Penicillin-binding Proteins (PBP) and Lmo0441 (a PBP-like protein) play a role in Beta-lactam sensitivity of Listeria monocytogenes
© Van de Velde et al; licensee BioMed Central Ltd. 2009
Received: 3 December 2009
Accepted: 15 December 2009
Published: 15 December 2009
While seven penicillin-binding proteins (PBPs) or PBP-like proteins have been identified either by radiolabelled penicillin binding studies or genomic analysis, only PBP3 has been considered of interest for Beta-lactams activity against Listeria monocytogenes. Herein we reveal that both PBP4 and Lmo0441 (a PBP-like protein) play a direct role in cephalosporin activity in L. monocytogenes while PBP4 additionally has a protective affect against both penicillin and carbapenem.
The Gram-positive foodborne pathogen Listeria monocytogenes is a causative agent of gastroenteritis  and in severe cases, listeriosis, which ranges from a mild flu-like illness to meningitis in non-pregnant individuals, or as infection of the foetus in pregnant women . Ampicillin, either alone or in combination with gentamicin, remains the gold standard treatment for L. monocytogenes infection . Conversely, cephalosporins are usually poorly active and thus not recommended in the treatment of listeriosis [4, 5]. This dissociation in susceptibilities between β-lactams has been rationalized by the observation that cephalosporins have poor affinity for PBP3 compared to penicillin, suggesting that PBP3 is the primary lethal target for β-lactams in L. monocytogenes . However, at least five different PBPs have been identified in L. monocytogenes based on their ability to bind radioactive penicillin [6, 7]. Moreover, genome analysis revealed seven distinct genes with homologies to PBPs in L. monocytogenes . With the exception of pbpB, each of the remaining genes has been disrupted by insertional mutagenesis  without loss of cell viability, suggesting that they are not critically required for normal synthesis of cell wall peptidoglycan. Based on a previous study by Guinane et al.,  in which an increase in the activity of cephalosporins was observed for L. monocytogenes disrupted in lmo2229 and lmo0441, we investigated the role of their corresponding proteins (PBP4 and Lmo0441 respectively) in protecting the pathogen against a penicillin (ampicillin, AMP), a carbapenem (meropenem, MEM) and a cephalosporin (cefuroxime, CFX).
MIC measurements, regression parametersa and statistical analysis of the dose-response curves illustrated in Figure 1
Δlog from original inoculum
Δlog from original inoculum
log from original inoculum
-1.53 (-1.7 to -1.3)
-1.85 (-2.2 to -1.5)
-3.65 (-4.2 to -3.1)
-2.51 (-2.7 to -2.3)d
-2.92 (-3.3 to -2.5)d
-2.29 (-2.6 to -1.9)d
-1.74 (-1.9 to -1.5)
-2.02 (-2.5 to -1.6)
-1.92 (-2.2 to -1.7)d
This study also reveals that loss of PBP4 or Lmo0441 proteins allows a decrease of maximal activity of cephalosporin, a class of antibiotics which, due to their poor affinity for the PBP3, are not particularly effective against L. monocytogenes . This observation, which runs contrary to what was observed for AMP and MEM may be explained by the fact that PBP3 may increase its relative action in peptydoglycan synthesis when PBP4 or lmo0441 are deleted: conferring resistance to cephalosporin. Thus, even if PBP3 remains the principal target of β-lactam antibiotics, as is the current dictum, PBP4 and Lmo0441 also appear to play an important role in the activity of β-lactam antibiotics.
We are grateful to Charlotte Misson who provided skillful technical assistance and the manufacturers for the kind gift of their antibiotics. This work was supported by a FIRST-Doctorate fellowship awarded to SVDV by the Direction Générale de la Recherche et des Technologies of the Région Wallonne, Belgium and co-financed by Eumedica s.a.. RDS is a Health Research Board (HRB) and Alimentary Pharmabiotic Centre (APC) Principal Investigator.
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