Involvement of Reactive Oxygen Intermediate in the Enhanced Expression of Virulence-Associated Genes ofListeria monocytogenesinside Activated Macrophages

Signals behind Listeria monocytogenes virulence mechanisms

The tight and coordinated regulation of virulence gene expression is crucial to ensure the survival and persistence of bacterial pathogens in different contexts within their hosts. Considering this, bacteria do not express virulence factors homogenously in time and space, either due to their associated fitness cost or to their detrimental effect at specific infection stages. To efficiently infect and persist into their hosts, bacteria have thus to monitor environmental cues or chemical cell-to-cell signaling mechanisms that allow their transition from the external environment to the host, and therefore adjust gene expression levels, intrinsic biological activities, and appropriate behaviors. Listeria monocytogenes (Lm), a major Gram-positive facultative intracellular pathogen, stands out for its adaptability and capacity to thrive in a wide range of environments. Because of that, Lm presents itself as a significant concern in food safety and public health, that can lead to potentially life-threatening infections in humans. A deeper understanding of the intricate bacterial virulence mechanisms and the signals that control them provide valuable insights into the dynamic interplay between Lm and the host. Therefore, this review addresses the role of some crucial signals behind Lm pathogenic virulence mechanisms and explores how the ability to assimilate and interpret these signals is fundamental for pathogenesis, identifying potential targets for innovative antimicrobial strategies.

Hacking the Immune Response to Solid Tumors: Harnessing the Anti-Cancer Capacities of Oncolytic Bacteria

Oncolytic bacteria are a classification of bacteria with a natural ability to specifically target solid tumors and, in the process, stimulate a potent immune response. Currently, these include species of Klebsiella, Listeria, Mycobacteria, Streptococcus/Serratia (Coley's Toxin), Proteus, Salmonella, and Clostridium. Advancements in techniques and methodology, including genetic engineering, create opportunities to "hijack" typical host-pathogen interactions and subsequently harness oncolytic capacities. Engineering, sometimes termed "domestication", of oncolytic bacterial species is especially beneficial when solid tumors are inaccessible or metastasize early in development. This review examines reported oncolytic bacteria-host immune interactions and details the known mechanisms of these interactions to the protein level. A synopsis of the presented membrane surface molecules that elicit particularly promising oncolytic capacities is paired with the stimulated localized and systemic immunogenic effects. In addition, oncolytic bacterial progression toward clinical translation through engineering efforts are discussed, with thorough attention given to strains that have accomplished Phase III clinical trial initiation. In addition to therapeutic mitigation after the tumor has formed, some bacterial species, referred to as "prophylactic", may even be able to prevent or "derail" tumor formation through anti-inflammatory capabilities. These promising species and their particularly favorable characteristics are summarized as well. A complete understanding of the bacteria-host interaction will likely be necessary to assess anti-cancer capacities and unlock the full cancer therapeutic potential of oncolytic bacteria.

Listeria monocytogenes isolates from Cornu aspersum snails: Whole genome-based characterization and host-pathogen interactions in a snail infection model

Even though Listeria monocytogenes is an extensive-studied foodborne pathogen, genome analysis of isolates from snails that may represent a reservoir of L. monocytogenes are still scarce. Here, we use whole-genome sequencing (WGS) to assess the genomic diversity of hypervirulent, virulent and non-virulent phenotypes of 15 L. monocytogenes isolated from snails to unveil their survival, virulence, and host-pathogen mechanisms of interactions in a snail infection model. Most of isolates (66.7%) were characterized as multidrug resistant (MDR) and belonged to clonal complexes (CCs) which are strongly associated with cases of human infection. All isolates contained intact genes associated with invasion and infection while hypervirulent isolates are adapted to host environment, possessing genes which are involved in teichoic acid biosynthesis, peptidoglycan modification and biofilm formation, correlating with their tolerance to haemolymph plasma phenotype and biofilm formation ability. A snail infection model showed that hypervirulent isolates triggered programmed host cell death pathway by increasing up to 30% the circulating apoptotic hemocytes in combination with induced nitrate production and reactive oxygen species (ROS) generation in snails' haemolymph. In contrast, the administration of the non-virulent strain which possesses a truncated mogR gene that regulates flagellar motility gene expression led only to an increase of necrotic non-apoptotic cells. Overall, this study provides significant insights into the genetic diversity of L. monocytogenes from snails, the genomic features of them linked to their hypervirulent/non-virulent phenotype, and the mechanisms of host-pathogen interactions.

The next frontier of oncotherapy: accomplishing clinical translation of oncolytic bacteria through genetic engineering

The development of a 'smart' drug capable of distinguishing tumor from host cells has been sought for centuries, but the microenvironment of solid tumors continues to confound therapeutics. Solid tumors present several challenges for current oncotherapeutics, including aberrant vascularization, hypoxia, necrosis, abnormally high pH and local immune suppression. While traditional chemotherapeutics are limited by such an environment, oncolytic microbes are drawn to it - having an innate ability to selectively infect, colonize and eradicate solid tumors. Development of an oncolytic species would represent a shift in the cancer therapeutic paradigm, with ramifications reaching from the medical into the socio-economic. Modern genetic engineering techniques could be implemented to customize 'Frankenstein' bacteria with advantageous characteristics from several species.

The molecular mechanisms of listeriolysin O-induced lipid membrane damage

Listeria monocytogenes is an intracellular food-borne pathogen that causes listeriosis, a severe and potentially life-threatening disease. Listeria uses a number of virulence factors to proliferate and spread to various cells and tissues. In this process, three bacterial virulence factors, the pore-forming protein listeriolysin O and phospholipases PlcA and PlcB, play a crucial role. Listeriolysin O belongs to a family of cholesterol-dependent cytolysins that are mostly expressed by gram-positive bacteria. Its unique structural features in an otherwise conserved three-dimensional fold, such as the acidic triad and proline-glutamate-serine-threonine-like sequence, enable the regulation of its intracellular activity as well as distinct extracellular functions. The stability of listeriolysin O is pH- and temperature-dependent, and this provides another layer of control of its activity in cells. Moreover, many recent studies have demonstrated a unique mechanism of pore formation by listeriolysin O, i.e., the formation of arc-shaped oligomers that can subsequently fuse to form membrane defects of various shapes and sizes. During listerial invasion of host cells, these membrane defects can disrupt phagosome membranes, allowing bacteria to escape into the cytosol and rapidly multiply. The activity of listeriolysin O is profoundly dependent on the amount and accessibility of cholesterol in the lipid membrane, which can be modulated by the phospholipase PlcB. All these prominent features of listeriolysin O play a role during different stages of the L. monocytogenes life cycle by promoting the proliferation of the pathogen while mitigating excessive damage to its replicative niche in the cytosol of the host cell.

Oncolytic bacteria: past, present and future

More than a century ago, independent groups raised the possibility of using bacteria to selectively infect tumours. Such treatment induces an immune reaction that can cause tumour rejection and protect the patient against further recurrences. One of the first holistic approximations to use bacteria in cancer treatment was performed by William Coley, considered the father of immune-therapy, at the end of XIX century. Since then, many groups have used different bacteria to test their antitumour activity in animal models and patients. The basis for this reactivity implies that innate immune responses activated upon bacteria recognition, also react against the tumour. Different publications have addressed several aspects of oncolytic bacteria. In the present review, we will focus on revisiting the historical aspects using bacteria as oncolytic agents and how they led to the current clinical trials. In addition, we address the molecules present in oncolytic bacteria that induce specific toxic effects against the tumors as well as the activation of host immune responses in order to trigger antitumour immunity. Finally, we discuss future perspectives that could be considered in the different fields implicated in the implementation of this kind of therapy in order to improve the current use of bacteria as oncolytic agents.

Reactive oxygen species inhibit biofilm formation of Listeria monocytogenes

Although the level of reactive oxygen species (ROS) is altered upon the formation of bacterial biofilm, the relationship between ROS alteration and biofilm formation is still unclear. The aim of the present study is to use Listeria monocytogenes (L. monocytogenes) as a model organism to examine whether ROS have an effect on the biofilm formation. After eliminating ROS by treatment with NAD(P)H oxidase inhibitor Diphenyleneiodonium chloride (DPI) or scavenging reagents N-acetylcysteine (NAC), the biofilm formation of L. monocytogenes was examined. Our data demonstrate that DPI and NAC induced-reduction of ROS enhances the biofilm formation in L. monocytogenes without affecting bacterial growth and activity. These data provide the evidence that ROS produced by L. monocytogenes inhibit the biofilm formation.

Listeriolysin O: from bazooka to Swiss army knife

Listeria monocytogenes (Lm) is a Gram-positive facultative intracellular pathogen. Infections in humans can lead to listeriosis, a systemic disease with a high mortality rate. One important mechanism of Lm dissemination involves cell-to-cell spread after bacteria have entered the cytosol of host cells. Listeriolysin O (LLO; encoded by the hly gene) is a virulence factor present in Lm that plays a central role in the cell-to-cell spread process. LLO is a member of the cholesterol-dependent cytolysin (CDC) family of toxins that were initially thought to promote disease largely by inducing cell death and tissue destruction-essentially acting like a 'bazooka'. This view was supported by structural studies showing CDCs can form large pores in membranes. However, it is now appreciated that LLO has many subtle activities during Lm infection of host cells, and many of these likely do not involve large pores, but rather small membrane perforations. It is also appreciated that membrane repair pathways of host cells play a major role in limiting membrane damage by LLO and other toxins. LLO is now thought to represent a 'Swiss army knife', a versatile tool that allows Lm to induce many membrane alterations and cellular responses that promote bacterial dissemination during infection.This article is part of the themed issue 'Membrane pores: from structure and assembly, to medicine and technology'.

Restraining reactive oxygen species in Listeria monocytogenes promotes the apoptosis of glial cells

OBJECTIVES

Listeria monocytogenes is a facultative anaerobic foodborne pathogen that can traverse the blood-brain barrier and cause brain infection. L. monocytogenes infection induces host cell apoptosis in several cell types. In this study, we investigated the apoptosis of human glioma cell line U251 invaded by L. monocytogenes and evaluated the function of bacterial reactive oxygen species (ROS) during infection.

METHODS

Bacterial ROS level was reduced by carrying out treatment with N-acetyl cysteine (NAC) and diphenyleneiodonium chloride (DPI). After infection, the apoptosis of U251 cells was examined by flow cytometry assay and propidium iodide staining.

RESULTS

DPI and NAC efficiently decreased ROS level in L. monocytogenes without affecting bacterial growth. Moreover, the apoptosis of glial cells was enhanced upon invasion of DPI- and NAC-pretreated L. monocytogenes.

DISCUSSION

Results indicate that the apoptosis of glial cells can be induced by L. monocytogenes, and that the inhibition of bacterial ROS increases the apoptosis of host cells.

Listeriolysin O suppresses phospholipase C-mediated activation of the microbicidal NADPH oxidase to promote Listeria monocytogenes infection

The intracellular bacterial pathogen Listeria monocytogenes produces phospholipases C (PI-PLC and PC-PLC) and the pore-forming cytolysin listeriolysin O (LLO) to escape the phagosome and replicate within the host cytosol. We found that PLCs can also activate the phagocyte NADPH oxidase during L. monocytogenes infection, a response that would adversely affect pathogen survival. However, secretion of LLO inhibits the NADPH oxidase by preventing its localization to phagosomes. LLO-deficient bacteria can be complemented by perfringolysin O, a related cytolysin, suggesting that other pathogens may also use pore-forming cytolysins to inhibit the NADPH oxidase. Our studies demonstrate that while the PLCs induce antimicrobial NADPH oxidase activity, this effect is alleviated by the pore-forming activity of LLO. Therefore, the combined activities of PLCs and LLO on membrane lysis and the inhibitory effects of LLO on NADPH oxidase activity allow L. monocytogenes to efficiently escape the phagosome while avoiding the microbicidal respiratory burst.

Adaptation of Listeria monocytogenes to oxidative and nitrosative stress in IFN-γ-activated macrophages

IFN-γ-activated macrophages are considered to be the primary effector cells in host defense against Listeria monocytogenes infections. However despite the induction of the complex host defense mechanisms, survival of L. monocytogenes in activated macrophages is still observed. Here we used a whole genome-based transcriptome approach to examine for bacterial genes specifically induced in IFN-γ-activated macrophages. We demonstrated that cells activated by IFN-γ had elevated oxidative and nitrosative stress levels in both the activated macrophages as well as in the intracellular replicating bacteria isolated from these infected cells. We found that a subset of 21 transcripts were specifically differentially regulated in bacteria growing in cells pretreated with IFN-γ. Bioinformatics and functional analysis revealed that many of these genes have roles involved in overcoming oxidative stress and contribute to bacterial survival within activated macrophages. We detected increased transcription of the putative trpE gene of L. monocytogenes, encoding an anthranilate synthase, in bacteria growing in IFN-γ cells indicating host cell metabolic restriction of bacterial growth. Indeed we found enhanced activation of host cell genes involved in the kynurenine pathway indicating an increased need of L. monocytogenes for tryptophan during replication in IFN-γ-activated macrophages.

Peroxynitrite stress is exacerbated by flavohaemoglobin-derived oxidative stress in Salmonella Typhimurium and is relieved by nitric oxide

Oxidative and nitrosative stresses including nitric oxide (NO), superoxide () and peroxynitrite play key roles in determining the outcome of bacterial infections. In order to survive within the host and allow proliferation within immune cells such as macrophages, Salmonella isolates have a number of inducible proteins that are able to detoxify these highly reactive species, notably the anoxically functioning NO reductase NorVW, and the aerobically functioning flavohaemoglobin, Hmp, which catalyses the reaction between oxygen and NO to produce relatively inert nitrate. However, in the absence of NO but in the presence of reducing substrates and oxygen, is generated from Hmp-mediated electron transfer to bound oxygen and may form a variety of further oxidative species. Hence, Hmp expression is under tight negative regulation by the transcription factor NsrR, abolition of which causes an increase in the production of Hmp. In a previous study, this increase in Hmp levels conferred resistance to the nitrosating agent S-nitrosoglutathione but, perhaps surprisingly, the organism became more sensitive to killing by macrophages. Here, we report that an nsrR mutant that constitutively overexpresses Hmp is also hypersensitive to peroxynitrite in vitro. This sensitivity is alleviated by deletion of the hmp gene or pre-incubation of growing bacteria with NO-releasing agents. We hypothesize that Hmp-expressing cells, in the absence of NO, generate reactive oxygen species, the toxicity of which is exacerbated by peroxynitrite in vitro and in macrophages. RT-PCR confirmed that peroxynitrite causes oxidative stress and upregulation of katG and ahpC, whilst hmp and norV expression are affected very little. The katG gene upregulated by peroxynitrite encodes a catalase peroxidase enzyme with well-established roles in detoxifying peroxides. Here, we report that KatG is also able to enhance the breakdown of peroxynitrite, suggesting that the protective role of this enzyme may be wider than previously thought. These data suggest that spatial and temporal fluctuations in the levels of NO and reactive oxygen species will have important consequences for bacterial survival in the macrophage.

Alpha-galactosylceramide promotes killing of Listeria monocytogenes within the macrophage phagosome through invariant NKT-cell activation

alpha-Galactosylceramide (alpha-GalCer) has been exploited for the treatment of microbial infections. Although amelioration of infection by alpha-GalCer involves invariant natural killer T (iNKT)-cell activation, it remains to be determined whether macrophages (Mphi) participate in the control of microbial pathogens. In the present study, we examined the participation of Mphi in immune intervention in infection by alpha-GalCer using a murine model of listeriosis. Phagocytic and bactericidal activities of peritoneal Mphi from C57BL/6 mice, but not iNKT cell-deficient mice, were enhanced after intraperitoneal injection of alpha-GalCer despite the absence of iNKT cells in the peritoneal cavity. High levels of gamma interferon (IFN-gamma) and nitric oxide (NO) were detected in the peritoneal cavities of mice treated with alpha-GalCer and in culture supernatants of peritoneal Mphi from mice treated with alpha-GalCer, respectively. Although enhanced bactericidal activity of peritoneal Mphi by alpha-GalCer was abrogated by endogenous IFN-gamma neutralization, this was only marginally affected by NO inhibition. Similar results were obtained by using a listeriolysin O-deficient strain of Listeria monocytogenes. Moreover, respiratory burst in Mphi was increased after alpha-GalCer treatment. Our results suggest that amelioration of listeriosis by alpha-GalCer is, in part, caused by enhanced killing of L. monocytogenes within phagosomes of Mphi activated by IFN-gamma from iNKT cells residing in an organ(s) other than the peritoneal cavity.

Listeria monocytogenes sigmaB modulates PrfA-mediated virulence factor expression

Listeria monocytogenes sigma(B) and positive regulatory factor A (PrfA) are pleiotropic transcriptional regulators that coregulate a subset of virulence genes. A positive regulatory role for sigma(B) in prfA transcription has been well established; therefore, observations of increased virulence gene expression and hemolytic activity in a DeltasigB strain initially appeared paradoxical. To test the hypothesis that L. monocytogenes sigma(B) contributes to a regulatory network critical for appropriate repression as well as induction of virulence gene expression, genome-wide transcript profiling and follow-up quantitative reverse transcriptase PCR (qRT-PCR), reporter fusion, and phenotypic experiments were conducted using L. monocytogenes prfA*, prfA* DeltasigB, DeltaprfA, and DeltaprfA DeltasigB strains. Genome-wide transcript profiling and qRT-PCR showed that in the presence of active PrfA (PrfA*), sigma(B) is responsible for reduced expression of the PrfA regulon. sigma(B)-dependent modulation of PrfA regulon expression reduced the cytotoxic effects of a PrfA* strain in HepG2 cells, highlighting the functional importance of regulatory interactions between PrfA and sigma(B). The emerging model of the role of sigma(B) in regulating overall PrfA activity includes a switch from transcriptional activation at the P2(prfA) promoter (e.g., in extracellular bacteria when PrfA activity is low) to posttranscriptional downregulation of PrfA regulon expression (e.g., in intracellular bacteria when PrfA activity is high).

Expression patterns of genes induced by oxidative stress in Porphyromonas gingivalis

INTRODUCTION

Porphyromonas gingivalis, a gram-negative anaerobic bacterium, is a major periopathogen whose transmission from host to host involves exposure to atmospheric oxygen. P. gingivalis contains genetic factors that function in an oxidative stress response, but their expression has not been analyzed during exposure to atmospheric oxygen. The aim of this study was to obtain a better understanding of atmospheric adaptation of P. gingivalis.

METHODS

The aerotolerance of wild-type and oxyR mutant P. gingivalis strains were determined, and quantitative polymerase chain reaction was performed to analyze gene expression patterns in response to exposure to atmospheric oxygen. The analyzed P. gingivalis genes encoded proteins involved in oxidative response (oxyR, ahpC-F, batA, dps, ftn, tpx) as well as several major virulence factors (hagA, hagB, hagE, rgpA, rgpB, hem).

RESULTS

Our results demonstrated a critical role for the oxyR gene in the aerotolerance of P. gingivalis. The ahpC-F, batA, and hem genes were slightly overexpressed (between 1.65-fold and 2-fold) after exposure to atmospheric oxygen compared to anaerobic conditions. The level of transcription of dps, ftn, tpx, and rgpA genes increased more than 2.5-fold, and the expression of ahpC-F, dps, ftn, and tpx was partially or completely OxyR-dependent.

CONCLUSION

A different transcription pattern of P. gingivalis genes was observed, depending on the stimulus of oxidative stress. We present new evidence that the expression of tpx, encoding a thiol peroxidase, is partially OxyR-dependent and is induced after atmospheric oxygen exposure.

Listeriosis due to infection with a catalase-negative strain of Listeria monocytogenes

A strain of Listeria monocytogenes recovered from blood and cerebrospinal fluid had no detectable catalase activity, a characteristic used for primary identification. The sporadic occurrence of pathogenic catalase-negative strains highlights the need for a reconsideration of diagnostic criteria and questions the role of catalase in the pathogenesis of listeria infection.