogenes, Clostridium perfringens, Bacillus cereus and Staphylococcus aureus (Noonpakdee et al., 2003). Fermented fish roe known in Thai as som-kai-pla is a tradi- tionally fermented product prevalent in Northeast Thailand. Roe of freshwater fish is mixed with 0.3% sugar, 2% salt and roasted rice. The fermentation takes place at room temperature (30  C) for 48 h. LAB are predominant microorganisms in the product. LAB isolated from som-kai-pla and their metabolites, particularly bacteriocin, have not been systematically exploited. Most bacteriocins isolated from LAB share a common mode of action. They typically dissipate proton motive force (PMF) of the target cells by forming transmembrane channels permeable to various ions (Abee, Krockel, & Hill, 1995). However, the mecha- nisms through which they achieve this appear to differ among bacteriocins. Understanding the mechanism of a bacteriocin of in- terest would be important for its application. The objectives of this study were to identify the most potent strain of bacteriocin- producing LAB isolated from som-kai-pla and to characterize the bacteriocin produced by the strain. The bactericidal mechanism of the bacteriocin was also elucidated. * Corresponding author. Tel.: þ660 4422 4297; fax: þ660 4422 4633. E-mail address: jirawat@sut.ac.th (J. Yongsawatdigul). Contents lists available at ScienceDirect Food Control journal homepage: www.elsevier.com/locate/foodcont http://dx.doi.org/10.1016/j.foodcont.2015.02.010 0956-7135/© 2015 Elsevier Ltd. All rights reserved. Food Control 54 (2015) 308e316