Critical Role of a Ferritin-Like Protein in the Control of <i>Listeria monocytogenes</i> Cell Envelope Structure and Stability under β-lactam Pressure

<div><p>The human pathogen <i>Listeria monocytogenes</i> is susceptible to the β-lactam antibiotics penicillin G and ampicillin, and these are the drugs of choice for the treatment of listerial infections. However, these antibiotics exert only a bacteriostatic effect on this bacterium and consequently, <i>L. monocytogenes</i> is regarded as β-lactam tolerant. It is widely accepted that the phenomenon of bacterial tolerance to β-lactams is due to the lack of adequate autolysin activity, but the mechanisms of <i>L. monocytogenes</i> tolerance to this class of antibiotics are poorly characterized. A ferritin-like protein (Fri) was recently identified as a mediator of β-lactam tolerance in <i>L. monocytogenes</i>, but its function in this process remains unknown. The present study was undertaken to improve our understanding of <i>L. monocytogenes</i> tolerance to β-lactams and to characterize the role of Fri in this phenomenon. A comparative physiological analysis of wild-type <i>L. monocytogenes</i> and a <i>fri</i> deletion mutant provided evidence of a multilevel mechanism controlling autolysin activity in cells grown under β-lactam pressure, which leads to a reduction in the level and/or activity of cell wall-associated autolysins. This is accompanied by increases in the amount of teichoic acids, cell wall thickness and cell envelope integrity of <i>L. monocytogenes</i> grown in the presence of penicillin G, and provides the basis for the innate β-lactam tolerance of this bacterium. Furthermore, this study revealed the inability of the <i>L. monocytogenes</i><i>Δ</i><i>fri</i> mutant to deplete autolysins from the cell wall, to adjust the content of teichoic acids and to maintain their D-alanylation at the correct level when treated with penicillin G, thus providing further evidence that Fri is involved in the control of <i>L. monocytogenes</i> cell envelope structure and stability under β-lactam pressure.</p>