Cell-wall deficient L. monocytogenes L-forms feature abrogated pathogenicity.

Barbara Schnell

doi:10.3389/fcimb.2014.00060

Abstract

Stable L-forms are cell wall-deficient bacteria which are able to multiply and propagate indefinitely, despite the absence of a rigid peptidoglycan cell wall. We investigated whether L-forms of the intracellular pathogen L. monocytogenes possibly retain pathogenicity, and if they could trigger an innate immune response. While phagocytosis of L. monocytogenes L-forms by non-activated macrophages sometimes resulted in an unexpected persistence of the bacteria in the phagocytes, they were effectively eliminated by IFN-γ preactivated or bone marrow-derived macrophages (BMM). These findings were in line with the observed down-regulation of virulence factors in the cell-wall deficient L. monocytogenes. Absence of Interferon-β (IFN-β) triggering indicated inability of L-forms to escape from the phagosome into the cytosol. Moreover, abrogated cytokine response in MyD88-deficient dendritic cells (DC) challenged with L. monocytogenes L-forms suggested an exclusive TLR-dependent host response. Taken together, our data demonstrate a strong attenuation of Listeria monocytogenes L-form pathogenicity, due to diminished expression of virulence factors and innate immunity recognition, eventually resulting in elimination of L-form bacteria from phagocytes.

Highlights

Listeria monocytogenes is a Gram-positive pathogen ubiquitously found in natural environments
In order to quantify and compare the expression of virulence genes in L. monocytogenes L-forms and parental bacteria in cell culture, Real-Time qRT-PCR analysis was performed on samples collected 3 h post infection
Because ActA is proposed to be involved in autophagy recognition of bacteria (Yoshikawa et al, 2009), its presence on L-forms was further investigated by Western blot, revealing that ActA is below detectable levels on the surface of L-form cells (Figure 1B)

Summary

Introduction

Listeria monocytogenes is a Gram-positive pathogen ubiquitously found in natural environments. Pathogen recognition and induction of immune responses are important prerequisites in fighting bacterial infections. The innate immune system can sense pathogen-associated molecular patterns (PAMPs), by evolutionary conserved host sensors known as pathogen recognition receptors (PRRs) (Kumar et al, 2011). Pathogens such as Listeria monocytogenes have developed strategies to evade or modulate these immune defenses in order to ensure their survival (Corr and O’Neill, 2009).

Objectives

Methods

Results

Conclusion