The 5' untranslated region-mediated enhancement of intracellular listeriolysin O production is required for Listeria monocytogenes pathogenicity

Rli51 Attenuates Transcription of the Listeria Pathogenicity Island 1 Gene mpl and Functions as a Trans-Acting sRNA in Intracellular Bacteria

Listeria pathogenicity island 1 (LIPI-1) is a genetic region containing a cluster of genes essential for virulence of the bacterial pathogen Listeria monocytogenes. Main virulence factors in LIPI-1 include long 5' untranslated regions (5'UTRs), among which is Rli51, a small RNA (sRNA) in the 5'UTR of the Zn-metalloprotease-coding mpl. So far, Rli51 function and molecular mechanisms have remained obscure. Here, we show that Rli51 exhibits a dual mechanism of regulation, functioning as a cis- and as a trans-acting sRNA. Under nutrient-rich conditions, rli51-mpl transcription is prematurely terminated, releasing a short 121-nucleotide-long sRNA. Rli51 is predicted to function as a transcription attenuator that can fold into either a terminator or a thermodynamically more stable antiterminator. We show that the sRNA Rli21/RliI binds to a single-stranded RNA loop in Rli51, which is essential to mediate premature transcription termination, suggesting that sRNA binding could stabilize the terminator fold. During intracellular infection, rli51 transcription is increased, which generates a higher abundance of the short Rli51 sRNA and allows for transcriptional read-through into mpl. Comparative intracellular bacterial transcriptomics in rli51-null mutants and the wild-type reference strain EGD-e suggests that Rli51 upregulates iron-scavenging proteins and downregulates virulence factors from LIPI-1. MS2 affinity purification confirmed that Rli51 binds transcripts of the heme-binding protein Lmo2186 and Lmo0937 in vivo. These results prove that Rli51 functions as a trans-acting sRNA in intracellular bacteria. Our research shows a growth condition-dependent mechanism of regulation for Rli51, preventing unintended mpl transcription in extracellular bacteria and regulating genes important for virulence in intracellular bacteria.

RECON gene disruption enhances host resistance to enable genome-wide evaluation of intracellular pathogen fitness during infection

Transposon sequencing (Tn-seq) is a powerful genome-wide technique to assess bacterial fitness under varying growth conditions. However, screening via Tn-seq in vivo is challenging. Dose limitations and host restrictions create bottlenecks that diminish the transposon mutant pool being screened. Here, we have developed a murine model with a disruption in Akr1c13 that renders the resulting RECON-/- mouse resistant to high-dose infection. We leveraged this model to perform a Tn-seq screen of the human pathogen Listeria monocytogenes in vivo. We identified 135 genes which were required for L. monocytogenes growth in mice including novel genes not previously identified for host survival. We identified organ-specific requirements for L. monocytogenes survival and investigated the role of the folate enzyme FolD in L. monocytogenes liver pathogenesis. A mutant lacking folD was impaired for growth in murine livers by 2.5-log10 compared to wild type and failed to spread cell-to-cell in fibroblasts. In contrast, a mutant in alsR, which encodes a transcription factor that represses an operon involved in D-allose catabolism, was attenuated in both livers and spleens of mice by 4-log10 and 3-log10, respectively, but showed modest phenotypes in in vitro models. We confirmed that dysregulation of the D-allose catabolism operon is responsible for the in vivo growth defect, as deletion of the operon in the ∆alsR background rescued virulence. By undertaking an unbiased, genome-wide screen in mice, we have identified novel fitness determinants for L. monocytogenes host infection, which highlights the utility of the RECON-/- mouse model for future screening efforts.

IMPORTANCE

Listeria monocytogenes is the gram-positive bacterium responsible for the food-borne disease listeriosis. Although infections with L. monocytogenes are limiting in healthy hosts, vulnerable populations, including pregnant and elderly people, can experience high rates of mortality. Thus, understanding the breadth of genetic requirements for L. monocytogenes in vivo survival will present new opportunities for treatment and prevention of listeriosis. We developed a murine model of infection using a RECON-/- mouse that is restrictive to systemic L. monocytogenes infection. We utilized this model to screen for L. monocytogenes genes required in vivo via transposon sequencing. We identified the liver-specific gene folD and a repressor, alsR, that only exhibits an in vivo growth defect. AlsR controls the expression of the D-allose operon which is a marker in diagnostic techniques to identify pathogenic Listeria. A better understanding of the role of the D-allose operon in human disease may further inform diagnostic and prevention measures.

The major role of Listeria monocytogenes folic acid metabolism during infection is the generation of N-formylmethionine

IMPORTANCE

Folic acid is an essential vitamin for bacteria, plants, and animals. The lack of folic acid leads to various consequences such as a shortage of amino acids and nucleotides that are fundamental building blocks for life. Though antifolate drugs are widely used for antimicrobial treatments, the underlying mechanism of bacterial folate deficiency during infection is unclear. This study compares the requirements of different folic acid end-products during the infection of Listeria monocytogenes, a facultative intracellular pathogen of animals and humans. The results reveal the critical importance of N-formylmethionine, the amino acid used by bacteria to initiate protein synthesis. This work extends the current understanding of folic acid metabolism in pathogens and potentially provides new insights into antifolate drug development in the future.

Characterization of an Autoinducing Peptide Signal Reveals Highly Efficacious Synthetic Inhibitors and Activators of Quorum Sensing and Biofilm Formation in Listeria monocytogenes

Bacteria can use chemical signals to assess their local population density in a process called quorum sensing (QS). Many of these bacteria are common pathogens, including Gram-positive bacteria that utilize agr QS systems regulated by macrocyclic autoinducing peptide (AIP) signals. Listeria monocytogenes, an important foodborne pathogen, uses an agr system to regulate a variety of virulence factors and biofilm formation, yet little is known about the specific roles of agr in Listeria infection and its persistence in various environments. Herein, we report synthetic peptide tools that will enable the study of QS in Listeria. We identified a 6-mer AIP signal in L. monocytogenes supernatants and selected it as a scaffold around which a collection of non-native AIP mimics was designed and synthesized. These peptides were evaluated in cell-based agr reporter assays to generate structure-activity relationships for AIP-based agonism and antagonism in L. monocytogenes. We discovered synthetic agonists with increased potency relative to native AIP and a synthetic antagonist capable of reducing agr activity to basal levels. Notably, the latter peptide was able to reduce biofilm formation by over 90%, a first for a synthetic QS modulator in wild-type L. monocytogenes. The lead agr agonist and antagonist in L. monocytogenes were also capable of antagonizing agr signaling in the related pathogen Staphylococcus aureus, further extending their utility and suggesting different mechanisms of agr activation in these two pathogens. This study represents an important first step in the application of chemical methods to modulate QS and concomitant virulence outcomes in L. monocytogenes.

The Virulence and Infectivity of Listeria monocytogenes Are Not Substantially Altered by Elevated SigB Activity

Listeria monocytogenes is a bacterial pathogen capable of causing severe infections but also thriving outside the host. To respond to different stress conditions, L. monocytogenes mainly utilizes the general stress response regulon, which largely is controlled by the alternative sigma factor Sigma B (SigB). In addition, SigB is important for virulence gene expression and infectivity. Upon encountering stress, a large multicomponent protein complex known as the stressosome becomes activated, ultimately leading to SigB activation. RsbX is a protein needed to reset a "stressed" stressosome and prevent unnecessary SigB activation in nonstressed conditions. Consequently, absence of RsbX leads to constitutive activation of SigB even without prevailing stress stimulus. To further examine the involvement of SigB in the virulence of this pathogen, we investigated whether a strain with constitutively active SigB would be affected in virulence factor expression and/or infectivity in cultured cells and in a chicken embryo infection model. Our results suggest that increased SigB activity does not substantially alter virulence gene expression compared with the wild-type (WT) strain at transcript and protein levels. Bacteria lacking RsbX were taken up by phagocytic and nonphagocytic cells at a similar frequency to WT bacteria, both in stressed and nonstressed conditions. Finally, the absence of RsbX only marginally affected the ability of bacteria to infect chicken embryos. Our results suggest only a minor role of RsbX in controlling virulence factor expression and infectivity under these conditions.

Ampicillin Treatment of Intracellular Listeria monocytogenes Triggers Formation of Persistent, Drug-Resistant L-Form Cells

Listeria monocytogenes is an opportunistic intracellular pathogen causing an infection termed listeriosis. Despite the low incidence of listeriosis, the high mortality rate in individuals at risk makes this bacterium one of the most dangerous foodborne pathogens. Reports about a relapse of infection after antibiotic treatment suggest that the bacteria may be able to evade antibiotic treatment and persist as a dormant, antibiotic-tolerant subpopulation. In this study, we observed intracellular generation of antibiotic-resistant L-forms of Listeria monocytogenes following Ampicillin treatment of Listeria monocytogenes infected cells. Detection and identification of intracellular Listeria L-forms was performed by a combination of fluorescence in-situ hybridization and confocal laser scanning microscopy. Using micromanipulation, it was possible to isolate single intracellular L-form cells that following transfer into fresh medium gave rise to pure cultures. In conclusion, the results obtained here provide strong evidence that antibiotic treatment of infected host cells can induce the formation of L-forms from intracellular Listeria monocytogenes. Furthermore, our results suggest that intracellular L-forms persist inside host cells and that they represent viable bacteria, which are still able to grow and proliferate.

Listeria monocytogenes requires cellular respiration for NAD+ regeneration and pathogenesis

Cellular respiration is essential for multiple bacterial pathogens and a validated antibiotic target. In addition to driving oxidative phosphorylation, bacterial respiration has a variety of ancillary functions that obscure its contribution to pathogenesis. We find here that the intracellular pathogen Listeria monocytogenes encodes two respiratory pathways which are partially functionally redundant and indispensable for pathogenesis. Loss of respiration decreased NAD+ regeneration, but this could be specifically reversed by heterologous expression of a water-forming NADH oxidase (NOX). NOX expression fully rescued intracellular growth defects and increased L. monocytogenes loads >1000-fold in a mouse infection model. Consistent with NAD+ regeneration maintaining L. monocytogenes viability and enabling immune evasion, a respiration-deficient strain exhibited elevated bacteriolysis within the host cytosol and NOX expression rescued this phenotype. These studies show that NAD+ regeneration represents a major role of L. monocytogenes respiration and highlight the nuanced relationship between bacterial metabolism, physiology, and pathogenesis.

Listeria exploits IFITM3 to suppress antibacterial activity in phagocytes

The type I interferon (IFN) signaling pathway has important functions in resistance to viral infection, with the downstream induction of interferon stimulated genes (ISG) protecting the host from virus entry, replication and spread. Listeria monocytogenes (Lm), a facultative intracellular foodborne pathogen, can exploit the type I IFN response as part of their pathogenic strategy, but the molecular mechanisms involved remain unclear. Here we show that type I IFN suppresses the antibacterial activity of phagocytes to promote systemic Lm infection. Mechanistically, type I IFN suppresses phagosome maturation and proteolysis of Lm virulence factors ActA and LLO, thereby promoting phagosome escape and cell-to-cell spread; the antiviral protein, IFN-induced transmembrane protein 3 (IFITM3), is required for this type I IFN-mediated alteration. Ifitm3^-/- mice are resistant to systemic infection by Lm, displaying decreased bacterial spread in tissues, and increased immune cell recruitment and pro-inflammatory cytokine signaling. Together, our findings show how an antiviral mechanism in phagocytes can be exploited by bacterial pathogens, and implicate IFITM3 as a potential antimicrobial therapeutic target.

Evolution of Listeria monocytogenes During a Persistent Human Prosthetic Hip Joint Infection

Listeria monocytogenes associated prosthetic joint infections (PJI) are a rare but increasing clinical problem of listeriosis. We characterized two isolates of the same L. monocytogenes strain isolated within five years of each other from a recurrent human prosthetic joint infection. The two isolates although clonally identical were phenotypically distinct confirming that the original infection strain had evolved within the human host PJI environment giving rise to a phenotypically distinct variant. The recurrent PJI isolate displayed various phenotypic differences compared to the parental original PJI isolate including diminished growth and carbon source metabolism, as well as altered morphology and increased stress sensitivity. The PJI isolates were both diminished in virulence due to an identical truncation mutation in the major virulence regulator PrfA. Genome wide sequence comparison provided conclusive evidence that the two isolates were identical clonal descendants of the same L. monocytogenes strain that had evolved through acquisition of various single nucleotide polymorphisms (SNPs) as well as insertion and deletion events (InDels) during a persistent human PJI. Acquired genetic changes included a specific mutation causing premature stop codon (PMSC) and truncation of RNAse J1 protein. Based on analysis of this naturally truncated as well as other complete RNAse J1 deletion mutants we show that the long-term survival of this specific L. monocytogenes strain within the prosthetic joint might in part be explained by the rnjA PMSC mutation that diminishes virulence and activation of the host immune system in a zebrafish embryo localized infection model. Overall our analysis of this special natural case provides insights into random mutation events and molecular mechanisms that might be associated with the adaptation and short-term evolution of this specific L. monocytogenes strain within a persistent human PJI environment.

Rapidly dynamic host cell heterogeneity in bacterial adhesion governs susceptibility to infection by Listeria monocytogenes

Interactions between host cells and individual pathogenic bacteria determine the clinical severity of disease during systemic infection in humans. Vascular endothelial cells, which line the lumen of blood vessels, represent a critical barrier for a bacterium in the bloodstream. These cells adopt a myriad of phenotypes that may modulate their susceptibility to infection; however, the precise determinants of their heterogeneity in susceptibility are not known. Here, we show that heterogeneity in susceptibility to Listeria monocytogenes infection among primary human vascular endothelial cells can be attributed entirely to robust, preexisting host cell heterogeneity in bacterial adhesion, and we find no evidence for significant heterogeneity in later steps of infection. High susceptibility to adhesion decays rapidly, within 30–60 min. Thus, rapidly fluctuating, nongenetic variability in bacterial adhesion diversifies susceptibility to infection, both among host cells and within individual cells over time.

Direct binding of polymeric GBP1 to LPS disrupts bacterial cell envelope functions

In the outer membrane of gram‐negative bacteria, O‐antigen segments of lipopolysaccharide (LPS) form a chemomechanical barrier, whereas lipid A moieties anchor LPS molecules. Upon infection, human guanylate binding protein‐1 (hGBP1) colocalizes with intracellular gram‐negative bacterial pathogens, facilitates bacterial killing, promotes activation of the lipid A sensor caspase‐4, and blocks actin‐driven dissemination of the enteric pathogen Shigella. The underlying molecular mechanism for hGBP1's diverse antimicrobial functions is unknown. Here, we demonstrate that hGBP1 binds directly to LPS and induces “detergent‐like” LPS clustering through protein polymerization. Binding of polymerizing hGBP1 to the bacterial surface disrupts the O‐antigen barrier, thereby unmasking lipid A, eliciting caspase‐4 recruitment, enhancing antibacterial activity of polymyxin B, and blocking the function of the Shigella outer membrane actin motility factor IcsA. These findings characterize hGBP1 as an LPS‐binding surfactant that destabilizes the rigidity of the outer membrane to exert pleiotropic effects on the functionality of gram‐negative bacterial cell envelopes.

RNAi screen reveals a role for PACSIN2 and caveolins during bacterial cell-to-cell spread

Listeria monocytogenes is a human bacterial pathogen that disseminates through host tissues using a process called cell-to-cell spread. This critical yet understudied virulence strategy resembles a vesicular form of intercellular trafficking that allows L. monocytogenes to move between host cells without escaping the cell. Interestingly, eukaryotic cells can also directly exchange cellular components via intercellular communication pathways (e.g., trans-endocytosis) using cell-cell adhesion, membrane trafficking, and membrane remodeling proteins. Therefore, we hypothesized that L. monocytogenes would hijack these types of host proteins during spread. Using a focused RNA interference screen, we identified 22 host genes that are important for L. monocytogenes spread. We then found that caveolins (CAV1 and CAV2) and the membrane sculpting F-BAR protein PACSIN2 promote L. monocytogenes protrusion engulfment during spread, and that PACSIN2 specifically localizes to protrusions. Overall, our study demonstrates that host intercellular communication pathways may be coopted during bacterial spread and that specific trafficking and membrane remodeling proteins promote bacterial protrusion resolution.

Regulation of Listeria monocytogenes Virulence

Whereas obligate human and animal bacterial pathogens may be able to depend upon the warmth and relative stability of their chosen replication niche, environmental bacteria such as Listeria monocytogenes that harbor the ability to replicate both within animal cells and in the outside environment must maintain the capability to manage life under a variety of disparate conditions. Bacterial life in the outside environment requires adaptation to wide ranges of temperature, available nutrients, and physical stresses such as changes in pH and osmolarity as well as desiccation. Following ingestion by a susceptible animal host, the bacterium must adapt to similar changes during transit through the gastrointestinal tract and overcome a variety of barriers associated with host innate immune responses. Rapid alteration of patterns of gene expression and protein synthesis represent one strategy for quickly adapting to a dynamic host landscape. Here, we provide an overview of the impressive variety of strategies employed by the soil-dwelling, foodborne, mammalian pathogen L. monocytogenes to straddle diverse environments and optimize bacterial fitness both inside and outside host cells.

Listeriolysin O: A phagosome-specific cytolysin revisited

Listeriolysin O (LLO) is an essential determinant of Listeria monocytogenes pathogenesis that mediates the escape of L. monocytogenes from host cell vacuoles, thereby allowing replication in the cytosol without causing appreciable cell death. As a member of the cholesterol-dependent cytolysin (CDC) family of pore-forming toxins, LLO is unique in that it is secreted by a facultative intracellular pathogen, whereas all other CDCs are produced by pathogens that are largely extracellular. Replacement of LLO with other CDCs results in strains that are extremely cytotoxic and 10,000-fold less virulent in mice. LLO has structural and regulatory features that allow it to function intracellularly without causing cell death, most of which map to a unique N-terminal region of LLO referred to as the proline, glutamic acid, serine, threonine (PEST)-like sequence. Yet, while LLO has unique properties required for its intracellular site of action, extracellular LLO, like other CDCs, affects cells in a myriad of ways. Because all CDCs form pores in cholesterol-containing membranes that lead to rapid Ca²⁺ influx and K⁺ efflux, they consequently trigger a wide range of host cell responses, including mitogen-activated protein kinase activation, histone modification, and caspase-1 activation. There is no debate that extracellular LLO, like all other CDCs, can stimulate multiple cellular activities, but the primary question we wish to address in this perspective is whether these activities contribute to L. monocytogenes pathogenesis.

An ATG16L1-dependent pathway promotes plasma membrane repair and limits Listeria monocytogenes cell-to-cell spread

Plasma membrane integrity is essential for the viability of eukaryotic cells. In response to bacterial pore-forming toxins, disrupted regions of the membrane are rapidly repaired. However, the pathways that mediate plasma membrane repair are unclear. Here we show that autophagy-related (ATG) protein ATG16L1 and its binding partners ATG5 and ATG12 are required for plasma membrane repair through a pathway independent of macroautophagy. ATG16L1 is required for lysosome fusion with the plasma membrane and blebbing responses that promote membrane repair. ATG16L1 deficiency causes accumulation of cholesterol in lysosomes that contributes to defective membrane repair. Cell-to-cell spread by Listeria monocytogenes requires membrane damage by the bacterial toxin listeriolysin O, which is restricted by ATG16L1-dependent membrane repair. Cells harbouring the ATG16L1 T300A allele associated with inflammatory bowel disease were also found to accumulate cholesterol and be defective in repair, linking a common inflammatory disease to plasma membrane integrity. Thus, plasma membrane repair could be an important therapeutic target for the treatment of bacterial infections and inflammatory disorders.

The VirAB ABC Transporter Is Required for VirR Regulation of Listeria monocytogenes Virulence and Resistance to Nisin

Listeria monocytogenes is a Gram-positive intracellular pathogen that causes a severe invasive disease. Upon infecting a host cell, L. monocytogenes upregulates the transcription of numerous factors necessary for productive infection. VirR is the response regulator component of a two-component regulatory system in L. monocytogenes In this report, we have identified the putative ABC transporter encoded by genes lmo1746-lmo1747 as necessary for VirR function. We have designated lmo1746-lmo1747 virAB We constructed an in-frame deletion of virAB and determined that the ΔvirAB mutant exhibited reduced transcription of VirR-regulated genes. The ΔvirAB mutant also showed defects in in vitro plaque formation and in vivo virulence that were similar to those of a ΔvirR deletion mutant. Since VirR is important for innate resistance to antimicrobial agents, we determined the MICs of nisin and bacitracin for ΔvirAB bacteria. We found that VirAB expression was necessary for nisin resistance but was dispensable for resistance to bacitracin. This result suggested a VirAB-independent mechanism of VirR regulation in response to bacitracin. Lastly, we found that the ΔvirR and ΔvirAB mutants had no deficiency in growth in broth culture, intracellular replication, or production of the ActA surface protein, which facilitates actin-based motility and cell-to-cell spread. However, the ΔvirR and ΔvirAB mutants produced shorter actin tails during intracellular infection, which suggested that these mutants have a reduced ability to move and spread via actin-based motility. These findings have demonstrated that L. monocytogenes VirAB functions in a pathway with VirR to regulate the expression of genes necessary for virulence and resistance to antimicrobial agents.

Detection of Cytosolic Shigella flexneri via a C-Terminal Triple-Arginine Motif of GBP1 Inhibits Actin-Based Motility

Dynamin-like guanylate binding proteins (GBPs) are gamma interferon (IFN-γ)-inducible host defense proteins that can associate with cytosol-invading bacterial pathogens. Mouse GBPs promote the lytic destruction of targeted bacteria in the host cell cytosol, but the antimicrobial function of human GBPs and the mechanism by which these proteins associate with cytosolic bacteria are poorly understood. Here, we demonstrate that human GBP1 is unique among the seven human GBP paralogs in its ability to associate with at least two cytosolic Gram-negative bacteria, Burkholderia thailandensis and Shigella flexneri Rough lipopolysaccharide (LPS) mutants of S. flexneri colocalize with GBP1 less frequently than wild-type S. flexneri does, suggesting that host recognition of O antigen promotes GBP1 targeting to Gram-negative bacteria. The targeting of GBP1 to cytosolic bacteria, via a unique triple-arginine motif present in its C terminus, promotes the corecruitment of four additional GBP paralogs (GBP2, GBP3, GBP4, and GBP6). GBP1-decorated Shigella organisms replicate but fail to form actin tails, leading to their intracellular aggregation. Consequentially, the wild type but not the triple-arginine GBP1 mutant restricts S. flexneri cell-to-cell spread. Furthermore, human-adapted S. flexneri, through the action of one its secreted effectors, IpaH9.8, is more resistant to GBP1 targeting than the non-human-adapted bacillus B. thailandensis These studies reveal that human GBP1 uniquely functions as an intracellular "glue trap," inhibiting the cytosolic movement of normally actin-propelled Gram-negative bacteria. In response to this powerful human defense program, S. flexneri has evolved an effective counterdefense to restrict GBP1 recruitment.IMPORTANCE Several pathogenic bacterial species evolved to invade, reside in, and replicate inside the cytosol of their host cells. One adaptation common to most cytosolic bacterial pathogens is the ability to coopt the host's actin polymerization machinery in order to generate force for intracellular movement. This actin-based motility enables Gram-negative bacteria, such as Shigella species, to propel themselves into neighboring cells, thereby spreading from host cell to host cell without exiting the intracellular environment. Here, we show that the human protein GBP1 acts as a cytosolic "glue trap," capturing cytosolic Gram-negative bacteria through a unique protein motif and preventing disseminated infections in cell culture models. To escape from this GBP1-mediated host defense, Shigella employs a virulence factor that prevents or dislodges the association of GBP1 with cytosolic bacteria. Thus, therapeutic strategies to restore GBP1 binding to Shigella may lead to novel treatment options for shigellosis in the future.

Cold-Shock Domain Family Proteins (Csps) Are Involved in Regulation of Virulence, Cellular Aggregation, and Flagella-Based Motility in Listeria monocytogenes

Cold shock-domain family proteins (Csps) are highly conserved nucleic acid binding proteins regulating the expression of various genes including those involved in stress resistance and virulence in bacteria. We show here that Csps are involved in virulence, cell aggregation and flagella-based extracellular motility of Listeria monocytogenes. A L. monocytogenes mutant deleted in all three csp genes (ΔcspABD) is attenuated with respect to human macrophage infection as well as virulence in a zebrafish infection model. Moreover, this mutant is incapable of aggregation and fails to express surface flagella or exhibit swarming motility. An evaluation of double csp gene deletion mutant (ΔcspBD, ΔcspAD and ΔcspAB) strains that produce single csp genes showed that there is redundancy as well as functional differences among the three L. monocytogenes Csps in their contributions to virulence, cellular aggregation, flagella production, and swarming motility. Protein and mRNA expression analysis further showed impaired expression of key virulence and motility genes in the csp mutants. Our observations at protein and mRNA level suggest Csp-dependent expression regulation of these genes at transcriptional and post-transcriptional levels. In a mutant lacking all csp genes (ΔcspABD) as well as those possessing single csp genes (ΔcspBD, ΔcspAD, and ΔcspAB) we detected reduced levels of proteins or activity as well as transcripts from the prfA, hly, mpl, and plcA genes suggesting a Csp-dependent transcriptional regulation of these genes. These csp mutants also had reduced or completely lacked ActA proteins and cell surface flagella but contained elevated actA and flaA mRNA levels compared to the parental wild type strain suggesting Csp involvement in post-transcriptional regulation of these genes. Overall, our results suggest that Csps contribute to the expression regulation of virulence and flagella-associated genes thereby promoting host pathogenicity, cell aggregation and flagella-based motility processes in L. monocytogenes.

Adhesion to the host cell surface is sufficient to mediate Listeria monocytogenes entry into epithelial cells

Listeria monocytogenes invades epithelial cells by binding to the host cell receptor E-cadherin, a component of the adherens junction. E-cadherin serves primarily as an adhesive to mediate bacterial invasion; the canonical E-cadherin/catenin/F-actin complex is not required for this process.

The intestinal epithelium is the first physiological barrier breached by the Gram-positive facultative pathogen Listeria monocytogenes during an in vivo infection. Listeria monocytogenes binds to the epithelial host cell receptor E-cadherin, which mediates a physical link between the bacterium and filamentous actin (F-actin). However, the importance of anchoring the bacterium to F-actin through E-cadherin for bacterial invasion has not been tested directly in epithelial cells. Here we demonstrate that depleting αE-catenin, which indirectly links E-cadherin to F-actin, did not decrease L. monocytogenes invasion of epithelial cells in tissue culture. Instead, invasion increased due to increased bacterial adhesion to epithelial monolayers with compromised cell–cell junctions. Furthermore, expression of a mutant E-cadherin lacking the intracellular domain was sufficient for efficient L. monocytogenes invasion of epithelial cells. Importantly, direct biotin-mediated binding of bacteria to surface lipids in the plasma membrane of host epithelial cells was sufficient for uptake. Our results indicate that the only requirement for L. monocytogenes invasion of epithelial cells is adhesion to the host cell surface, and that E-cadherin–mediated coupling of the bacterium to F-actin is not required.

Lactococcus lactis and Lactobacillus salivarius differently modulate early immunological response of Wistar rats co-administered with Listeria monocytogenes

In the light of the increasing resistance of bacterial pathogens to antibiotics, one of the main global strategies in applied science is development of alternative treatments, which would be safe both for the host and from the environmental perspective. Accordingly, the aim of this study was to test whether two lactic acid bacteria (LAB) strains, Lactococcus lactis BGBU1-4 and Lactobacillus salivarius BGHO1, could be applied as safe supplements for Listeria infection. Two major research objectives were set: to compare the effects of BGBU1-4 and BGHO1 on early immune response in gut tissue of Wistar rats co-administered with Listeria monocytogenes ATCC19111 and next, to test how this applies to their usage as therapeutics in acute ATCC19111 infection. Intestinal villi (IV), Peyer's patches (PP) and mesenteric lymph nodes (MLN) were used for the analysis. The results showed that BGHO1 increased the mRNA expression of innate immune markers CD14, interleukin (IL)-1β and tumour necrosis factor (TNF)-α in PP and IV, and, in parallel, caused a decrease of listeriolysin O (LLO) mRNA expression in same tissues. In MLN of BGHO1 treated rats, LLO expression was increased, along with an increase of the expression of OX-62 mRNA and CD69, pointing to the activation of adaptive immunity. On the other hand, in BGBU1-4 treated rats, there was no reduction of LLO mRNA expression and no induction of innate immunity markers in intestinal tissue. Additionally, CD14 and IL-1β, as well as LLO, but not OX-62 mRNA and CD69 expression, were elevated in MLN of BGBU1-4 treated rats. However, when applied therapeutically, both, BGBU1-4 and BGHO1, lowered Listeria count in spleens of infected rats. Our results not only reveal the potential of LAB to ameliorate Listeria infections, but suggest different immunological effects of two different LAB strains, both of which could be effective in Listeria elimination.

Limited Colonization Undermined by Inadequate Early Immune Responses Defines the Dynamics of Decidual Listeriosis

The bacterial pathogen Listeria monocytogenes causes foodborne systemic disease in pregnant women, which can lead to preterm labor, stillbirth, or severe neonatal disease. Colonization of the maternal decidua appears to be an initial step in the maternal component of the disease as well as bacterial transmission to the placenta and fetus. Host-pathogen interactions in the decidua during this early stage of infection remain poorly understood. Here, we assessed the dynamics of L. monocytogenes infection in primary human decidual organ cultures and in the murine decidua in vivo A high inoculum was necessary to infect both human and mouse deciduas, and the data support the existence of a barrier to initial colonization of the murine decidua. If successful, however, colonization in both species was followed by significant bacterial expansion associated with an inability of the decidua to mount appropriate innate cellular immune responses. The innate immune deficits included the failure of bacterial foci to attract macrophages and NK cells, cell types known to be important for early defenses against L. monocytogenes in the spleen, as well as a decrease in the tissue density of inflammatory Ly6C^hi monocytes in vivo These results suggest that the infectivity of the decidua is not the result of an enhanced recruitment of L. monocytogenes to the gestational uterus but rather is due to compromised local innate cellular immune responses.

Dual functionality nanobioconjugates targeting intracellular bacteria in cancer cells with enhanced antimicrobial activity

Bacterial drug resistance has emerged as a serious global threat mandating the development of novel methodologies that allow facile modulation of antimicrobial action in a controlled fashion. Conjugating antibiotics to nanoparticles helps to meet this goal by increasing the drug’s overall avidity, bioavailability and easier internalisation into mammalian cells, targeting bacteria that otherwise escape antibacterial action by host cell-localisation. We used polymyxin B sulfate (PMB) and sushi peptide as model drugs against Gram-negative bacteria and established their enhanced antimicrobial activity on Escherichia coli (E. coli) cells after conjugation to gold nanoparticles (AuNPs). The efficacy of the bioconjugates was also tested on Salmonella typhi (S. typhi) bacteria infected into cervical cancer cells (HeLa) and further improved through specific targeting via folate receptors. Our results demonstrate significantly lower inhibitory concentration values for sushi-NP assemblies as compared to free drug, especially at optimal drug loading levels. No major cytotoxicity was observed in mammalian cells alone.

Listeria monocytogenes Internalizes in Romaine Lettuce Grown in Greenhouse Conditions

Listeria monocytogenes has been implicated in a number of outbreaks involving fresh produce, including an outbreak in 2016 resulting from contaminated packaged salads. The persistence and internalization potential of L. monocytogenes in romaine lettuce was evaluated, and the persistence of two L. monocytogenes strains was assessed on three romaine lettuce cultivars. Seeds were germinated, and plants grown in three soil types (i.e., standard potting mix, autoclaved potting mix, and top soil) and sterile soft-top agar for up to 21 days. Average CFU per gram of L. monocytogenes on seeds and plants was calculated from five replicates per harvest day. Up to 8.2 log CFU/g L. monocytogenes persisted on romaine lettuce plants (Braveheart cultivar) grown in soft-top agar, while those grown in commercial potting mix (initial soil aerobic plate count of 4.0 × 104 CFU/g) had a final concentration of 5.4 log CFU/g, and autoclaved commercial potting mix had a final concentration of 3.8 ± 0.2 log CFU/g after a 21-day period. Pathogen levels dropped below the limit of detection (2 log CFU/g) by day 18 in 75% topsoil (initial soil aerobic plate count of 4.0 × 101 CFU/g); this did not occur in sterile media. Although L. monocytogenes strain differences and presence of a clay coating on seeds did not affect persistence, differences were observed in L. monocytogenes growth and survival among cultivars. To assess internalization, seeds were inoculated with L. monocytogenes expressing green fluorescent protein. Three plants were fixed, paraffin embedded, and sectioned; localization was studied by using standard immunohistochemistry techniques. A total of 539 internalized L. monocytogenes cells were visualized among three 20-day seedlings. L. monocytogenes cells were located in all major tissue types (pith followed by cortex, xylem, phloem, and epidermis). The presence of L. monocytogenes in the plant vasculature suggests potential for transport throughout the plant into edible tissue.

An Effective Counterselection System for Listeria monocytogenes and Its Use To Characterize the Monocin Genomic Region of Strain 10403S

Construction of Listeria monocytogenes mutants by allelic exchange has been laborious and time-consuming due to lack of proficient selection markers for the final recombination event, that is, a marker conveying substance sensitivity to the bacteria bearing it, enabling the exclusion of merodiploids and selection for plasmid loss. In order to address this issue, we engineered a counterselection marker based on a mutated phenylalanyl-tRNA synthetase gene (pheS*). This mutation renders the phenylalanine-binding site of the enzyme more promiscuous and allows the binding of the toxic p-chloro-phenylalanine analog (p-Cl-phe) as a substrate. When pheS* is introduced into L. monocytogenes and highly expressed under control of a constitutively active promoter, the bacteria become sensitive to p-Cl-phe supplemented in the medium. This enabled us to utilize pheS* as a negative selection marker and generate a novel, efficient suicide vector for allelic exchange in L. monocytogenes We used this vector to investigate the monocin genomic region in L. monocytogenes strain 10403S by constructing deletion mutants of the region. We have found this region to be active and to cause bacterial lysis upon mitomycin C treatment. The future applications of such an effective counterselection system, which does not require any background genomic alterations, are vast, as it can be modularly used in various selection systems (e.g., genetic screens). We expect this counterselection marker to be a valuable genetic tool in research on L. monocytogenesIMPORTANCEL. monocytogenes is an opportunistic intracellular pathogen and a widely studied model organism. An efficient counterselection marker is a long-standing need in Listeria research for improving the ability to design and perform various genetic manipulations and screening systems for different purposes. We report the construction and utilization of an efficient suicide vector for allelic exchange which can be conjugated, leaves no marker in the bacterial chromosome, and does not require the use of sometimes leaky inducible promoters. This highly efficient genome editing tool for L. monocytogenes will allow for rapid sequential mutagenesis, introduction of point mutations, and design of screening systems. We anticipate that it will be extensively used by the research community and yield novel insights into the diverse fields studied using this model organism.

Rickettsia Sca4 Reduces Vinculin-Mediated Intercellular Tension to Promote Spread

Spotted fever group (SFG) rickettsiae are human pathogens that infect cells in the vasculature. They disseminate through host tissues by a process of cell-to-cell spread that involves protrusion formation, engulfment, and vacuolar escape. Other bacterial pathogens rely on actin-based motility to provide a physical force for spread. Here, we show that SFG species Rickettsia parkeri typically lack actin tails during spread and instead manipulate host intercellular tension and mechanotransduction to promote spread. Using transposon mutagenesis, we identified surface cell antigen 4 (Sca4) as a secreted effector of spread that specifically promotes protrusion engulfment. Sca4 interacts with the cell-adhesion protein vinculin and blocks association with vinculin's binding partner, α-catenin. Using traction and monolayer stress microscopy, we show that Sca4 reduces vinculin-dependent mechanotransduction at cell-cell junctions. Our results suggest that Sca4 relieves intercellular tension to promote protrusion engulfment, which represents a distinctive strategy for manipulating cytoskeletal force generation to enable spread.

Proliferation of Listeria monocytogenes L-form cells by formation of internal and external vesicles

L-forms are cell wall-deficient bacteria that divide through unusual mechanisms, involving dynamic perturbations of the cellular shape and generation of vesicles, independently of the cell-division protein FtsZ. Here we describe FtsZ-independent mechanisms, involving internal and external vesicles, by which Listeria monocytogenesL-forms proliferate. Using micromanipulation of single cells and vesicles, we show that small vesicles are formed by invagination within larger intracellular vesicles, receive cytoplasmic content, and represent viable progeny. In addition, the L-forms can reproduce by pearling, that is, generation of extracellular vesicles that remain transiently linked to their mother cell via elastic membranous tubes. Using photobleaching and fluorescence recovery, we demonstrate cytoplasmic continuity and transfer through these membranous tubes. Our findings indicate that L-forms' polyploidy and extended interconnectivity through membranous tubes contribute to the generation of viable progeny independently of dedicated division machinery, and further support L-forms as models for studies of potential multiplication mechanisms of hypothetical primitive cells.

L-forms are cell wall-deficient bacteria that divide through unusual mechanisms, potentially resembling those of primitive cells. Here the authors describe how Listeria monocytogenesL-forms proliferate by generation of internal and external vesicles.

Control of Candida albicans morphology and pathogenicity by post-transcriptional mechanisms

Candida albicans is a major human fungal pathogen responsible for both systemic and mucosal infections in a wide variety of immunocompromised individuals. Because the ability of C. albicans to undergo a reversible morphological transition from yeast to filaments is important for virulence, significant research efforts have focused on mechanisms that control this transition. While transcriptional and post-translational mechanisms have been well-studied, considerably less is known about the role of post-transcriptional mechanisms. However, in recent years several discoveries have begun to shed light on this important, but understudied, area. Here, I will review a variety of post-transcriptional mechanisms that have recently been shown to control C. albicans morphology, virulence and/or virulence-related processes, including those involving alternative transcript localization, mRNA stability and translation. I will also discuss the role that these mechanisms play in other pathogens as well as the potential they may hold to serve as targets for new antifungal strategies. Ultimately, gaining a better understanding of C. albicans post-transcriptional mechanisms will significantly improve our knowledge of how morphogenesis and virulence are controlled in fungal pathogens and open new avenues for the development of novel and more effective antifungals.

An In Vivo Selection Identifies Listeria monocytogenes Genes Required to Sense the Intracellular Environment and Activate Virulence Factor Expression

Listeria monocytogenes is an environmental saprophyte and facultative intracellular bacterial pathogen with a well-defined life-cycle that involves escape from a phagosome, rapid cytosolic growth, and ActA-dependent cell-to-cell spread, all of which are dependent on the master transcriptional regulator PrfA. The environmental cues that lead to temporal and spatial control of L. monocytogenes virulence gene expression are poorly understood. In this study, we took advantage of the robust up-regulation of ActA that occurs intracellularly and expressed Cre recombinase from the actA promoter and 5' untranslated region in a strain in which loxP sites flanked essential genes, so that activation of actA led to bacterial death. Upon screening for transposon mutants that survived intracellularly, six genes were identified as necessary for ActA expression. Strikingly, most of the genes, including gshF, spxA1, yjbH, and ohrA, are predicted to play important roles in bacterial redox regulation. The mutants identified in the genetic selection fell into three broad categories: (1) those that failed to reach the cytosolic compartment; (2) mutants that entered the cytosol, but failed to activate the master virulence regulator PrfA; and (3) mutants that entered the cytosol and activated transcription of actA, but failed to synthesize it. The identification of mutants defective in vacuolar escape suggests that up-regulation of ActA occurs in the host cytosol and not the vacuole. Moreover, these results provide evidence for two non-redundant cytosolic cues; the first results in allosteric activation of PrfA via increased glutathione levels and transcriptional activation of actA while the second results in translational activation of actA and requires yjbH. Although the precise host cues have not yet been identified, we suggest that intracellular redox stress occurs as a consequence of both host and pathogen remodeling their metabolism upon infection.

RNA- and protein-mediated control of Listeria monocytogenes virulence gene expression

The model opportunistic pathogen Listeria monocytogenes has been the object of extensive research, aiming at understanding its ability to colonize diverse environmental niches and animal hosts. Bacterial transcriptomes in various conditions reflect this efficient adaptability. We review here our current knowledge of the mechanisms allowing L. monocytogenes to respond to environmental changes and trigger pathogenicity, with a special focus on RNA-mediated control of gene expression. We highlight how these studies have brought novel concepts in prokaryotic gene regulation, such as the ‘excludon’ where the 5′-UTR of a messenger also acts as an antisense regulator of an operon transcribed in opposite orientation, or the notion that riboswitches can regulate non-coding RNAs to integrate complex metabolic stimuli into regulatory networks. Overall, the Listeria model exemplifies that fine RNA tuners act together with master regulatory proteins to orchestrate appropriate transcriptional programmes.

Comparative Bioinformatics and Experimental Analysis of the Intergenic Regulatory Regions of Bacillus cereus hbl and nhe Enterotoxin Operons and the Impact of CodY on Virulence Heterogeneity

Bacillus cereus is a food contaminant with greatly varying enteropathogenic potential. Almost all known strains harbor the genes for at least one of the three enterotoxins Nhe, Hbl, and CytK. While some strains show no cytotoxicity, others have caused outbreaks, in rare cases even with lethal outcome. The reason for these differences in cytotoxicity is unknown. To gain insight into the origin of enterotoxin expression heterogeneity in different strains, the architecture and role of 5′ intergenic regions (5′ IGRs) upstream of the nhe and hbl operons was investigated. In silico comparison of 142 strains of all seven phylogenetic groups of B. cereus sensu lato proved the presence of long 5′ IGRs upstream of the nheABC and hblCDAB operons, which harbor recognition sites for several transcriptional regulators, including the virulence regulator PlcR, redox regulators ResD and Fnr, the nutrient-sensitive regulator CodY as well as the master regulator for biofilm formation SinR. By determining transcription start sites, unusually long 5′ untranslated regions (5′ UTRs) upstream of the nhe and hbl start codons were identified, which are not present upstream of cytK-1 and cytK-2. Promoter fusions lacking various parts of the nhe and hbl 5′ UTR in B. cereus INRA C3 showed that the entire 331 bp 5′ UTR of nhe is necessary for full promoter activity, while the presence of the complete 606 bp hbl 5′ UTR lowers promoter activity. Repression was caused by a 268 bp sequence directly upstream of the hbl transcription start. Luciferase activity of reporter strains containing nhe and hbl 5′ IGR lux fusions provided evidence that toxin gene transcription is upregulated by the depletion of free amino acids. Electrophoretic mobility shift assays showed that the branched-chain amino acid sensing regulator CodY binds to both nhe and hbl 5′ UTR downstream of the promoter, potentially acting as a nutrient-responsive roadblock repressor of toxin gene transcription. PlcR binding sites are highly conserved among all B. cereus sensu lato strains, indicating that this regulator does not significantly contribute to the heterogeneity in virulence potentials. The CodY recognition sites are far less conserved, perhaps conferring varying strengths of CodY binding, which might modulate toxin synthesis in a strain-specific manner.

Endothelial Cells Use a Formin-Dependent Phagocytosis-Like Process to Internalize the Bacterium Listeria monocytogenes

Vascular endothelial cells act as gatekeepers that protect underlying tissue from blood-borne toxins and pathogens. Nevertheless, endothelial cells are able to internalize large fibrin clots and apoptotic debris from the bloodstream, although the precise mechanism of such phagocytosis-like uptake is unknown. We show that cultured primary human endothelial cells (HUVEC) internalize both pathogenic and non-pathogenic Listeria bacteria comparably, in a phagocytosis-like process. In contrast with previously studied host cell types, including intestinal epithelial cells and hepatocytes, we find that endothelial internalization of Listeria is independent of all known pathogenic bacterial surface proteins. Consequently, we exploited the internalization and intracellular replication of L. monocytogenes to identify distinct host cell factors that regulate phagocytosis-like uptake in HUVEC. Using siRNA screening and subsequent genetic and pharmacologic perturbations, we determined that endothelial infectivity was modulated by cytoskeletal proteins that normally modulate global architectural changes, including phosphoinositide-3-kinase, focal adhesions, and the small GTPase Rho. We found that Rho kinase (ROCK) is acutely necessary for adhesion of Listeria to endothelial cells, whereas the actin-nucleating formins FHOD1 and FMNL3 specifically regulate internalization of bacteria as well as inert beads, demonstrating that formins regulate endothelial phagocytosis-like uptake independent of the specific cargo. Finally, we found that neither ROCK nor formins were required for macrophage phagocytosis of L. monocytogenes, suggesting that endothelial cells have distinct requirements for bacterial internalization from those of classical professional phagocytes. Our results identify a novel pathway for L. monocytogenes uptake by human host cells, indicating that this wily pathogen can invade a variety of tissues by using a surprisingly diverse suite of distinct uptake mechanisms that operate differentially in different host cell types.

Tolerance is established in polyclonal CD4(+) T cells by distinct mechanisms, according to self-peptide expression patterns

Studies of repertoires of mouse monoclonal CD4(+) T cells have revealed several mechanisms of self-tolerance; however, which mechanisms operate in normal repertoires is unclear. Here we studied polyclonal CD4(+) T cells specific for green fluorescent protein expressed in various organs, which allowed us to determine the effects of specific expression patterns on the same epitope-specific T cells. Peptides presented uniformly by thymic antigen-presenting cells were tolerated by clonal deletion, whereas peptides excluded from the thymus were ignored. Peptides with limited thymic expression induced partial clonal deletion and impaired effector T cell potential but enhanced regulatory T cell potential. These mechanisms were also active for T cell populations specific for endogenously expressed self antigens. Thus, the immunotolerance of polyclonal CD4(+) T cells was maintained by distinct mechanisms, according to self-peptide expression patterns.

Listeria monocytogenes and the Inflammasome: From Cytosolic Bacteriolysis to Tumor Immunotherapy

Inflammasomes are cytosolic innate immune surveillance systems that recognize a variety of danger signals, including those from pathogens. Listeria monocytogenes is a Gram-positive intracellular bacterium evolved to live within the harsh environment of the host cytosol. Further, L. monocytogenes can activate a robust cell-mediated immune response that is being harnessed as an immunotherapeutic platform. Access to the cytosol is critical for both causing disease and inducing a protective immune response, and it is hypothesized that the cytosolic innate immune system, including the inflammasome, is critical for both host protection and induction of long-term immunity. L. monocytogenes can activate a variety of inflammasomes via its pore-forming toxin listeriolysin-O, flagellin, or DNA released through bacteriolysis; however, inflammasome activation attenuates L. monocytogenes, and as such, L. monocytogenes has evolved a variety of ways to limit inflammasome activation. Surprisingly, inflammasome activation also impairs the host cell-mediated immune response. Thus, understanding how L. monocytogenes activates or avoids detection by the inflammasome is critical to understand the pathogenesis of L. monocytogenes and improve the cell-mediated immune response generated to L. monocytogenes for more effective immunotherapies.

Listeria monocytogenes: Strain Heterogeneity, Methods, and Challenges of Subtyping

Listeria monocytogenes is a food-borne bacterial pathogen that is associated with 20% to 30% case fatality rate. L. monocytogenes is a genetically heterogeneous species, with a small fraction of strains (serotypes 1/2a, 1/2b, 4b) implicated in human listeriosis. Monitoring and source tracking of L. monocytogenes involve the use of subtyping methods, with the performance of genetic-based methods found to be superior to phenotypic-based ones. Various methods have been used to subtype L. monocytogenes isolates, with the pulsed-field gel electrophoresis (PFGE) being the gold standard. Although PFGE has had a massive impact on food safety through the establishment of the PulseNet, there is no doubt that whole genome sequence (WGS) typing is accurate, has a discriminatory power superior to any known method, and allows genome-wide differences between strains to be quantified through the comparison of nucleotide sequences. This review focuses on the different techniques that have been used to type L. monocytogenes strains, their performance challenges, and the tremendous impact WGS typing could have on the food safety landscape.

Macrophages mediate flagellin induced inflammasome activation and host defense in zebrafish

The inflammasome is an innate immune complex whose rapid inflammatory outputs play a critical role in controlling infection; however, the host cells that mediate inflammasome responses in vivo are not well defined. Using zebrafish larvae, we examined the cellular immune responses to inflammasome activation during infection. We compared the host responses with two Listeria monocytogenes strains: wild type and Lm-pyro, a strain engineered to activate the inflammasome via ectopic expression of flagellin. Infection with Lm-pyro led to activation of the inflammasome, macrophage pyroptosis and ultimately attenuation of virulence. Depletion of caspase A, the zebrafish caspase-1 homolog, restored Lm-pyro virulence. Inflammasome activation specifically recruited macrophages to infection sites, whereas neutrophils were equally recruited to wild type and Lm-pyro infections. Similar to caspase A depletion, macrophage deficiency rescued Lm-pyro virulence to wild-type levels, while defective neutrophils had no specific effect. Neutrophils were, however, important for general clearance of L. monocytogenes, as both wild type and Lm-pyro were more virulent in larvae with defective neutrophils. This study characterizes a novel model for inflammasome studies in an intact host, establishes the importance of macrophages during inflammasome responses and adds importance to the role of neutrophils in controlling L. monocytogenes infections.

A Dual Microscopy-Based Assay To Assess Listeria monocytogenes Cellular Entry and Vacuolar Escape

Listeria monocytogenes is a Gram-positive bacterium and a facultative intracellular pathogen that invades mammalian cells, disrupts its internalization vacuole, and proliferates in the host cell cytoplasm. Here, we describe a novel image-based microscopy assay that allows discrimination between cellular entry and vacuolar escape, enabling high-content screening to identify factors specifically involved in these two steps. We first generated L. monocytogenes and Listeria innocua strains expressing a β-lactamase covalently attached to the bacterial cell wall. These strains were then incubated with HeLa cells containing the Förster resonance energy transfer (FRET) probe CCF4 in their cytoplasm. The CCF4 probe was cleaved by the bacterial surface β-lactamase only in cells inoculated with L. monocytogenes but not those inoculated with L. innocua, thereby demonstrating bacterial access to the host cytoplasm. Subsequently, we performed differential immunofluorescence staining to distinguish extracellular versus total bacterial populations in samples that were also analyzed by the FRET-based assay. With this two-step analysis, bacterial entry can be distinguished from vacuolar rupture in a single experiment. Our novel approach represents a powerful tool for identifying factors that determine the intracellular niche of L. monocytogenes.

From genome to toxicity: a combinatory approach highlights the complexity of enterotoxin production in Bacillus cereus

In recent years Bacillus cereus has gained increasing importance as a food poisoning pathogen. It is the eponymous member of the B. cereus sensu lato group that consists of eight closely related species showing impressive diversity of their pathogenicity. The high variability of cytotoxicity and the complex regulatory network of enterotoxin expression have complicated efforts to predict the toxic potential of new B. cereus isolates. In this study, comprehensive analyses of enterotoxin gene sequences, transcription, toxin secretion and cytotoxicity were performed. For the first time, these parameters were compared in a whole set of B. cereus strains representing isolates of different origin (food or food poisoning outbreaks) and of different toxic potential (enteropathogenic and apathogenic) to elucidate potential starting points of strain-specific differential toxicity. While toxin gene sequences were highly conserved and did not allow for differentiation between high and low toxicity strains, comparison of nheB and hblD enterotoxin gene transcription and Nhe and Hbl protein titers revealed not only strain-specific differences but also incongruence between toxin gene transcripts and toxin protein levels. With one exception all strains showed comparable capability of protein secretion and so far, no secretion patterns specific for high and low toxicity strains were identified. These results indicate that enterotoxin expression is more complex than expected, possibly involving the orchestrated interplay of different transcriptional regulator proteins, as well as posttranscriptional and posttranslational regulatory mechanisms plus additional influences of environmental conditions.

Avoidance of autophagy mediated by PlcA or ActA is required for Listeria monocytogenes growth in macrophages

Listeria monocytogenes is a facultative intracellular pathogen that escapes from phagosomes and grows in the cytosol of infected host cells. Most of the determinants that govern its intracellular life cycle are controlled by the transcription factor PrfA, including the pore-forming cytolysin listeriolysin O (LLO), two phospholipases C (PlcA and PlcB), and ActA. We constructed a strain that lacked PrfA but expressed LLO from a PrfA-independent promoter, thereby allowing the bacteria to gain access to the host cytosol. This strain did not grow efficiently in wild-type macrophages but grew normally in macrophages that lacked ATG5, a component of the autophagy LC3 conjugation system. This strain colocalized more with the autophagy marker LC3 (42% ± 7%) at 2 h postinfection, which constituted a 5-fold increase over the colocalization exhibited by the wild-type strain (8% ± 6%). While mutants lacking the PrfA-dependent virulence factor PlcA, PlcB, or ActA grew normally, a double mutant lacking both PlcA and ActA failed to grow in wild-type macrophages and colocalized more with LC3 (38% ± 5%). Coexpression of LLO and PlcA in a PrfA-negative strain was sufficient to restore intracellular growth and decrease the colocalization of the bacteria with LC3. In a cell-free assay, purified PlcA protein blocked LC3 lipidation, a key step in early autophagosome biogenesis, presumably by preventing the formation of phosphatidylinositol 3-phosphate (PI3P). The results of this study showed that avoidance of autophagy by L. monocytogenes primarily involves PlcA and ActA and that either one of these factors must be present for L. monocytogenes growth in macrophages.

The Listeria Small RNA Rli27 Regulates a Cell Wall Protein inside Eukaryotic Cells by Targeting a Long 5'-UTR Variant

Listeria monocytogenes is a bacterial pathogen whose genome encodes many cell wall proteins that bind covalently to peptidoglycan. Some members of this protein family have a key role in virulence, and recent studies show that some of these, such as Lmo0514, are upregulated in bacteria that colonize eukaryotic cells. The regulatory mechanisms that lead to these changes in cell wall proteins remain poorly characterized. Here we studied the regulation responsible for increased Lmo0514 protein levels in intracellular bacteria. The amount of this protein increased markedly in intracellular bacteria (>200-fold), which greatly exceeded the increase in lmo0514 transcript levels (∼6-fold). Rapid amplification of 5'-cDNA ends (RACE) assays identified two lmo0514 transcripts with 5'-untranslated regions (5'-UTR) of 28 and 234 nucleotides. The transcript containing the long 5'-UTR is upregulated by intracellular bacteria. The 234-nucleotide 5'-UTR is also the target of a small RNA (sRNA) denoted Rli27, which we identified by bioinformatics analysis as having extensive base pairing potential with the long 5'-UTR. The interaction is predicted to increase accessibility of the Shine-Dalgarno sequence occluded in the long 5'-UTR and thus to promote Lmo0514 protein production inside the eukaryotic cell. Real-time quantitative PCR showed that Rli27 is upregulated in intracellular bacteria. In vivo experiments indicated a decrease in Lmo0514 protein levels in intracellular bacteria that lacked Rli27. Wild-type Lmo0514 levels were restored by expressing the wild-type Rli27 molecule but not a mutated version unable to interact with the lmo0514 long 5'-UTR. These findings emphasize how 5'-UTR length affects regulation by defined sRNA. In addition, they demonstrate how alterations in the relative abundance of two transcripts with distinct 5'-UTR confine the action of an sRNA for a specific target to bacteria that occupy the intracellular eukaryotic niche.

Listeria monocytogenes exploits efferocytosis to promote cell-to-cell spread

Efferocytosis, the process by which dying or dead cells are removed by phagocytosis, has an important role in development, tissue homeostasis and innate immunity. Efferocytosis is mediated, in part, by receptors that bind to exofacial phosphatidylserine (PS) on cells or cellular debris after loss of plasma membrane asymmetry. Here we show that a bacterial pathogen, Listeria monocytogenes, can exploit efferocytosis to promote cell-to-cell spread during infection. These bacteria can escape the phagosome in host cells by using the pore-forming toxin listeriolysin O (LLO) and two phospholipase C enzymes. Expression of the cell surface protein ActA allows L. monocytogenes to activate host actin regulatory factors and undergo actin-based motility in the cytosol, eventually leading to formation of actin-rich protrusions at the cell surface. Here we show that protrusion formation is associated with plasma membrane damage due to LLO's pore-forming activity. LLO also promotes the release of bacteria-containing protrusions from the host cell, generating membrane-derived vesicles with exofacial PS. The PS-binding receptor TIM-4 (encoded by the Timd4 gene) contributes to efficient cell-to-cell spread by L. monocytogenes in macrophages in vitro and growth of these bacteria is impaired in Timd4(-/-) mice. Thus, L. monocytogenes promotes its dissemination in a host by exploiting efferocytosis. Our results indicate that PS-targeted therapeutics may be useful in the fight against infections by L. monocytogenes and other bacteria that use similar strategies of cell-to-cell spread during infection.

A 5' UTR-mediated translational efficiency mechanism inhibits the Candida albicans morphological transition

While virulence properties of Candida albicans, the most commonly isolated human fungal pathogen, are controlled by transcriptional and post-translational mechanisms, considerably little is known about the role of post-transcriptional, and particularly translational, mechanisms. We demonstrate that UME6, a key filament-specific transcriptional regulator whose expression level is sufficient to determine C. albicans morphology and promote virulence, has one of the longest 5' untranslated regions (UTRs) identified in fungi to date, which is predicted to form a complex and extremely stable secondary structure. The 5' UTR inhibits the ability of UME6, when expressed at constitutive high levels, to drive complete hyphal growth, but does not cause a reduction in UME6 transcript. Deletion of the 5' UTR increases C. albicans filamentation under a variety of conditions but does not affect UME6 transcript level or induction kinetics. We show that the 5' UTR functions to inhibit Ume6 protein expression under several filament-inducing conditions and specifically reduces association of the UME6 transcript with polysomes. Overall, our findings suggest that translational efficiency mechanisms, known to regulate diverse biological processes in bacterial and viral pathogens as well as higher eukaryotes, have evolved to inhibit and fine-tune morphogenesis, a key virulence trait of many human fungal pathogens.

Expression of fluorescent proteins in bifidobacteria for analysis of host-microbe interactions

Bifidobacteria are an important component of the human gastrointestinal microbiota and are frequently used as probiotics. The genetic inaccessibility and lack of molecular tools commonly used in other bacteria have hampered a detailed analysis of the genetic determinants of bifidobacteria involved in their adaptation to, colonization of, and interaction with the host. In the present study, a range of molecular tools were developed that will allow the closing of some of the gaps in functional analysis of bifidobacteria. A number of promoters were tested for transcriptional activity in Bifidobacterium bifidum S17 using pMDY23, a previously published promoter probe vector. The promoter of the gap gene (Pgap) of B. bifidum S17 yielded the highest promoter activity among the promoters tested. Thus, this promoter and the pMDY23 backbone were used to construct a range of vectors for expression of different fluorescent proteins (FPs). Successful expression of cyan fluorescent protein (CFP), green fluorescent protein (GFP), yellow fluorescent protein (YFP), and mCherry could be shown for three strains representing three different Bifidobacterium spp. The red fluorescent B. bifidum S17/pVG-mCherry was further used to demonstrate application of fluorescent bifidobacteria for adhesion assays and detection in primary human macrophages cultured in vitro. Furthermore, pMGC-mCherry was cloned by combining a chloramphenicol resistance marker and expression of the FP mCherry under the control of Pgap. The chloramphenicol resistance marker of pMGC-mCherry was successfully used to determine gastrointestinal transit time of B. bifidum S17. Moreover, B. bifidum S17/pMGC-mCherry could be detected in fecal samples of mice after oral administration.

Multifaceted activity of listeriolysin O, the cholesterol-dependent cytolysin of Listeria monocytogenes

The cholesterol-dependent cytolysins (CDCs) are a large family of pore-forming toxins that are produced by numerous Gram-positive bacterial pathogens. These toxins are released in the extracellular environment as water-soluble monomers or dimers that bind to cholesterol-rich membranes and assemble into large pore complexes. Depending upon their concentration, the nature of the host cell and membrane (cytoplasmic or intracellular) they target, the CDCs can elicit many different cellular responses. Among the CDCs, listeriolysin O (LLO), which is a major virulence factor of the facultative intracellular pathogen Listeria monocytogenes, is involved in several stages of the intracellular lifecycle of the bacterium and displays unique characteristics. It has long been known that following L. monocytogenes internalization into host cells, LLO disrupts the internalization vacuole, enabling the bacterium to replicate into the host cell cytosol. LLO is then used by cytosolic bacteria to spread from cell to cell, avoiding bacterial exposure to the extracellular environment. Although LLO is continuously produced during the intracellular lifecycle of L. monocytogenes, several processes limit its toxicity to ensure the survival of infected cells. It was previously thought that LLO activity was limited to mediating vacuolar escape during bacterial entry and cell to cell spreading. This concept has been challenged by compelling evidence suggesting that LLO secreted by extracellular L. monocytogenes perforates the host cell plasma membrane, triggering important host cell responses. This chapter provides an overview of the well-established intracellular activity of LLO and the multiple roles attributed to LLO secreted by extracellular L. monocytogenes.

Rickettsia actin-based motility occurs in distinct phases mediated by different actin nucleators

Many intracellular bacterial pathogens undergo actin-based motility to promote cell-cell spread during infection [1]. For each pathogen, motility was assumed to be driven by a single actin polymerization pathway. Curiously, spotted fever group Rickettsia differ from other pathogens in possessing two actin-polymerizing proteins. RickA, an activator of the host Arp2/3 complex, was initially proposed to drive motility [2, 3]. Sca2, a mimic of host formins [4, 5], was later shown to be required for motility [6]. Whether and how their activities are coordinated has remained unclear. Here, we show that each protein directs an independent mode of Rickettsia parkeri motility at different times during infection. Early after invasion, motility is slow and meandering, generating short, curved actin tails that are enriched with Arp2/3 complex and cofilin. Early motility requires RickA and Arp2/3 complex and is correlated with transient RickA localization to the bacterial pole. Later in infection, motility is faster and directionally persistent, resulting in long, straight actin tails. Late motility is independent of Arp2/3 complex and RickA and requires Sca2, which accumulates at the bacterial pole. Both motility pathways facilitate cell-to-cell spread. The ability to exploit two actin assembly pathways may allow Rickettsia to establish an intracellular niche and spread between diverse cells throughout a prolonged infection.

A differential fluorescence-based genetic screen identifies Listeria monocytogenes determinants required for intracellular replication

Listeria monocytogenes is a Gram-positive, facultative intracellular pathogen capable of causing severe invasive disease with high mortality rates in humans. While previous studies have largely elucidated the bacterial and host cell mechanisms necessary for invasion, vacuolar escape, and subsequent cell-to-cell spread, the L. monocytogenes factors required for rapid replication within the restrictive environment of the host cell cytosol are poorly understood. In this report, we describe a differential fluorescence-based genetic screen utilizing fluorescence-activated cell sorting (FACS) and high-throughput microscopy to identify L. monocytogenes mutants defective in optimal intracellular replication. Bacteria harboring deletions within the identified gene menD or pepP were defective for growth in primary murine macrophages and plaque formation in monolayers of L2 fibroblasts, thus validating the ability of the screening method to identify intracellular replication-defective mutants. Genetic complementation of the menD and pepP deletion strains rescued the in vitro intracellular infection defects. Furthermore, the menD deletion strain displayed a general extracellular replication defect that could be complemented by growth under anaerobic conditions, while the intracellular growth defect of this strain could be complemented by the addition of exogenous menaquinone. As prior studies have indicated the importance of aerobic metabolism for L. monocytogenes infection, these findings provide further evidence for the importance of menaquinone and aerobic metabolism for L. monocytogenes pathogenesis. Lastly, both the menD and pepP deletion strains were attenuated during in vivo infection of mice. These findings demonstrate that the differential fluorescence-based screening approach provides a powerful tool for the identification of intracellular replication determinants in multiple bacterial systems.

Characterization of the promoter, MxiE box and 5' UTR of genes controlled by the activity of the type III secretion apparatus in Shigella flexneri

Activation of the type III secretion apparatus (T3SA) of Shigella flexneri, upon contact of the bacteria with host cells, and its deregulation, as in ipaB mutants, specifically increases transcription of a set of effector-encoding genes controlled by MxiE, an activator of the AraC family, and IpgC, the chaperone of the IpaB and IpaC translocators. Thirteen genes carried by the virulence plasmid (ospB, ospC1, ospD2, ospD3, ospE1, ospE2, ospF, ospG, virA, ipaH1.4, ipaH4.5, ipaH7.8 and ipaH9.8) and five genes carried by the chromosome (ipaHa-e) are regulated by the T3SA activity. A conserved 17-bp MxiE box is present 5' of most of these genes. To characterize the promoter activity of these MxiE box-containing regions, similar ∼67-bp DNA fragments encompassing the MxiE box of 14 MxiE-regulated genes were cloned 5' of lacZ in a promoter probe plasmid; β-galactosidase activity detected in wild-type and ipaB strains harboring these plasmids indicated that most MxiE box-carrying regions contain a promoter regulated by the T3SA activity and that the relative strengths of these promoters cover an eight-fold range. The various MxiE boxes exhibiting up to three differences as compared to the MxiE box consensus sequence were introduced into the ipaH9.8 promoter without affecting its activity, suggesting that they are equally efficient in promoter activation. In contrast, all nucleotides conserved among MxiE boxes were found to be involved in MxiE-dependent promoter activity. In addition, we present evidence that the 5' UTRs of four MxiE-regulated genes enhance expression of the downstream gene, presumably by preventing degradation of the mRNA, and the 5' UTRs of two other genes carry an ancillary promoter.

The non-coding RNA world of the bacterial pathogen Listeria monocytogenes

In the past ten years, Listeria monocytogenes has emerged as a model organism in infection biology and also become an attractive system for the study of regulatory RNAs in pathogenic bacteria. Due to the recent completion of several transcriptomic studies, it is now clear that L. monocytogenes possesses a large repertoire of both cis- and trans-encoded RNAs. These include numerous small RNAs (sRNAs) expressed during infection, widespread transcription of both short and long antisense RNAs (asRNAs) and an array of cis-acting regulatory RNA elements. In this review we highlight the recent advances in non-coding RNA research in L. monocytogenes with a particular focus on emerging themes of RNA-based regulation.

The Listeria monocytogenes σB regulon and its virulence-associated functions are inhibited by a small molecule

The stress-responsive alternative sigma factor σ^B is conserved across diverse Gram-positive bacterial genera. In Listeria monocytogenes, σ^B regulates transcription of >150 genes, including genes contributing to virulence and to bacterial survival under host-associated stress conditions, such as those encountered in the human gastrointestinal lumen. An inhibitor of L. monocytogenes σ^B activity was identified by screening ~57,000 natural and synthesized small molecules using a high-throughput cell-based assay. The compound fluoro-phenyl-styrene-sulfonamide (FPSS) (IC₅₀ = 3.5 µM) downregulated the majority of genes previously identified as members of the σ^B regulon in L. monocytogenes 10403S, thus generating a transcriptional profile comparable to that of a 10403S ΔsigB strain. Specifically, of the 208 genes downregulated by FPSS, 75% had been identified previously as positively regulated by σ^B. Downregulated genes included key virulence and stress response genes, such as inlA, inlB, bsh, hfq, opuC, and bilE. From a functional perspective, FPSS also inhibited L. monocytogenes invasion of human intestinal epithelial cells and bile salt hydrolase activity. The ability of FPSS to inhibit σ^B activity in both L. monocytogenes and Bacillus subtilis indicates its utility as a specific inhibitor of σ^B across multiple Gram-positive genera.

The σ^B transcription factor regulates expression of genes responsible for bacterial survival under changing environmental conditions and for virulence; therefore, this alternative sigma factor is important for transmission of L. monocytogenes and other Gram-positive bacteria. Regulation of σ^B activity is complex and tightly controlled, reflecting the key role of this factor in bacterial metabolism. We present multiple lines of evidence indicating that fluoro-phenyl-styrene-sulfonamide (FPSS) specifically inhibits activity of σ^B across Gram-positive bacterial genera, i.e., in both Listeria monocytogenes and Bacillus subtilis. Therefore, FPSS is an important new tool that will enable novel approaches for exploring complex regulatory networks in L. monocytogenes and other Gram-positive pathogens and for investigating small-molecule applications for controlling pathogen transmission.