Role of the SRRz/Rz1 lambdoid lysis cassette in the pathoadaptive evolution of Shigella

The Shigella flexneri virulence factor apyrase is released inside eukaryotic cells to hijack host cell fate

IMPORTANCE

In this paper, we demonstrated that apyrase is released within the host cell cytoplasm during infection to target the intracellular ATP pool. By degrading intracellular ATP, apyrase contributes to prevent caspases activation, thereby inhibiting the activation of pyroptosis in infected cells. Our results show, for the first time, that apyrase is involved in the modulation of host cell survival, thereby aiding this pathogen to dampen the inflammatory response. This work adds a further piece to the puzzle of Shigella pathogenesis. Due to its increased spread worldwide, prevention and controlling strategies are urgently needed. Overall, this study highlighted apyrase as a suitable target for an anti-virulence therapy to tackle this pathogen.

Diffusible signal factors (DSFs) bind and repress VirF, the leading virulence activator of Shigella flexneri

Shigella, the aetiological agent of human bacillary dysentery, controls the expression of its virulence determinants through an environmentally stimulated cascade of transcriptional activators. VirF is the leading activator and is essential for proper virulence expression. In this work, we report on in vitro and in vivo experiments showing that two autoinducers of the DSF family, XcDSF and BDSF interact with the jelly roll module of VirF causing its inhibition and affecting the expression of the entire virulence system of Shigella, including its ability to invade epithelial cells. We propose a molecular model explaining how the binding of XcDSF and BDSF causes inhibition of VirF by preventing its dimerization. Overall, our experimental results suggest that XcDSF and BDSF may contribute to "colonisation resistance" in the human gut or, alternatively, may be exploited for the fine-tuning of Shigella virulence expression as the bacterium migrates from the lumen to approach the intestinal mucosa. Our findings also stress how a detailed understanding of the interaction of DSF ligands with VirF may contribute to the rational development of innovative antivirulence drugs to treat shigellosis.

Fatty Acids Abolish Shigella Virulence by Inhibiting Its Master Regulator, VirF

The pathogenicity of Shigella, the intracellular pathogen responsible for human bacillary dysentery, depends on a coordinated and tightly regulated expression of its virulence determinants. This is the result of a cascade organization of its positive regulators, with VirF, a transcriptional activator belonging to the AraC-XylS family, in a pivotal position. VirF itself is submitted to several well-known regulations at the transcriptional level. In this work, we present evidence for a novel posttranslational regulatory mechanism of VirF mediated by the inhibitory interaction with specific fatty acids. By homology modeling and molecular docking analyses, we identify a jelly roll motif in the structure of ViF capable of interacting with medium-chain saturated and long-chain unsaturated fatty acids. In vitro and in vivo assays show that capric, lauric, myristoleic, palmitoleic, and sapienic acids interact effectively with the VirF protein, abolishing its transcription-promoting activity. This silences the virulence system of Shigella, leading to a drastic reduction in its ability to invade epithelial cells and proliferate in their cytoplasm. IMPORTANCE In the absence of a valid vaccine, the main therapeutic approach currently used to treat shigellosis is based on the use of antibiotics. The emergence of antibiotic resistance jeopardizes the future effectiveness of this approach. The importance of the present work resides both in the identification of a new level of posttranslational regulation of the Shigella virulence system and in the characterization of a mechanism offering new opportunities for the design of antivirulence compounds, which may change the treatment paradigm of Shigella infections by limiting the emergence of antibiotic-resistant bacteria.

Role of the MDR Efflux Pump AcrAB in Epithelial Cell Invasion by Shigella flexneri

The tripartite complex AcrAB-TolC is the major RND pump in Escherichia coli and other Enterobacteriaceae, including Shigella, the etiological agent of bacillary dysentery. In addition to conferring resistance to many classes of antibiotics, AcrAB plays a role in the pathogenesis and virulence of several bacterial pathogens. Here, we report data demonstrating that AcrAB specifically contributes to Shigella flexneri invasion of epithelial cells. We found that deletion of both acrA and acrB genes causes reduced survival of S. flexneri M90T strain within Caco-2 epithelial cells and prevents cell-to-cell spread of the bacteria. Infections with single deletion mutant strains indicate that both AcrA and AcrB favor the viability of the intracellular bacteria. Finally, we were able to further confirm the requirement of the AcrB transporter activity for intraepithelial survival by using a specific EP inhibitor. Overall, the data from the present study expand the role of the AcrAB pump to an important human pathogen, such as Shigella, and add insights into the mechanism governing the Shigella infection process.

In Vitro Probiotic Properties and In Vivo Anti-Ageing Effects of Lactoplantibacillus plantarum PFA2018AU Strain Isolated from Carrots on Caenorhabditis elegans

Lactic acid bacteria (LAB) share and provide several beneficial effects on human health, such as the release of bioactive metabolites, pathogen competition, and immune stimulation. The two major reservoirs of probiotic microorganisms are the human gastro-intestinal tract and fermented dairy products. However, other sources, such as plant-based foods, represent important alternatives thanks to their large distribution and nutritive value. Here, the probiotic potential of autochthonous Lactiplantibacillus plantarum PFA2018AU, isolated from carrots harvested in Fucino highland, Abruzzo (Italy), was investigated through in vitro and in vivo approaches. The strain was sent to the biobank of Istituto Zooprofilattico Sperimentale della Lombardia ed Emilia Romagna in Italy for the purpose of patent procedures under the Budapest Treaty. The isolate showed high survival capability under in vitro simulated gastro-intestinal conditions, antibiotic susceptibility, hydrophobicity, aggregation, and the ability to inhibit the in vitro growth of Salmonella enterica serovar Typhimurium, Listeria monocytogenes, Pseudomonas aeruginosa, and Staphylococcus aureus pathogens. Caenorhabditis elegans was used as the in vivo model in order to analyse prolongevity and anti-ageing effects. L. plantarum PFA2018AU significantly colonised the gut of the worms, extended their lifespan, and stimulated their innate immunity. Overall, these results showed that autochthonous LAB from vegetables, such as carrots, have functional features that can be considered novel probiotic candidates.

AcrAB efflux pump impacts on the survival of adherent-invasive Escherichia coli strain LF82 inside macrophages

The tripartite complex AcrAB-TolC is the major RND pump in Escherichia coli and other Enterobacteriaceae. It consists of the AcrB transporter, which is embedded in the inner membrane, the AcrA adapter located in the periplasm, and the channel protein TolC responsible for the transport of substrates towards the extracellular environment. Besides conferring resistance to many classes of antibiotics, AcrAB plays a role in the pathogenesis and virulence of several bacterial pathogens. Here we report that the AcrAB pump heavily affects the infection process of the LF82 strain, the prototype of Adherent-Invasive Escherichia coli (AIEC) which are highly abundant in the ileal mucosa of Chron disease patients. We found that the deletion of genes encoding AcrA and/or AcrB leads to decreased survival of LF82 within macrophages. Ectopic AcrAB expression in a acrAB defective mutant restores the wild type condition. Furthermore, we demonstrate that inhibition of AcrB and replacement of the transporter with an unfunctional AcrB also interfere with bacterial viability inside macrophages. Overall, these data suggest a pivotal role of the AcrAB efflux pump in bacteria-host cell interactions also in AIEC.

Roles of Two-Component Signal Transduction Systems in Shigella Virulence

Two-component signal transduction systems (TCSs) are widespread types of protein machinery, typically consisting of a histidine kinase membrane sensor and a cytoplasmic transcriptional regulator that can sense and respond to environmental signals. TCSs are responsible for modulating genes involved in a multitude of bacterial functions, including cell division, motility, differentiation, biofilm formation, antibiotic resistance, and virulence. Pathogenic bacteria exploit the capabilities of TCSs to reprogram gene expression according to the different niches they encounter during host infection. This review focuses on the role of TCSs in regulating the virulence phenotype of Shigella, an intracellular pathogen responsible for severe human enteric syndrome. The pathogenicity of Shigella is the result of the complex action of a wide number of virulence determinants located on the chromosome and on a large virulence plasmid. In particular, we will discuss how five TCSs, EnvZ/OmpR, CpxA/CpxR, ArcB/ArcA, PhoQ/PhoP, and EvgS/EvgA, contribute to linking environmental stimuli to the expression of genes related to virulence and fitness within the host. Considering the relevance of TCSs in the expression of virulence in pathogenic bacteria, the identification of drugs that inhibit TCS function may represent a promising approach to combat bacterial infections.

Host - Bacterial Pathogen Communication: The Wily Role of the Multidrug Efflux Pumps of the MFS Family

Bacterial pathogens are able to survive within diverse habitats. The dynamic adaptation to the surroundings depends on their ability to sense environmental variations and to respond in an appropriate manner. This involves, among others, the activation of various cell-to-cell communication strategies. The capability of the bacterial cells to rapidly and co-ordinately set up an interplay with the host cells and/or with other bacteria facilitates their survival in the new niche. Efflux pumps are ubiquitous transmembrane transporters, able to extrude a large set of different molecules. They are strongly implicated in antibiotic resistance since they are able to efficiently expel most of the clinically relevant antibiotics from the bacterial cytoplasm. Besides antibiotic resistance, multidrug efflux pumps take part in several important processes of bacterial cell physiology, including cell to cell communication, and contribute to increase the virulence potential of several bacterial pathogens. Here, we focus on the structural and functional role of multidrug efflux pumps belonging to the Major Facilitator Superfamily (MFS), the largest family of transporters, highlighting their involvement in the colonization of host cells, in virulence and in biofilm formation. We will offer an overview on how MFS multidrug transporters contribute to bacterial survival, adaptation and pathogenicity through the export of diverse molecules. This will be done by presenting the functions of several relevant MFS multidrug efflux pumps in human life-threatening bacterial pathogens as Staphylococcus aureus, Listeria monocytogenes, Klebsiella pneumoniae, Shigella/E. coli, Acinetobacter baumannii.

Modulation of OMV Production by the Lysis Module of the DLP12 Defective Prophage of Escherichia coli K12

Outer membrane vesicles (OMVs) are nanostructures mostly produced by blebbing of the outer membrane in Gram negative bacteria. They contain biologically active proteins and perform a variety of processes. OMV production is also a typical response to events inducing stress in the bacterial envelope. In these cases, hypervesiculation is regarded as a strategy to avoid the dangerous accumulation of undesired products within the periplasm. Several housekeeping genes influence the biogenesis of OMVs, including those correlated with peptidoglycan and cell wall dynamics. In this work, we have investigated the relationship between OMV production and the lysis module of the E. coli DLP12 cryptic prophage. This module is an operon encoding a holin, an endolysin and two spannins, and is known to be involved in cell wall maintenance. We find that deleting the lysis module increases OMV production, suggesting that during evolution this operon has been domesticated to regulate vesiculation, likely through the elimination of non-recyclable peptidoglycan fragments. We also show that the expression of the lysis module is negatively regulated by environmental stress stimuli as high osmolarity, low pH and low temperature. Our data further highlight how defective prophages finely contribute to bacterial host fitness.

Expression Profile of Multidrug Resistance Efflux Pumps During Intracellular Life of Adherent-Invasive Escherichia coli Strain LF82

Efflux pumps (EPs) are present in all living cells and represent a large and important group of transmembrane proteins involved in transport processes. In bacteria, multidrug resistance efflux pumps (MDR EPs) confer resistance to antibiotics at different levels and are deeply implicated in the fast and dramatic emergence of antibiotic resistance. Recently, several reports have outlined the great versatility of MDR EPs in exporting a large variety of compounds other than antibiotics, thus promoting bacterial adaptation to a wide range of habitats. In several bacterial pathogens, MDR EPs contribute to increase the virulence potential and are directly involved in the crosstalk with host cells. In this work, we have investigated the possible role of MDR EPs in the infectious process of the adherent-invasive Escherichia coli (AIEC), a group of pathogenic E. coli that colonize the ileal mucosa of Crohn disease (CD) patients causing a strong intestinal inflammation. The results we have obtained indicate that, with the exception of mdtM, all MDR-EPs encoding genes present in E.coli K12 are conserved in the AIEC prototype strain LF82. The analysis of MDR EP expression during LF82 infection of macrophages and epithelial cells reveals that their transcription is highly modulated during the bacterial intracellular life. Notably, some EP genes are regulated in a cell-type specific manner, strongly suggesting that their function is required for LF82 successful infection. AIEC are able to adhere to and invade intestinal epithelial cells and, importantly, to survive and multiply within macrophages. Thus, we further investigated the role of EPs specifically induced by macrophage environment. We present evidence indicating that deletion of mdtEF genes, encoding an MDR EP belonging to the resistance nodulation division (RND) family, significantly impairs survival of LF82 in macrophages and that the wild type phenotype can be restored by trans-complementation with functional MdtEF pump. Altogether, our results indicate a strong involvement of MDR EPs in host pathogen interaction also in AIEC and highlight the contribution of MdtEF to the fitness of LF82 in the macrophage environment.

The MFS efflux pump EmrKY contributes to the survival of Shigella within macrophages

Efflux pumps are membrane protein complexes conserved in all living organisms. Beyond being involved in antibiotic extrusion in several bacteria, efflux pumps are emerging as relevant players in pathogen-host interactions. We have investigated on the possible role of the efflux pump network in Shigella flexneri, the etiological agent of bacillary dysentery. We have found that S. flexneri has retained 14 of the 20 pumps characterized in Escherichia coli and that their expression is differentially modulated during the intracellular life of Shigella. In particular, the emrKY operon, encoding an efflux pump of the Major Facilitator Superfamily, is specifically and highly induced in Shigella-infected U937 macrophage-like cells and is activated in response to a combination of high K⁺ and acidic pH, which are sensed by the EvgS/EvgA two-component system. Notably, we show that following S. flexneri infection, macrophage cytosol undergoes a mild reduction of intracellular pH, permitting EvgA to trigger the emrKY activation. Finally, we present data suggesting that EmrKY is required for the survival of Shigella in the harsh macrophage environment, highlighting for the first time the key role of an efflux pump during the Shigella invasive process.

The Intriguing Evolutionary Journey of Enteroinvasive E. coli (EIEC) toward Pathogenicity

Among the intestinal pathogenic Escherichia coli, enteroinvasive E. coli (EIEC) are a group of intracellular pathogens able to enter epithelial cells of colon, multiplicate within them, and move between adjacent cells with a mechanism similar to Shigella, the ethiological agent of bacillary dysentery. Despite EIEC belong to the same pathotype of Shigella, they neither have the full set of traits that define Shigella nor have undergone the extensive gene decay observed in Shigella. Molecular analysis confirms that EIEC are widely distributed among E. coli phylogenetic groups and correspond to bioserotypes found in many E. coli serogroups. Like Shigella, also in EIEC the critical event toward a pathogenic life-style consisted in the acquisition by horizontal gene transfer of a large F-type plasmid (pINV) containing the genes required for invasion, intracellular survival, and spreading through the intestinal mucosa. In Shigella, the ample gain in virulence determinants has been counteracted by a substantial loss of functions that, although important for the survival in the environment, are redundant or deleterious for the life inside the host. The pathoadaptation process that has led Shigella to modify its metabolic profile and increase its pathogenic potential is still in infancy in EIEC, although maintenance of some features typical of E. coli might favor their emerging relevance as intestinal pathogens worldwide, as documented by recent outbreaks in industrialized countries. In this review, we will discuss the evolution of EIEC toward Shigella-like invasive forms going through the epidemiology, including the emergence of new virulent strains, their genome organization, and the complex interactions they establish with the host.