Studies on the pathogenesis and survival of different culture forms of Listeria monocytogenes to pulsed UV-light irradiation after exposure to mild-food processing stresses Derek Bradley a , Brian McNeil b , John G. Laffey c , Neil J. Rowan a, * a Department of Nursing and Health Science, Athlone Institute of Technology, Dublin Road, Athlone, Co. Westmeath, Ireland b Strathclyde Institute of Biomedical and Pharmaceutical Sciences, Strathclyde University, Glasgow, Scotland, UK c Department of Anaesthesia and Intensive Care Medicine, School of Medicine, National University of Ireland Galway, Ireland article info Article history: Received 23 August 2011 Received in revised form 27 December 2011 Accepted 28 December 2011 Available online 3 January 2012 Keywords: Listeria monocytogenes Morphotypes Food-related stresses Invasiveness Pulsed UV light abstract The effects of mild conventional food-processing conditions on Listeria monocytogenes survival to pulsed UV (PUV) irradiation and virulence-associated characteristics were investigated. Specifically, this study describes the inability of 10 strains representative of 3 different culture forms or morphotypes of L. monocytogenes to adapt to normally lethal levels of PUV-irradiation after exposure to sub-lethal concentrations of salt (7.5% (w/v) NaCl for 1 h), acid (pH 5.5 for 1 h), heating (48  C for 1 h) or PUV (UV dose 0.08 mJ/cm 2 ). Findings showed that the order of increasing sensitivity of L. monocytogenes of non-adapted and stressed morphotypes to low pH (pH 3.5 for 5 h, adjusted with lactic), high salt (17.5% w/v NaCl for 5 h), heating (60  C for 1 h) and PUV-irradiation (100 pulses at 7.2 J and 12.8 J, equivalent to UV doses of 2.7 and 8.4 mJ/cm 2 respectively) was typical wild-type smooth (S/WT), atypical filamentous rough (FR) and atypical multiple-cell-chain (MCR) variants. Exposure of L. monocytogenes cells to sub- lethal acid, salt or heating conditions resulted in similar or increased susceptibility to PUV treatments. Only prior exposure to mild heat stressing significantly enhanced invasion of Caco-2 cells, whereas subjection of L. monocytogenes cells to combined sub-lethal salt, acid and heating conditions produced the greatest reduction in invasiveness. Implications of these findings are discussed. This constitutes the first study to show that pre-exposure to mild conventional food-processing stresses enhances sensitivity of different culture morphotypes of L. monocytogenes to PUV, which is growing in popularity as an alternative or complementary approach for decontamination in the food environment. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction The opportunistic pathogen Listeria monocytogenes causes liste- riosis, a serious infection with high hosptialization and mortality rates, with consumption of contaminated food principle mode of its transmission to humans (Tompkin, 2002). Increased ready-to-eat (RTE) food consumption makes L. monocytogenes a serious threat since the pathogen is able to survive a wide range of environmental stress conditions, such as low temperature, acidic pH, and high osmolarity (Sue et al., 2004). According to the European Centre for Disease Control and Prevention, listeriosis was the fifth most common zoonotic infection in Europe in 2006 (EFSA-EDSC, 2007), while it is estimated to account for approximately 28% of the deaths resulting from food-borne illnesses in the UK (cited in Chorianopoulus et al., 2011). Because L. monocytogenes is ubiquitous it may be introduced into food-processing plants through many different routes. L. monocytogenes has been shown to colonize pro- cessing environments and to contaminate products during pro- cessing. Certain strains may become persistent in a plant and cause continuous contamination (Lünden et al., 2008; Van Boeijen et al., 2008; Van Boeijen et al., 2010). Although the origin of persistence can be different, overall persisters can have specific qualities, such as acid and heat tolerance and adherence to surfaces, contributing to the establishment of house strains. A number of studies have shown persistence of L. monocytogenes in various food-processing plants (Lünden et al., 2008). Some of these persistent strains dominated and persisted in a plant or production line for years and caused food contamination and human disease. Adaptation to hostile environmental conditions has been demonstrated to alter cellular physiology of a number of food-borne persister pathogens such that they become resistant to further extreme food-processing stresses (Hill et al., 2002; Chorianopoulus * Corresponding author. E-mail address: nrowan@ait.ie (N.J. Rowan). Contents lists available at SciVerse ScienceDirect Food Microbiology journal homepage: www.elsevier.com/locate/fm 0740-0020/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.fm.2011.12.024 Food Microbiology 30 (2012) 330e339