Comparison of widely used Listeria monocytogenes strains EGD, 10403S, and EGD-e highlights genomic variations underlying differences in pathogenicity

Interferon γ-induced GTPase promotes invasion of Listeria monocytogenes into trophoblast giant cells

Listeria monocytogenes is well known for having the ability to cross the placental barrier, leading to fetal infections and abortion. However, the mechanisms leading to infectious abortion are poorly understood. In this study, we demonstrate that interferon γ-induced GTPase (IGTP) contributes to the invasion of L. monocytogenes into trophoblast giant (TG) cells, which are placental immune cells. Knockdown of IGTP in TG cells decreased the relative efficiencies of L. monocytogenes invasion. Moreover, IGTP accumulated around infected L. monocytogenes in TG cells. Treatment of TG cells with phosphatidylinositol 3-kinase (PI3K)/Akt inhibitors also reduced bacterial invasion. PI3K/Akt inhibitor or IGTP knockdown reduced the amount of phosphorylated Akt. Monosialotetrahexosylganglioside (GM1) gangliosides, lipid raft markers, accumulated in the membrane of L. monocytogenes-containing vacuoles in TG cells. Furthermore, treatment with a lipid raft inhibitor reduced bacterial invasion. These results suggest that IGTP-induced activation of the PI3K/Akt signaling pathway promotes bacterial invasion into TG cells.

PI3-kinase activation is critical for host barrier permissiveness to Listeria monocytogenes

Invasion of nonphagocytic cells, a critical property of Listeria monocytogenes (Lm) that enables it to cross host barriers, is mediated by the interaction of two bacterial surface proteins, InlA and InlB, with their respective receptors E-cadherin and c-Met. Although InlA-E-cadherin interaction is necessary and sufficient for Lm crossing of the intestinal barrier, both InlA and InlB are required for Lm crossing of the placental barrier. The mechanisms underlying these differences are unknown. Phosphoinositide 3-kinase (PI3-K) is involved in both InlA- and InlB-dependent pathways. Indeed, InlA-dependent entry requires PI3-K activity but does not activate it, whereas InlB-c-Met interaction activates PI3-K. We show that Lm intestinal target cells exhibit a constitutive PI3-K activity, rendering InlB dispensable for InlA-dependent Lm intestinal barrier crossing. In contrast, the placental barrier does not exhibit constitutive PI3-K activity, making InlB necessary for InlA-dependent Lm placental invasion. Here, we provide the molecular explanation for the respective contributions of InlA and InlB to Lm host barrier invasion, and reveal the critical role of InlB in rendering cells permissive to InlA-mediated invasion. This study shows that PI3-K activity is critical to host barrier permissiveness to microbes, and that pathogens exploit both similarities and differences of host barriers to disseminate.

The Listeria monocytogenes hibernation-promoting factor is required for the formation of 100S ribosomes, optimal fitness, and pathogenesis

During exposure to certain stresses, bacteria dimerize pairs of 70S ribosomes into translationally silent 100S particles in a process called ribosome hibernation. Although the biological roles of ribosome hibernation are not completely understood, this process appears to represent a conserved and adaptive response that contributes to optimal survival during stress and post-exponential-phase growth. Hibernating ribosomes are formed by the activity of one or more highly conserved proteins; gammaproteobacteria produce two relevant proteins, ribosome modulation factor (RMF) and hibernation promoting factor (HPF), while most Gram-positive bacteria produce a single, longer HPF protein. Here, we report the formation of 100S ribosomes by an HPF homolog in Listeria monocytogenes. L. monocytogenes 100S ribosomes were observed by sucrose density gradient centrifugation of bacterial extracts during mid-logarithmic phase, peaked at the transition to stationary phase, and persisted at lower levels during post-exponential-phase growth. 100S ribosomes were undetectable in bacteria carrying an hpf::Himar1 transposon insertion, indicating that HPF is required for ribosome hibernation in L. monocytogenes. Additionally, epitope-tagged HPF cosedimented with 100S ribosomes, supporting its previously described direct role in 100S formation. We examined hpf mRNA by quantitative PCR (qPCR) and identified several conditions that upregulated its expression, including carbon starvation, heat shock, and exposure to high concentrations of salt or ethanol. Survival of HPF-deficient bacteria was impaired under certain conditions both in vitro and during animal infection, providing evidence for the biological relevance of 100S ribosome formation.

Divergent evolution of the activity and regulation of the glutamate decarboxylase systems in Listeria monocytogenes EGD-e and 10403S: roles in virulence and acid tolerance

The glutamate decarboxylase (GAD) system has been shown to be important for the survival of Listeria monocytogenes in low pH environments. The bacterium can use this faculty to maintain pH homeostasis under acidic conditions. The accepted model for the GAD system proposes that the antiport of glutamate into the bacterial cell in exchange for γ-aminobutyric acid (GABA) is coupled to an intracellular decarboxylation reaction of glutamate into GABA that consumes protons and therefore facilitates pH homeostasis. Most strains of L. monocytogenes possess three decarboxylase genes (gadD1, D2 & D3) and two antiporter genes (gadT1 & gadT2). Here, we confirm that the gadD3 encodes a glutamate decarboxylase dedicated to the intracellular GAD system (GAD_i), which produces GABA from cytoplasmic glutamate in the absence of antiport activity. We also compare the functionality of the GAD system between two commonly studied reference strains, EGD-e and 10403S with differences in terms of acid resistance. Through functional genomics we show that EGD-e is unable to export GABA and relies exclusively in the GAD_i system, which is driven primarily by GadD3 in this strain. In contrast 10403S relies upon GadD2 to maintain both an intracellular and extracellular GAD system (GAD_i/GAD_e). Through experiments with a murinised variant of EGD-e (EGDm) in mice, we found that the GAD system plays a significant role in the overall virulence of this strain. Double mutants lacking either gadD1D3 or gadD2D3 of the GAD system displayed reduced acid tolerance and were significantly affected in their ability to cause infection following oral inoculation. Since EGDm exploits GAD_i but not GAD_e the results indicate that the GAD_i system makes a contribution to virulence within the mouse. Furthermore, we also provide evidence that there might be a separate line of evolution in the GAD system between two commonly used reference strains.

Quantitative proteome analyses identify PrfA-responsive proteins and phosphoproteins in Listeria monocytogenes

Protein phosphorylation is a major mechanism of signal transduction in bacteria. Here, we analyzed the proteome and phosphoproteome of a wild-type strain of the food-borne pathogen Listeria monocytogenes that was grown in either chemically defined medium or rich medium containing glucose. We then compared these results with those obtained from an isogenic prfA* mutant that produced a constitutively active form of PrfA, the main transcriptional activator of virulence genes. In the prfA* mutant grown in rich medium, we identified 256 peptides that were phosphorylated on serine (S), threonine (T), or tyrosine (Y) residues, with a S/T/Y ratio of 155:75:12. Strikingly, we detected five novel phosphosites on the virulence protein ActA. This protein was known to be phosphorylated by a cellular kinase in the infected host, but phosphorylation by a listerial kinase had not previously been reported. Unexpectedly, SILAC experiments with the prfA* mutant grown in chemically defined medium revealed that, in addition to previously described PrfA-regulated proteins, several other proteins were significantly overproduced, among them were several proteins involved in purine biosynthesis. This work provides new information for our understanding of the correlation among protein phosphorylation, virulence mechanisms, and carbon metabolism.

The roles of noncoding RNA Rli60 in regulating the virulence of Listeria monocytogenes

BACKGROUND

Listeria monocytogenes (LM) is an important zoonotic foodborne pathogen. Noncoding RNA (ncRNA) has an important role in regulating its virulence. As a member of ncRNA, however, the function of Rli60 in regulating LM virulence remain unclear. The aim of this study was to investigate the role of Rli60 in regulating LM virulence.

METHODS

Using a homologous recombination method, a LM EGD-e rli60 gene deletion strain (LM-Δrli60) was constructed and compared with a LM EGD-e strain in the following respects: (1) adhesiveness, invasion ability, intracellular survival, proliferation, and transcription of virulence genes in the mouse macrophage cell line RAW264.7; (2) 50% lethal dose (LD50) to the BALB/c mouse; and (3) the amount in the mouse liver and spleen and the effects on pathology of mouse liver, spleen, and kidney after inoculation.

RESULTS

The LM-Δrli60 strain had a significantly higher adhesion rate and lower invasion rate with significantly lower intracellular survival and proliferation rates in the RAW264.7 cell line, compared to the LM EGD-e strain. Inoculation with LM-Δrli60 strain significantly affected the transcription of virulence genes. The LD50 of LM-Δrli60 to BALB/c mouse was increased by 2.12 logarithmic magnitude, which indicated that the virulence in LM-Δrli60 is significantly decreased (p < 0.05). The amount of LM-Δrli60 in the liver and spleen was significantly lower than the amount of LM EGD-e in these organs (p < 0.05). The pathological damage due to LM-Δrli60 infection in the mouse liver, spleen, and kidney was lower than the damage due to LM EGD-e infection.

CONCLUSION

This study confirmed that the rli60 deletion could significantly affect LM virulence, adhesion, invasion, survival, and proliferation. This suggests that Rli60 has an important role in regulating LM virulence.

Detection of very long antisense transcripts by whole transcriptome RNA-Seq analysis of Listeria monocytogenes by semiconductor sequencing technology

The Gram-positive bacterium Listeria monocytogenes is the causative agent of listeriosis, a severe food-borne infection characterised by abortion, septicaemia, or meningoencephalitis. L. monocytogenes causes outbreaks of febrile gastroenteritis and accounts for community-acquired bacterial meningitis in humans. Listeriosis has one of the highest mortality rates (up to 30%) of all food-borne infections. This human pathogenic bacterium is an important model organism for biomedical research to investigate cell-mediated immunity. L. monocytogenes is also one of the best characterised bacterial systems for the molecular analysis of intracellular parasitism. Recently several transcriptomic studies have also made the ubiquitous distributed bacterium as a model to understand mechanisms of gene regulation from the environment to the infected host on the level of mRNA and non-coding RNAs (ncRNAs). We have used semiconductor sequencing technology for RNA-seq to investigate the repertoire of listerial ncRNAs under extra- and intracellular growth conditions. Furthermore, we applied a new bioinformatic analysis pipeline for detection, comparative genomics and structural conservation to identify ncRNAs. With this work, in total, 741 ncRNA locations of potential ncRNA candidates are now known for L. monocytogenes, of which 611 ncRNA candidates were identified by RNA-seq. 441 transcribed ncRNAs have never been described before. Among these, we identified novel long non-coding antisense RNAs with a length of up to 5,400 nt e.g. opposite to genes coding for internalins, methylases or a high-affinity potassium uptake system, namely the kdpABC operon, which were confirmed by qRT-PCR analysis. RNA-seq, comparative genomics and structural conservation of L. monocytogenes ncRNAs illustrate that this human pathogen uses a large number and repertoire of ncRNA including novel long antisense RNAs, which could be important for intracellular survival within the infected eukaryotic host.

A trip in the "New Microbiology" with the bacterial pathogen Listeria monocytogenes

Listeria monocytogenes is a food-borne pathogen causing an opportunistic disease called listeriosis. This bacterium invades and replicates in most cell types, due to its multiple strategies to exploit host molecular mechanisms. Research aiming at unravelling Listeria invasion and intracellular lifestyle has led to a number of key discoveries in infection biology, cell biology and also microbiology. In this review, we report on our most recent advances in understanding the intimate crosstalk between the bacterium and its host, resulting from in-depth studies performed over the past five years. We specifically highlight new concepts in RNA-based regulation in bacteria and discuss important findings in cell biology, including a new role for clathrin and an atypical mitochondrial fragmentation mechanism. We also illustrate the notion that bacterial infection regulates host gene expression at the chromatin level, contributing to an emerging field called patho-epigenetics. This review corresponds to the lecture given by one of us (P.C.) on the occasion of the 2014 FEBS|EMBO Woman in Science Award.

Genome Sequences of Three Frequently Used Listeria monocytogenes and Listeria ivanovii Strains

We present the complete de novo assembled genome sequences of Listeria monocytogenes strains WSLC 1001 (ATCC 19112) and WSLC 1042 (ATCC 23074) and Listeria ivanovii WSLC 3009, three strains frequently used for the propagation and study of bacteriophages because they are presumed to be free of inducible prophages.

Internalization assays for Listeria monocytogenes

Listeria monocytogenes is a model intracellular pathogen that can invade the cytoplasm of host mammalian cells. Cellular invasion can be measured using standard techniques such as the classical gentamicin protection assay, based on the quantification of colony-forming units from lysates of infected cells. In addition, there are methods based on immunofluorescence microscopy which allow for assaying invasion in a medium- to high-throughput manner. In the following sections we detail two different assays that can be used alone or in combination to quantify the internalization of L. monocytogenes in host cells.