Re-evaluation of the significance of penicillin binding protein 3 in the susceptibility of Listeria monocytogenes to β-lactam antibiotics

In this study, overexpression of lmo1438 was achieved using an inducible (nisin-controlled) expression system. This permitted the direct demonstration that lmo1438 encodes PBP3. PBP3 overexpression was accompanied by slightly elevated PBP4 expression. The recombinant strain overexpressing PBP3 displayed significant growth retardation and greatly reduced cell length in the stationary phase of growth in culture. In antibiotic susceptibility assays, the strain overexpressing PBP3 displayed increased sensitivity to subinhibitory concentrations of several β-lactams and decreased survival in the presence of a lethal dose of penicillin G. However, the MIC values of the tested β-lactams for this recombinant strain were unchanged compared to the parent strain.The present study allows a reevaluation of the importance of PBP3 in the susceptibility of L. monocytogenes to β-lactams. It is clear that PBP3 is not the primary lethal target for β-lactams, since neither the absence nor an excess of this protein affect the susceptibility of L. monocytogenes to these antibiotics. The elevated level of PBP4 expression observed in the recombinant strain overexpressing PBP3 demonstrates that the composition of the L. monocytogenes cell wall is subject to tight regulation. The observed changes in the morphology of stationary phase cells in response to PBP3 overexpression suggests the involvement of this protein in cell division during this phase of growth.Listeria monocytogenes is a ubiquitous gram-positive opportunistic pathogen that can cause very serious food-borne infections in humans, with symptoms including meningitis, frequently accompanied by septicemia and meningoencephalitis, which are particularly severe for newborns and immunocompromised individuals [1]. The antibiotics of choice in the treatment of listeriosis are the β-lactams penicillin G or ampicillin, alone or in combination with an aminoglycoside [2]. The classical target enzymes for β-lactam antibiotics are the penicilli