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 specific amino acid. When the purified EPS was subjected to attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, the spectra obtained did not match the profile of any of the 12 reference compounds used. An x-ray diffraction (XRD) analysis showed that the EPS was amorphous and a nuclear magnetic resonance (NMR) analysis detected the presence of glycerol. An elemental energy dispersive x-ray (EDX) analysis showed traces of phosphorous as a major component. In conclusion, it is proposed that the non-proteinaceous component may be phospholipid in nature, possibly derived from the cell wall lipoteichoic acid.
Highlights
The organism Listeria monocytogenes is one of the two pathogenic species among the genus Listeria [1] that has 20 species [2]
To help understand why L. monocytogenes may persist in food industry equipment and premises, Carpentier and Cerf [13] suggested a conceptual model of persistence based on the relative weight of growth and the outcome of cleaning and disinfection, which requires a minimum initial bacterial load necessary for bacteria to persist in harbourage sites
L. monocytogenes were grown in three media, BHI and two defined minimal media; namely, D10 and MCDB 202
Summary
The organism Listeria monocytogenes is one of the two pathogenic species among the genus Listeria [1] that has 20 species [2]. The reported occurrence of listeriosis is rare compared with other foodborne infections, the fatality rate is high [3,4] and the economic losses associated with an outbreak of listeriosis support the need for active surveillance and control measures [5]. The persistent presence of L. monocytogenes has been widely reported in food processing plants. The organism has been found in processing plants of meat [6,7]. The strains recovered from the processing environment can be resistant to sanitation chemicals [11] and show multi-drug resistance [12]. It is believed that after sanitation procedures, cells that are not recovered by Biomolecules 2021, 11, 331.
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