Short communication Acid-induced injury renders Salmonella Enteritidis PT4 sensitive to the antimicrobial action of Filipendula ulmaria plant extract Ioannis S. Boziaris, 1 * Charalampos Proestos, 2 Maria Kapsokefalou 3 & Michael Komaitis 3 1 Department of Agriculture, Ichthyology and Aquatic Environment, School of Agricultural Sciences, University of Thessaly, Fitokou street, 38446 N. Ionia, Volos, Greece 2 Laboratory of Food Chemistry, Department of Chemistry, University of Athens, Zografou, 157 84 Athens, Greece 3 Laboratory of Food Chemistry, Department of Food Science and Technology, Agricultural University of Athens, Iera Odos 75, 118 55 Athens, Greece (Received 4 November 2011; Accepted in revised form 25 February 2012) Keywords Filipendula ulmaria, lactic, plant extract, Salmonella, sublethal injury. Introduction Plant antimicrobials consist mainly of phenolic com- pounds and exhibit antimicrobial activity (Nychas et al., 2003). Filipendula ulmaria methanol ⁄ water extract con- tains a number of phenolic compounds that exhibits antimicrobial action against selected food-borne bacte- ria (Rauha et al., 2000; Proestos et al., 2008). The plant, commonly known as meadowsweet, belongs to Rosa- ceae family. The stems are about 1–2 m tall, with dark green leaves on the upper side and whitish and downy underneath. The flowers are creamy-white clustered close together in handsome irregularly branched cymes, having a very strong, sweet smell. Gram-negative bacteria, such as salmonellae, are surrounded by the outer membrane (OM). OM serves as a permeability barrier to the penetration of a variety of antimicrobial compounds making cells resistant to those compounds (Vaara, 1992; Mann et al., 2000). The antimicrobial action of Filipendula ulmaria extract against Salmonella Enteritidis PT4, is reduced compared to Gram-positive pathogens such as Listeria monocytogenes (Proestos et al., 2008; Boziaris et al., 2011). However, organic acids can permeabilise OM and sensitise Gram negatives (Alakomi et al., 2000). The aim of this work was to study the possibility of increasing antimicrobial activity of Filipendula ulmaria plant extract against Salmonella Enteritidis PT4, an important Gram-negative enteric pathogen, by inflict- ing acid-induced injury on bacterial cell using lactic acid. Materials and methods Preparation of plant extract Samples of meadowsweet, were collected from mount Immitos in Attica, Greece, at 350 m altitude in May 2005. Samples (leaves and flowers) were dried in the air (at 25 °C in the dark) and used within 3 months of collection. The extraction method, the identification and quantification of phenolic compounds of Filipendula ulmaria have been reported in a previous work (Proestos et al., 2008). Microorganism Salmonella Enteritidis PT4 was provided by culture collection of Laboratory of Microbiology and Biotechnology of Foods of Agricultural University of Athens. The cultures were resuscitated twice in Tryp- tone Soy Broth supplemented with 0.5% (w ⁄ v) yeast extract (TSBYE) after incubation at 37 °C for 20– 24 h. The cells were harvested by centrifugation (1500 g for 15 min at 20 °C); the pellets were washed in sterile 0.85% (w ⁄ v) NaCl and resuspended in TSBYE. Effect of acidic pH, lactic acid and PMBN Minimum inhibitory concentration of plant extract against Salmonella Enteritidis PT4 in TSBYE was determined in a previous work and found to be 10% (v ⁄ v) (Boziaris et al., 2011). Hence, aliquots of TSBYE with 0.0 and 10% (v ⁄ v) plant extract were prepared, with or without addition of 100 mm lactic acid. The pH *Correspondent. Fax: +30 24210 93157; e-mail: boziaris@uth.gr International Journal of Food Science and Technology 2012, 47, 1784–1787 1784 doi:10.1111/j.1365-2621.2012.03018.x Ó 2012 The Authors. International Journal of Food Science and Technology Ó 2012 Institute of Food Science and Technology