biomolecules Article Properties of the Extracellular Polymeric Substance Layer from Minimally Grown Planktonic Cells of Listeria monocytogenes Ogueri Nwaiwu 1,*, Lawrence Wong 1,2, Mita Lad 1, Timothy Foster 1, William MacNaughtan 1 and Catherine Rees 1 1 Division of Food, Nutrition and Dietetics, School of Biosciences, University of Nottingham, Nottingham LE12 5RD, UK;
[email protected] (L.W.);
[email protected] (M.L.);
[email protected] (T.F.);
[email protected] (W.M.);
[email protected] (C.R.) 2 Senthink Science & Technology, Hangzhou 310000, China * Correspondence:
[email protected] Abstract: The bacterium Listeria monocytogenes is a serious concern to food processing facilities because of its persistence. When liquid cultures of L. monocytogenes were prepared in defined media, it was noted that planktonic cells rapidly dropped out of suspension. Zeta potential and hydrophobicity assays found that the cells were more negatively charged (−22, −18, −10 mV in defined media D10, MCDB 202 and brain heart infusion [BHI] media, respectively) and were also more hydrophobic. A SEM analysis detected a capsular-like structure on the surface of cells grown in D10 media. A crude extract of the extracellular polymeric substance (EPS) was found to contain cell-associated proteins. The proteins were removed with pronase treatment. The remaining non- proteinaceous component was not stained by Coomassie blue dye and a further chemical analysis of the EPS did not detect significant amounts of sugars, DNA, polyglutamic acid or any other Citation: Nwaiwu, O.; Wong, L.; specific amino acid. When the purified EPS was subjected to attenuated total reflectance-Fourier Lad, M.; Foster, T.; MacNaughtan, W.; Rees, C.