A Novel Salmonella Periplasmic Protein Controlling Cell Wall Homeostasis and Virulence

Morphogenetic penicillin-binding proteins control virulence-associated type III secretion systems in Salmonella

Type III protein secretion systems (T3SSs) function as multiprotein devices that span the envelope of Gram-negative bacteria using the peptidoglycan (PG) layer as scaffold. This spatial arrangement explains why modifications in PG structure can alter T3SS activity. In Salmonella, incorporation of non-canonical D-amino acids in the PG was shown to decrease the activity of the T3SS encoded by the pathogenicity island-1 (SPI-1) without affecting other T3SS, like the flagellum apparatus. Enigmatically, following invasion of host cell Salmonella enterica serovar Typhimurium modifies PG synthesis by upregulating two pathogen-specific enzymes, the penicillin-binding proteins PBP2SAL and PBP3SAL, with roles in cell elongation and division, respectively. In the mouse typhoid model, the amount of PBP2SAL and PBP3SAL produced by the pathogen exceeds by large those of the canonical enzymes PBP2 and PBP3. This change responds to acidity and high osmolarity, the same cues that intra-phagosomal S. Typhimurium perceives to switch the SPI-1 T3SS by that encoded in SPI-2. Using isogenic mutants lacking each of the four morphogenetic PBPs, we tested whether their activities and those of the T3SS encoded by SPI-1 and SPI-2, are interconnected. Our data show that PBP2 is required for proper function of SPI-1 T3SS but dispensable for motility, whereas the lack of any of the morphogenetic PBPs increases SPI-2 T3SS activity. The positive control exerted by PBP2 on SPI-1 takes place via the SPI-1-specific regulators HilA and InvF. To our knowledge, these findings provide the first evidence linking morphogenetic enzymes that synthesize PG with T3SS associated to virulence.

Exploring nagZ as a virulence biomarker and treatment target in Enterobacter cloacae

BACKGROUND

Enterobacter cloacae is increasingly prevalent and resistant to multiple antibiotics, making it a significant pathogen in healthcare settings with high mortality rates. However, its pathogenic mechanisms are not fully understood.

RESULTS

In this study, we explored the role of nagZ in regulating the virulence of E. cloacae and its potential as a therapeutic target. Our research showed that pathogenic strains of E. cloacae express higher levels of nagZ than colonizing strains, particularly in simulated infection environments. Deleting nagZ significantly reduced E. cloacae virulence in various infection models, including Galleria mellonella larvae, mice, and RAW264.7 cells. Moreover, nagZ knockout decreased the bacterium's ability to induce inflammatory factor levels, while complementing nagZ in knockout strains partially rescued this ability. Importantly, the absence of nagZ also enhanced the antibacterial efficacy of ceftazidime against E. cloacae.

CONCLUSIONS

These findings underscore the crucial role of nagZ in E. cloacae pathogenesis and highlight its potential as a novel therapeutic target for treating infections caused by this pathogen.

Tackling salmonellosis: A comprehensive exploration of risks factors, impacts, and solutions

Salmonellosis, caused by Salmonella species, is one of the most common foodborne illnesses worldwide with an estimated 93.8 million cases and about 155,00 fatalities. In both industrialized and developing nations, Salmonellosis has been reported to be one of the most prevalent foodborne zoonoses and is linked with arrays of illness syndromes such as acute and chronic enteritis, and septicaemia. The two major and most common Salmonella species implicated in both warm-blooded and cold-blooded animals are Salmonella bongori and Salmonella enterica. To date, more than 2400 S. enterica serovars which affect both humans and animals have been identified. Salmonella is further classified into serotypes based on three primary antigenic determinants: somatic (O), flagella (H), and capsular (K). The capacity of nearly all Salmonella species to infect, multiply, and survive in human host cells with the aid of their pathogenic and virulence arsenals makes them deadly and important public health pathogens. Primarily, food-producing animals such as poultry, swine, cattle, and their products have been identified as important sources of salmonellosis. Additionally, raw fruits and vegetables are among other food types that have been linked to the spread of Salmonella spp. Based on the clinical manifestation of human salmonellosis, Salmonella strains can be categorized as either non-typhoidal Salmonella (NTS) and typhoidal Salmonella. The detection of aseptically collected Salmonella in necropsies, environmental samples, feedstuffs, rectal swabs, and food products serves as the basis for diagnosis. In developing nations, typhoid fever due to Salmonella Typhi typically results in the death of 5%-30% of those affected. The World Health Organization (WHO) calculated that there are between 16 and 17 million typhoid cases worldwide each year, with scaring 600,000 deaths as a result. The contagiousness of a Salmonella outbreak depends on the bacterial strain, serovar, growth environment, and host susceptibility. Risk factors for Salmonella infection include a variety of foods; for example, contaminated chicken, beef, and pork. Globally, there is a growing incidence and emergence of life-threatening clinical cases, especially due to multidrug-resistant (MDR) Salmonella spp, including strains exhibiting resistance to important antimicrobials such as beta-lactams, fluoroquinolones, and third-generation cephalosporins. In extreme cases, especially in situations involving very difficult-to-treat strains, death usually results. The severity of the infections resulting from Salmonella pathogens is dependent on the serovar type, host susceptibility, the type of bacterial strains, and growth environment. This review therefore aims to detail the nomenclature, etiology, history, pathogenesis, reservoir, clinical manifestations, diagnosis, epidemiology, transmission, risk factors, antimicrobial resistance, public health importance, economic impact, treatment, and control of salmonellosis.

Peptidoglycan NlpC/P60 peptidases in bacterial physiology and host interactions

The bacterial cell wall is composed of a highly crosslinked matrix of glycopeptide polymers known as peptidoglycan that dictates bacterial cell morphology and protects against environmental stresses. Regulation of peptidoglycan turnover is therefore crucial for bacterial survival and growth and is mediated by key protein complexes and enzyme families. Here, we review the prevalence, structure, and activity of NlpC/P60 peptidases, a family of peptidoglycan hydrolases that are crucial for cell wall turnover and division as well as interactions with antibiotics and different hosts. Understanding the molecular functions of NlpC/P60 peptidases should provide important insight into bacterial physiology, their interactions with different kingdoms of life, and the development of new therapeutic approaches.

Bacterial virulence regulation through soluble peptidoglycan fragments sensing and response: knowledge gaps and therapeutic potential

Given the growing clinical-epidemiological threat posed by the phenomenon of antibiotic resistance, new therapeutic options are urgently needed, especially against top nosocomial pathogens such as those within the ESKAPE group. In this scenario, research is pushed to explore therapeutic alternatives and, among these, those oriented toward reducing bacterial pathogenic power could pose encouraging options. However, the first step in developing these antivirulence weapons is to find weak points in the bacterial biology to be attacked with the goal of dampening pathogenesis. In this regard, during the last decades some studies have directly/indirectly suggested that certain soluble peptidoglycan-derived fragments display virulence-regulatory capacities, likely through similar mechanisms to those followed to regulate the production of several β-lactamases: binding to specific transcriptional regulators and/or sensing/activation of two-component systems. These data suggest the existence of intra- and also intercellular peptidoglycan-derived signaling capable of impacting bacterial behavior, and hence likely exploitable from the therapeutic perspective. Using the well-known phenomenon of peptidoglycan metabolism-linked β-lactamase regulation as a starting point, we gather and integrate the studies connecting soluble peptidoglycan sensing with fitness/virulence regulation in Gram-negatives, dissecting the gaps in current knowledge that need filling to enable potential therapeutic strategy development, a topic which is also finally discussed.

Inflammation Regulation by Bacterial Molecular Patterns

Stimulation of innate immunity by bacterial molecular patterns can induce an enhanced cellular immune response to pathogens that are associated with innate immune memory shaped by epigenetic changes. Immunological memory can be expressed in the acceleration/intensification of inflammation, as well as in the exact opposite-to maintain tolerance and non-response to a repeated stimulus. Tolerance is one of the central concepts of immunity and is ensured by the consistency of all parts of the immune response. The severe consequences of inflammation force researchers to study in detail all stages of the downstream pathways that are activated after exposure to a stimulus, while the formation of non-response to a pro-inflammatory stimulus has not yet received a detailed description. Elucidation of the mechanism of tolerance is an urgent task for the prevention and treatment of inflammatory diseases. The aim of this investigation was to study the dynamic changes in the gene expression of A20 and ATF3, the inflammation suppressors, against the background of the expression of the genes of the innate immunity receptors TLR4 and NOD2 and the pro-inflammatory cytokine TNF-α under the influence of TLR4 and NOD2 agonists, lipopolysaccharide (LPS) and glucosaminylmuramyl dipeptide (GMDP). The mechanism of inflammation regulation by bioregulators of bacterial origin-LPS and GMDP-was evaluated in vitro in human peripheral blood mononuclear cells and in vivo after i.p. administration of LPS and GMDP to mice. Gene expression was assessed by RT-PCR. Innate immune receptors and the pro-inflammatory cytokine TNF-α were found to develop early in response to LPS and GMDP, both in vitro and in vivo. Genes of cytosolic proteins controlling inflammation (A20 and ATF3) were expressed later. Prior exposure of the innate immune system to LPS and muramyl peptides may modulate host defense against acute inflammation. As a result of the study, new data were obtained on dynamic changes in deubiquitinase A20 and the transcription factor ATF3, which are involved in the limitation and suppression of inflammatory reactions caused by fragments of bacterial cell walls-LPS and GMDP. Thus, bioregulators of bacterial origin LPS and GMDP, along with pro-inflammatory factors, activate the expression of genes that suppress inflammation, which should be considered when analyzing data from studies of the pro-inflammatory properties of LPS and GMDP and when developing drugs based on them.

OmpR and Prc contribute to switch the Salmonella morphogenetic program in response to phagosome cues

Salmonella enterica serovar Typhimurium infects eukaryotic cells residing within membrane-bound phagosomes. In this compartment, the pathogen replaces the morphogenetic penicillin-binding proteins 2 and 3 (PBP2/PBP3) with PBP2_SAL /PBP3_SAL , two proteins absent in Escherichia coli. The basis for this switch is unknown. Here, we show that PBP3 protein levels drop drastically when S. Typhimurium senses acidity, high osmolarity and nutrient scarcity, cues that activate virulence functions required for intra-phagosomal survival and proliferation. The protease Prc and the transcriptional regulator OmpR contribute to lower PBP3 levels whereas OmpR stimulates PBP2_SAL /PBP3_SAL production. Surprisingly, despite being essential for division in E. coli, PBP3 levels also drop in non-pathogenic and pathogenic E. coli exposed to phagosome cues. Such exposure alters E. coli morphology resulting in very long bent and twisted filaments indicative of failure in the cell division and elongation machineries. None of these aberrant shapes are detected in S. Typhimurium. Expression of PBP3_SAL restores cell division in E. coli exposed to phagosome cues although the cells retain elongation defects in the longitudinal axis. By switching the morphogenetic program, OmpR and Prc allow S. Typhimurium to properly divide and elongate inside acidic phagosomes maintaining its cellular dimensions and the rod shape.

Muramyl dipeptide-based analogs as potential anticancer compounds: Strategies to improve selectivity, biocompatibility, and efficiency

According to the WHO, cancer is the second leading cause of death in the world. This is an important global problem and a major challenge for researchers who have been trying to find an effective anticancer therapy. A large number of newly discovered compounds do not exert selective cytotoxic activity against tumorigenic cells and have too many side effects. Therefore, research on muramyl dipeptide (MDP) analogs has attracted interest due to the urgency for finding more efficient and safe treatments for oncological patients. MDP is a ligand of the cytosolic nucleotide-binding oligomerization domain 2 receptor (NOD2). This molecule is basic structural unit that is responsible for the immune activity of peptidoglycans and exhibits many features that are important for modern medicine. NOD2 is a component of the innate immune system and represents a promising target for enhancing the innate immune response as well as the immune response against cancer cells. For this reason, MDP and its analogs have been widely used for many years not only in the treatment of immunodeficiency diseases but also as adjuvants to support improved vaccine delivery, including for cancer treatment. Unfortunately, in most cases, both the MDP molecule and its synthesized analogs prove to be too pyrogenic and cause serious side effects during their use, which consequently exclude them from direct clinical application. Therefore, intensive research is underway to find analogs of the MDP molecule that will have better biocompatibility and greater effectiveness as anticancer agents and for adjuvant therapy. In this paper, we review the MDP analogs discovered in the last 10 years that show promise for antitumor therapy. The first part of the paper compiles the achievements in the field of anticancer vaccine adjuvant research, which is followed by a description of MDP analogs that exhibit promising anticancer and antiproliferative activity and their structural changes compared to the original MDP molecule.

Strategies for Using Muramyl Peptides - Modulators of Innate Immunity of Bacterial Origin - in Medicine

The spread of infectious diseases is rampant. The emergence of new infections, the irrational use of antibiotics in medicine and their widespread use in agriculture contribute to the emergence of microorganisms that are resistant to antimicrobial drugs. By 2050, mortality from antibiotic-resistant strains of bacteria is projected to increase up to 10 million people per year, which will exceed mortality from cancer. Mutations in bacteria and viruses are occurring faster than new drugs and vaccines are being introduced to the market. In search of effective protection against infections, new strategies and approaches are being developed, one of which is the use of innate immunity activators in combination with etiotropic chemotherapy drugs. Muramyl peptides, which are part of peptidoglycan of cell walls of all known bacteria, regularly formed in the body during the breakdown of microflora and considered to be natural regulators of immunity. Their interaction with intracellular receptors launches a sequence of processes that ultimately leads to the increased expression of genes of MHC molecules, pro-inflammatory mediators, cytokines and their soluble and membrane-associated receptors. As a result, all subpopulations of immunocompetent cells are activated: macrophages and dendritic cells, neutrophils, T-, B- lymphocytes and natural killer cells for an adequate response to foreign or transformed antigens, manifested both in the regulation of the inflammatory response and in providing immunological tolerance. Muramyl peptides take part in the process of hematopoiesis, stimulating production of colony-stimulating factors, which is the basis for their use in the treatment of oncological diseases. In this review we highlight clinical trials of drugs based on muramyl peptides, as well as clinical efficacy of drugs mifamurtide, lycopid, liasten and polimuramil. Such a multifactorial effect of muramyl peptides and a well-known mechanism of activity make them promising drugs in the treatment and preventing of infectious, allergic and oncological diseases, and in the composition of vaccines.