Suppression of Virulence of Toxigenic Vibrio cholerae by Anethole through the Cyclic AMP (cAMP)-cAMP Receptor Protein Signaling System

Vibrio cholerae virulence and its suppression through the quorum-sensing system

Vibrio cholerae is a cholera-causing pathogen known to instigate severe contagious diarrhea that affects millions globally. Survival of vibrios depend on a combination of multicellular responses and adapt to changes that prevail in the environment. This process is achieved through a strong communication at the cellular level, the process has been recognized as quorum sensing (QS). The severity of infection is highly dependent on the QS of vibrios in the gut milieu. The quorum may exist in a low/high cell density (LCD/HCD) state to exert a positive or negative response to control the regulatory pathogenic networks. The impact of this regulation reflects on the transition of pathogenic V. cholerae from the environment to infect humans and cause outbreaks or epidemics of cholera. In this context, the review portrays various regulatory processes and associated virulent pathways, which maneuver and control LCD and HCD states for their survival in the host. Although several treatment options are existing, promotion of therapeutics by exploiting the virulence network may potentiate ineffective antibiotics to manage cholera. In addition, this approach is also useful in resource-limited settings, where the accessibility to antibiotics or conventional therapeutic options is limited.

The cAMP receptor protein gene contributes to growth, stress resistance, and colonization of Actinobacillus pleuropneumoniae

Porcine infectious pleuropneumonia (PCP) is a severe disease of porcine caused by Actinobacillus pleuropneumoniae (APP). The spread of PCP remains a threat to the porcine farms and has been known to cause severe economic losses. The cAMP receptor protein (CRP) serves as a pivotal player in helping bacteria adapt to shifts in their environment, particularly when facing the challenges posed by bacterial infections. In this study, we investigated the role of CRP in APP. Our results revealed that crp mutant (Δcrp) strains were more sensitive to acidic and osmotic stress resistance and had lower biofilm formation ability than wild-type (WT) strains. Furthermore, the Δcrp strains showed deficiencies in anti-phagocytosis, adhesion, and invasion upon interaction with host cells. Mice infected with the Δcrp strains demonstrated reduced bacterial loads in their lungs compared to those infected with the WT strains. This study reveals the pivotal role of crp gene expression in regulating pleuropneumonia growth, stress resistance, iron utilization, biofilm formation, phagocytosis, adhesion, invasion and colonization. Our discoveries offer novel perspectives on understanding the development and progression of APP infections.

Using next generation antimicrobials to target the mechanisms of infection

The remarkable impact of antibiotics on human health is being eroded at an alarming rate by the emergence of multidrug resistant pathogens. There is a recognised consensus that new strategies to tackle infection are urgently needed to limit the devasting impact of antibiotic resistance on our global healthcare infrastructure. Next generation antimicrobials (NGAs) are compounds that target bacterial virulence factors to disrupt pathogenic potential without impacting bacterial viability. By disabling the key virulence factors required to establish and maintain infection, NGAs make pathogens more vulnerable to clearance by the immune system and can potentially render them more susceptible to traditional antibiotics. In this review, we discuss the developing field of NGAs and how advancements in this area could offer a viable standalone alternative to traditional antibiotics or an effective means to prolong antibiotic efficacy when used in combination.

The Response Regulator VC1795 of Vibrio Pathogenicity Island-2 Contributes to Intestinal Colonization by Vibrio cholerae

Vibrio cholerae is an intestinal pathogen that can cause severe diarrheal disease. The disease has afflicted millions of people since the 19th century and has aroused global concern. The Vibrio Pathogenicity Island-2 (VPI-2) is a 57.3 kb region, VC1758-VC1809, which is present in choleragenic V. cholerae. At present, little is known about the function of VC1795 in the VPI-2 of V. cholerae. In this study, the intestinal colonization ability of the ΔVC1795 strain was significantly reduced compared to that of the wild-type strain, and the colonization ability was restored to the wild-type strain after VC1795 gene replacement. This result indicated that the VC1795 gene plays a key role in the intestinal colonization and pathogenicity of V. cholerae. Then, we explored the upstream and downstream regulation mechanisms of the VC1795 gene. Cyclic adenylate receptor protein (CRP) was identified as being located upstream of VC1795 by a DNA pull-down assay and electrophoretic mobility shift assays (EMSAs) and negatively regulating the expression of VC1795. In addition, the results of Chromatin immunoprecipitation followed by sequencing (ChIP-seq), EMSAs, and Quantitative Real-Time Polymerase Chain Reaction (qRT-PCR) indicated that VC1795 directly negatively regulates the expression of its downstream gene, VC1794. Furthermore, by using qRT-PCR, we hypothesized that VC1795 indirectly positively regulates the toxin-coregulated pilus (TCP) cluster to influence the colonization ability of V. cholerae in intestinal tracts. In short, our findings support the key regulatory role of VC1795 in bacterial pathogenesis as well as lay the groundwork for the further determination of the complex regulatory network of VC1795 in bacteria.

Piperine, an active ingredient of white pepper suppresses growth of multidrug resistant toxigenic Vibrio cholerae and other pathogenic bacteria

Emergence and rapid spread of multi-drug resistant (MDR) bacteria including Vibrio cholerae are a global public health issue. Much attention has been paid to natural compounds, such as spices and herbs to find novel antimicrobial compounds as they are considered to be cheaper alternatives to develop as a drug. Here, we show that methanol extract of white pepper could inhibit growth of V. cholerae O1 El Tor variant, responsible for the recent outbreaks/epidemics. Furthermore, we demonstrate for the first time that piperine, the major component of white pepper showed a dose-dependent bactericidal effect on V. cholerae growth irrespective of their biotypes and serogroups in presence of 200 and 300 µg ml^-1 of piperine, respectively. Piperine also inhibited growth of MDR strains of Pseudomonas aeruginosa, Escherichia coli isolated from poultry and enterohemorrhagic/enteroaggregative E. coli O104 in presence of 200 µg ml^-1 . Interestingly, we did not observe any significant inhibitory effect of piperine on E. coli strains isolated from healthy person even up to 200 µg ml^-1 . Our data suggest that piperine could be a novel antimicrobial agent in therapeutic and preventive applications against infections caused by pathogenic bacteria including MDR strains.

Dietary Isothiocyanates, Sulforaphane and 2-Phenethyl Isothiocyanate, Effectively Impair Vibrio cholerae Virulence

Vibrio cholerae represents a constant threat to public health, causing widespread infections, especially in developing countries with a significant number of fatalities and serious complications every year. The standard treatment by oral rehydration does not eliminate the source of infection, while increasing antibiotic resistance among pathogenic V. cholerae strains makes the therapy difficult. Thus, we assessed the antibacterial potential of plant-derived phytoncides, isothiocyanates (ITC), against V. cholerae O365 strain. Sulforaphane (SFN) and 2-phenethyl isothiocyanate (PEITC) ability to inhibit bacterial growth was assessed. Minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values indicate that these compounds possess antibacterial activity and are also effective against cells growing in a biofilm. Tested ITC caused accumulation of stringent response alarmone, ppGpp, which indicates induction of the global stress response. It was accompanied by bacterial cytoplasm shrinkage, the inhibition of the DNA, and RNA synthesis as well as downregulation of the expression of virulence factors. Most importantly, ITC reduced the toxicity of V. cholerae in the in vitro assays (against Vero and HeLa cells) and in vivo, using Galleria mellonella larvae as an infection model. In conclusion, our data indicate that ITCs might be considered promising antibacterial agents in V. cholerae infections.

Effect of cAMP Receptor Protein Gene on Growth Characteristics and Stress Resistance of Haemophilus parasuis Serovar 5

Haemophilus parasuis (HPS), a member of the family Pasteurellaceae, is a common bacteria in the upper respiratory tract of pigs but under certain circumstances can cause serious systemic disease (Glasser's disease) characterized by severe infection of the upper respiratory tract, fibrinous polyserositis, polyarthritis, and meningitis. cAMP receptor protein (CRP) is among the most important global regulators, playing a vital role in adapting to environmental changes during the process of bacterial infection. In order to investigate the function of the crp gene in the growth characteristics of H. parasuis serovar 5 (HPS5) and its ability to overcome adverse environmental stresses, a crp mutant strain (Δcrp) was constructed and verified. In this study, we found that the crp gene was involved in growth rate, biofilm formation, stress tolerance, serum resistance, and iron utilization. Compared with the wild type, both the growth rate of the crp mutant and its resistance to osmotic pressure decreased significantly. Similar phenomena were also found in biofilm formation and iron utilization. However, the resistance to heat shock and serum complement of the crp mutant were enhanced. This study aimed to reveal the function in growth characteristics and stress resistance of the crp gene in HPS5. Whether it relates to virulence requires additional in-depth research.

Current landscape in the discovery of novel antibacterial agents

BACKGROUND

Standard treatments against bacterial infections are becoming ineffective due to the rise of antibacterial resistance worldwide. Classical approaches to develop new antibacterial agents are not sufficient to fulfil the current pipeline, therefore new strategies are currently being devised in the field of antibacterial discovery.

OBJECTIVES

The objective of this narrative review is to compile the most successful strategies for drug discovery within the antibacterial context that are currently being pursued.

SOURCES

Peer-reviewed publications from the MEDLINE database with robust data addressing the discovery of new antibacterial agents in the current pipeline have been selected.

CONTENT

Several strategies to discover new antibacterials are described in this review: (i) derivatives of known antibacterial agents; the activity of a known antimicrobial agent can be improved through two strategies: (a) the modification of the original chemical structure of an antimicrobial agent to circumvent antibacterial resistance mechanisms and (b) the development of a compound that inhibits the mechanisms of resistance to an antibacterial agent; (ii) new antibacterial agents targeting new proteins; (iii) inhibitors of virulence factors; (iv) nanoparticles; (v) antimicrobial peptides and peptidomimetics; (vi) phage therapy and enzybiotics; and (vii) antisense oligonucleotides.

IMPLICATIONS

This review intends to provide a positive message affirming that several different strategies to design new antibacterial agents are currently being developed, and we are therefore confident that in the near future some of the most promising approaches will come to fruition.

Capsaicin inhibitory effects on Vibrio cholerae toxin genes expression

OBJECTIVE

Cholera is an acute secretory diarrhea caused by the Gram-negative bacterium, Vibrio cholerae mostly through production of cholera toxin (CT) and zonula occludens toxin (Zot). Isolates of V. cholerae have acquired resistance elements during the last decade. One of the most promising ways to treat resistant strains is to use antivirulence agents instead of killing the causative agent with conventional antibiotics. In this study, we examined whether different concentrations of capsaicin - the pungent fraction of red chili- can act as an antivirulence agent and inhibit V. cholerae toxin production.

MATERIALS AND METHODS

Two standard strains namely, V. cholerae ATCC 14035 and V. cholerae PTCC 1611 were used in this study. Minimum Inhibitory Concentration (MIC) of capsaicin was determined by broth microdilution method. Based on MIC results, the bacteria were cultured in the presence of sub-MIC concentrations of capsaicin and a negative control without capsaicin. Real-time PCR (RT-PCR) was carried out to determine the expression level of V. cholerae toxin genes at each concentration.

RESULTS

MIC test showed that 200 mg/mL of capsaicin in 2% dimethyl sulfoxide (DMSO) could inhibit the growth of the two standard strains of V. cholerae. The expression of V. cholerae toxin genes was significantly reduced following treatment with sub-MIC concentrations of capsaicin as assessed by RT-PCR.

CONCLUSION

Capsaicin showed great inhibitory effect against cholera toxin and reduced Zot production in the tested strains of V. cholerae. The results showed promising insights into antivirulence effects of capsaicin.

mTOR and ROS regulation by anethole on adipogenic differentiation in human mesenchymal stem cells

Background

Adipocyte differentiation of human mesenchymal stem cells (hMSCs) is dependent on mitochondrial metabolism and reactive oxygen species (ROS) to initiate adipocyte differentiation. Although anethole has been known as an anti-oxidant and lipid peroxidation inhibitor, there is little investigated about its role in adipogenic differentiation.

Methods

The effects on cytotoxicity and proliferation of anethole in hMSCs were measured by the MTT assay. The anti-adipogenic effect of anethole on hMSCs was analyzed by Oil Red O staining and western blot analysis. The anti-oxidant activity of anethole on hMSC was assessed by flowcytometry and fluorescence staining using 2',7' –dichlorofluorescin diacetate (DCFDA). The western blotting was used to detect of phospho-Akt, phospho-mTOR, phospho-p70S6K, PPARγ, and phsopho-AMP-activated kinase (AMPK).

Results

Anethole suppressed the adipogenic differentiation of hMSCs through down-regulation of Akt-mTOR-p70S6K-PPARγ and up-regulation of AMPK. Anethole affected oxidative conditions through ROS generation. Anethole also rescued AMPK activity and reduced activation of mTOR-p70S6K-PPARγ under oxidative conditions in presence of exogenous hydrogen peroxide.

Conclusion

ROS and mTOR regulation is a crucial factor in adipogenic differentiation, anethole has an important role in regulating activities of mTOR/PPARγ and ROS control in adipogenic differentiation of hMSCs.

Electronic supplementary material

The online version of this article (10.1186/s12860-018-0163-2) contains supplementary material, which is available to authorized users.

Edwardsiella piscicida lacking the cyclic AMP receptor protein (Crp) is avirulent and immunogenic in fish

Edwardsiella piscicida is a Gram-negative pathogen that generally causes lethal septicemia in marine and freshwater fish. We generated a E. piscicida CK216 Δcrp mutant to investigate various biological roles related to this organism, including pathogenesis. Lack of Crp in CK216 was demonstrated by immunoblotting using a Crp-specific antibody. Compared to the parental strain, the mutant exhibited changes in three biochemical phenotypes, including ornithine decarboxylation, citrate utilization, and H₂S production. Complementation of crp deletion in trans rescued the phenotype of the parental strain. This study proved that hemolytic activity in E. piscicida is controlled by Crp. In addition, significantly reduced motility of E. piscicida CK216 was observed, which resulted from a lack of flagella synthesis. To examine the virulence in fish, E. piscicida cells were injected into the goldfish (Carassius auratus) via intraperitoneal route. The LD₅₀ of CK216 was 9.25 × 10⁸ CFU, while that of the CK108 parental strain was 9.24 × 10⁵ CFU, attenuated 1000 fold in goldfish. Fish immunized with CK216 elicited IgM responses. Moreover, 80% of goldfish immunized with 1 × 10⁶ CFU survived after administration of a lethal dose (1 × 10⁷ CFU) of virulent E. piscicida CK41, suggesting the potential for E. piscicida CK216 to serve as a live attenuated vaccine in aquaculture.

Disarming the enemy: targeting bacterial toxins with small molecules

The rapid emergence of antibiotic-resistant bacterial strains has prompted efforts to find new and more efficacious treatment strategies. Targeting virulence factors produced by pathogenic bacteria has gained particular attention in the last few years. One of the inherent advantages of this approach is that it provides less selective pressure for the development of resistance mechanisms. In addition, antivirulence drugs could potentially be the answer for diseases in which the use of conventional antibiotics is counterproductive. That is the case for bacterial toxin-mediated diseases, in which the severity of the symptoms is a consequence of the exotoxins produced by the pathogen. Examples of these are haemolytic-uraemic syndrome produced by Shiga toxins, the profuse and dangerous dehydration caused by Cholera toxin or the life-threatening colitis occasioned by clostridial toxins. This review focuses on the recent advances on the development of small molecules with antitoxin activity against Enterohaemorrhagic Escherichia coli, Vibrio cholerae and Clostridium difficile given their epidemiological importance. The present work includes studies of small molecules with antitoxin properties that act directly on the toxin (direct inhibitors) or that act by preventing expression of the toxin (indirect inhibitors).

Anti-virulence Strategies to Target Bacterial Infections

Resistance of important bacterial pathogens to common antimicrobial therapies and the emergence of multidrug-resistant bacteria are increasing at an alarming rate and constitute one of our greatest challenges in the combat of bacterial infection and accompanied diseases. The current shortage of effective drugs, lack of successful prevention measures and only a few new antibiotics in the clinical pipeline demand the development of novel treatment options and alternative antimicrobial therapies. Our increasing understanding of bacterial virulence strategies and the induced molecular pathways of the infectious disease provides novel opportunities to target and interfere with crucial pathogenicity factors or virulence-associated traits of the bacteria while bypassing the evolutionary pressure on the bacterium to develop resistance. In the past decade, numerous new bacterial targets for anti-virulence therapies have been identified, and structure-based tailoring of intervention strategies and screening assays for small-molecule inhibitors of such pathways were successfully established. In this chapter, we will take a closer look at the bacterial virulence-related factors and processes that present promising targets for anti-virulence therapies, recently discovered inhibitory substances and their promises and discuss the challenges, and problems that have to be faced.