Beyond killing: Can we find new ways to manage infection?

Bacillus velezensis iturins inhibit the hemolytic activity of Staphylococcus aureus

Bovine mastitis caused by S. aureus has a major economic impact on the dairy sector. With the crucial need for new therapies, anti-virulence strategies have gained attention as alternatives to antibiotics. Here we aimed to identify novel compounds that inhibit the production/activity of hemolysins, a virulence factor of S. aureus associated with mastitis severity. We screened Bacillus strains obtained from diverse sources for compounds showing anti-hemolytic activity. Our results demonstrate that lipopeptides produced by Bacillus spp. completely prevented the hemolytic activity of S. aureus at certain concentrations. Following purification, both iturins, fengycins, and surfactins were able to reduce hemolysis caused by S. aureus, with iturins showing the highest anti-hemolytic activity (up to 76% reduction). The lipopeptides showed an effect at the post-translational level. Molecular docking simulations demonstrated that these compounds can bind to hemolysin, possibly interfering with enzyme action. Lastly, molecular dynamics analysis indicated general stability of important residues for hemolysin activity as well as the presence of hydrogen bonds between iturins and these residues, with longevous interactions. Our data reveals, for the first time, an anti-hemolytic activity of lipopeptides and highlights the potential application of iturins as an anti-virulence therapy to control bovine mastitis caused by S. aureus.

A neonatal piglet model reveals interactions between nasal microbiota and influenza A virus pathogenesis

While vaccination and therapeutics for prevention/treatment of influenza are available, new strategies are needed to combat influenza disease in susceptible populations, particularly young children and newborns. Host associated microbiota play an important role in modulating the virulence of numerous pathogens, including the influenza A virus. In this study, we examined microbiome-influenza interactions in a neonatal piglet model system. The nasal microbiome of newborn piglets was longitudinally sampled before and after intranasal infection with recombinant viruses expressing hemagglutinins (HAs) derived from distinct zoonotic H1 subtypes. We found that viruses expressing different parental HAs manifested unique patterns of pathogenicity, and varied impacts on microbial community diversity. Despite these virus specific differences, a consistent microbial signature of viral infection was detected. Our results indicate that influenza A virus infection associates with the restructuring of nasal microbiome and such shifts in microbial diversity may contribute to outcomes of viral infection in neonatal piglets.

Unveiling the hidden language of bacteria: anti-quorum sensing strategies for gram-negative bacteria infection control

Quorum sensing (QS) is a communication mechanism employed by many bacteria to regulate gene expression in a population density-dependent manner. It plays a crucial role in coordinating various bacterial behaviors, including biofilm formation, virulence factor production, and antibiotic resistance. However, the dysregulation of QS can lead to detrimental effects, making it an attractive target for developing novel therapeutic strategies. Anti-QS approaches aim to interfere with QS signaling pathways, inhibiting the communication between bacteria, and disrupting their coordinated activities. Various strategies have been explored to achieve this goal. Advances in understanding QS mechanisms and the discovery of new targets have paved the way for the development of innovative anti-QS approaches. Combining multiple anti-QS strategies or utilizing them in combination with traditional antibiotics holds great promise for combating bacterial infections and addressing the challenges posed by antibiotic resistance. Anti-QS approaches offer a diverse range of strategies including natural compounds, antibody-mediated quorum quenching (QQ), computer-aided drug design for QQ, repurposing of Drugs approved by FDA as anti-QS agents and modulating quorum-sensing molecules which were discussed in detail in this review. This review, comprehensively and for the first time, sheds light on the significance of diverse anti-QS strategies in solving antimicrobial resistance problem in Gram-negative microbial infection.

Exopolysaccharides produced by Bacillus spp. inhibit biofilm formation by Staphylococcus aureus strains associated with bovine mastitis

Bovine mastitis is a costly disease in the dairy sector worldwide. Here the objective was to identify and characterize anti-biofilm compounds produced by Bacillus spp. against S. aureus associated with bovine mastitis. Results showed that cell-free supernatants of three Bacillus strains (out of 33 analysed) reduced S. aureus biofilm formation by approximately 40 % without affecting bacterial growth. The anti-biofilm activity was associated with exopolysaccharides (EPS) secreted by Bacillus spp. The EPS decreased S. aureus biofilm formation in a dose-dependent manner, inhibiting biofilm formation by 83 % at 1 mg/mL. The EPS also showed some biofilm disruption activity (up to 36.4 %), which may be partially mediated by increased expression of the aur gene. The characterization of EPS produced by Bacillus velezensis 87 and B. velezensis TR47II revealed macromolecules with molecular weights of 31.2 and 33.7 kDa, respectively. These macromolecules were composed mainly of glucose (mean = 218.5 μg/mg) and mannose (mean = 241.5 μg/mg) and had similar functional groups (pyranose ring, beta-type glycosidic linkage, and alkynes) as revealed by FT-IR. In conclusion, this study shows the potential applications of EPS produced by B. velezensis as an anti-biofilm compound that could contribute to the treatment of bovine mastitis caused by S. aureus.

Synthetic non-toxic anti-biofilm agents as a strategy in combating bacterial resistance

The tremendous increase in the bacterial resistance to the available antibiotics is a serious problem for the treatment of various infections. Biofilm formation in bacteria significantly contributes to the bacterial survival in host cells, and is considered as an crucial factor, responsible for bacterial resistance. The response of the bacterial cells in the biofilm to antibiotics is completely different from that of the free floating planktonic cells of the same strain. The anti-biofilm agents that could inhibit the biofilm production without affecting the bacterial growth, apply less selective pressure over the bacterial strains than the traditional antibiotics; thus the development of bacterial resistance would be of low incidence. Many attempts have been performed to discover novel agents capable of interfering with the bacterial biofilm life cycle, and several compounds have shown promising activities in suppressing the biofilm production or in dispersing mature existing biofilms. This review describes the different chemical classes that have anti-biofilm effects against different Gram-positive and Gram-negative bacteria without affecting the bacterial growth.

An assembled bacterial community associated with Artemisia annua L. causes plant protection against a pathogenic fungus

The microorganisms associated with a plant influence its growth and fitness. These microorganisms accumulate on the aerial and root surfaces of plants, as well as within the plants, as endophytes, although how the interaction between microorganisms protects the plant from pathogens is still little understood. In the current study, the impact of assembled the bacterial communities against the pathogenic fungus to promote Artemisia annua L. growths was investigated. We established a model of bacterium-fungus-plant system. Eight bacterial strains and a fungal pathogen Globisporangium ultimum (Glo) were isolated from wild A. annua roots and leaves, respectively. We assembled the six-bacteria community (C6: Rhizobium pusense, Paracoccus sp., Flavobacterium sp., Brevundimonas sp., Stenotrophomonas sp., and Bacillus sp.) with inhibition, and eight-bacteria community (C8) composing of C6 plus another two bacteria (Brevibacillus nitrificans and Cupriavidus sp.) without inhibition against Glo in individually dual culture assays. Inoculation of seedlings with C8 significantly reduced impact of Glo. The growth and disease suppression of A. annua seedlings inoculated with C8 + Glo were significantly better than those of seedlings inoculated with only Glo. C8 had more inhibitory effects on Glo, and also enhanced the contents of four metabolites in seedling roots compared to Glo treatment only. Additionally, the inhibitory effects of root extracts from A. annua seedlings showed that Glo was most sensitive, the degree of eight bacteria sensitivity were various with different concentrations. Our findings suggested that the non-inhibitory bacteria played a vital role in the bacterial community composition and that some bacterial taxa were associated with disease suppression. The construction of a defined assembled bacterial community could be used as a biological fungicide, promoting biological disease control of plants.

Duox and Jak/Stat signalling influence disease tolerance in Drosophila during Pseudomonas entomophila infection

Disease tolerance describes an infected host's ability to maintain health independently of the ability to clear microbe loads. The Jak/Stat pathway plays a pivotal role in humoral innate immunity by detecting tissue damage and triggering cellular renewal, making it a candidate tolerance mechanism. Here, we find that in Drosophila melanogaster infected with Pseudomonas entomophila disrupting ROS-producing dual oxidase (duox) or the negative regulator of Jak/Stat Socs36E, render male flies less tolerant. Another negative regulator of Jak/Stat, G9a - which has previously been associated with variable tolerance of viral infections - did not affect the rate of mortality with increasing microbe loads compared to flies with functional G9a, suggesting it does not affect tolerance of bacterial infection as in viral infection. Our findings highlight that ROS production and Jak/Stat signalling influence the ability of flies to tolerate bacterial infection sex-specifically and may therefore contribute to sexually dimorphic infection outcomes in Drosophila.

Targeting multidrug resistant Staphylococcus infections with bacterial histidine kinase inhibitors

The emergence of drug-resistant bacteria, such as methicillin-resistant Staphylococcus aureus (MRSA), which are not susceptible to current antibiotics has necessitated the development of novel approaches and targets to tackle this growing challenge. Bacterial two-component systems (TCSs) play a central role in the adaptative response of bacteria to their ever-changing environment. They are linked to antibiotic resistance and bacterial virulence making the proteins of the TCSs, histidine kinases and response regulators, attractive for the development of novel antibacterial drugs. Here, we developed a suite of maleimide-based compounds that we evaluated against a model histidine kinase, HK853, in vitro and in silico. The most potent leads were then assessed for their ability to decrease the pathogenicity and virulence of MRSA, resulting in the identification of a molecule that decreased the lesion size caused by a methicillin-resistant S. aureus skin infection by 65% in a murine model.

Anti-virulence compounds against Staphylococcus aureus associated with bovine mastitis: A new therapeutic option?

Bovine mastitis represents a major economic burden faced by the dairy industry. S. aureus is an important and prevalent bovine mastitis-associated pathogen in dairy farms worldwide. The pathogenicity and persistence of S. aureus in the bovine mammary gland are associated with the expression of a range of virulence factors involved in biofilm formation and the production of several toxins. The traditional therapeutic approach to treating bovine mastitis includes the use of antibiotics, but the emergence of antibiotic-resistant strains has caused therapeutic failure. New therapeutic approaches targeting virulence factors of S. aureus rather than cell viability can have several advantages including lower selective pressure towards the development of resistance and little impact on the host commensal microbiota. This review summarizes the potential of anti-virulence therapies to control S. aureus associated with bovine mastitis focusing on anti-toxin, anti-biofilm, and anti-quorum sensing compounds. It also points to potential sources of new anti-virulence inhibitors and presents screening strategies for identifying these compounds.

Towards Translation of PqsR Inverse Agonists: From In Vitro Efficacy Optimization to In Vivo Proof-of-Principle

Pseudomonas aeruginosa (PA) is an opportunistic human pathogen, which is involved in a wide range of dangerous infections. It develops alarming resistances toward antibiotic treatment. Therefore, alternative strategies, which suppress pathogenicity or synergize with antibiotic treatments are in great need to combat these infections more effectively. One promising approach is to disarm the bacteria by interfering with their quorum sensing (QS) system, which regulates the release of various virulence factors as well as biofilm formation. Herein, this work reports the rational design, optimization, and in-depth profiling of a new class of Pseudomonas quinolone signaling receptor (PqsR) inverse agonists. The resulting frontrunner compound features a pyrimidine-based scaffold, high in vitro and in vivo efficacy, favorable pharmacokinetics as well as clean safety pharmacology characteristics, which provide the basis for potential pulmonary as well as systemic routes of administration. An X-ray crystal structure in complex with PqsR facilitated further structure-guided lead optimization. The compound demonstrates potent pyocyanin suppression, synergizes with aminoglycoside antibiotic tobramycin against PA biofilms, and is active against a panel of clinical isolates from bronchiectasis patients. Importantly, this in vitro effect translated into in vivo efficacy in a neutropenic thigh infection model in mice providing a proof-of-principle for adjunctive treatment scenarios.

Mechanisms of damage prevention, signalling and repair impact disease tolerance

The insect gut is frequently exposed to pathogenic threats and must not only clear these potential infections, but also tolerate relatively high microbe loads. In contrast to the mechanisms that eliminate pathogens, we currently know less about the mechanisms of disease tolerance. We investigated how well-described mechanisms that prevent, signal, control or repair damage during infection contribute to the phenotype of disease tolerance. We established enteric infections with the bacterial pathogen Pseudomonas entomophila in transgenic lines of Drosophila melanogaster fruit flies affecting dcy (a major component of the peritrophic matrix), upd3 (a cytokine-like molecule), irc (a negative regulator of reactive oxygen species) and egfr¹ (epithelial growth factor receptor). Flies lacking dcy experienced the highest mortality, while loss of function of either irc or upd3 reduced tolerance in both sexes. The disruption of egfr¹ resulted in a severe loss in tolerance in male flies but had no substantial effect on the ability of female flies to tolerate P. entomophila infection, despite carrying greater microbe loads than males. Together, our findings provide evidence for the role of damage limitation mechanisms in disease tolerance and highlight how sexual dimorphism in these mechanisms could generate sex differences in infection outcomes.

The cost of host genetic resistance on body condition: evidence from divergently selected sheep

Trade‐offs between host resistance to parasites and host growth or reproduction can occur due to allocation of limited available resources between competing demands. To predict potential trade‐offs arising from genetic selection for host resistance, a better understanding of the associated nutritional costs is required. Here, we studied resistance costs by using sheep from lines divergently selected on their resistance to a common blood‐feeding gastro‐intestinal parasite (Haemonchus contortus). First, we assessed the effects of selection for high or low host resistance on condition traits (body weight, back fat, and muscle thickness) and infection traits (parasite fecal egg excretion and loss in blood haematocrit) at various life stages, in particular during the periparturient period when resource allocation to immunity may limit host resistance. Second, we analysed the condition–infection relationship to detect a possible trade‐off, in particular during the periparturient period. We experimentally infected young females in four stages over their first 2 years of life, including twice around parturition (at 1 year and at 2 years of age). Linear mixed‐model analyses revealed a large and consistent between‐line difference in infection traits during growth and outside of the periparturient period, whereas this difference was strongly attenuated during the periparturient period. Despite their different responses to infection, lines had similar body condition traits. Using covariance decomposition, we then found that the phenotypic relationship between infection and condition was dominated by direct infection costs arising from parasite development within the host. Accounting for these within‐individual effects, a cost of resistance on body weight was detected among ewes during their first reproduction. Although this cost and the reproductive constraint on resistance are unlikely to represent a major concern for animal breeding in nutrient‐rich environments, this study provides important new insights regarding the nutritional costs of parasite resistance at different lifestages and how these may affect response to selection.

The Interplay between the Host Microbiome and Pathogenic Viral Infections

The microorganisms associated with an organism, the microbiome, have a strong and wide impact in their host biology. In particular, the microbiome modulates both the host defense responses and immunity, thus influencing the fate of infections by pathogens. Indeed, this immune modulation and/or interaction with pathogenic viruses can be essential to define the outcome of viral infections. Understanding the interplay between the microbiome and pathogenic viruses opens future venues to fight viral infections and enhance the efficacy of antiviral therapies. An increasing number of researchers are focusing on microbiome-virus interactions, studying diverse combinations of microbial communities, hosts, and pathogenic viruses. Here, we aim to review these studies, providing an integrative overview of the microbiome impact on viral infection across different pathosystems.

Review: Livestock disease resilience: from individual to herd level

Infectious diseases are a major threat to the sustainable production of high-producing animals. Control efforts, such as vaccination or breeding approaches often target improvements to individual resilience to infections, i.e., they strengthen an animal's ability to cope with infection, rather than preventing infection per se. There is increasing evidence for the contribution of non-clinical carriers (animals that become infected and are infectious but do not develop clinical signs) to the overall health and production of livestock populations for a wide range of infectious diseases. Therefore, we strongly advocate a shift of focus from increasing the disease resilience of individual animals to herd disease resilience as the appropriate target for sustainable disease control in livestock. Herd disease resilience not only captures the direct effects of vaccination or host genetics on the health and production performance of individuals but also the indirect effects on the environmental pathogen load that herd members are exposed to. For diseases primarily caused by infectious pathogens shed by herd members, these indirect effects on herd resilience are mediated both by individual susceptibility to infection and by characteristics (magnitude of infectiousness, duration of infectious period) that influence pathogen shedding from infected individuals. We review what is currently known about how vaccination and selective breeding affect herd disease resilience and its underlying components, and outline the changes required for improvement. To this purpose, we also seek to clarify and harmonise the terminology used in the different animal science disciplines to facilitate future collaborative approaches to infectious disease control in livestock.

Combining Colistin with Furanone C-30 Rescues Colistin Resistance of Gram-Negative Bacteria in Vitro and in Vivo

The spread of multidrug-resistant (MDR) Gram-negative bacteria (GNB) has led to serious public health problems worldwide. Colistin, as a “last resort” for the treatment of MDR bacterial infections, has been used significantly in recent years and has led to the continuous emergence of colistin-resistant strains. In this study, we aimed to investigate the synergistic effect on the antimicrobial and antibiofilm activities of a colistin/furanone C-30 combination against colistin-resistant GNB in vitro and in vivo. According to antimicrobial resistance profiles, most of the colistin-resistant strains we collected showed MDR phenotypes. The checkerboard method and time-kill curve showed that the combination with furanone C-30 increases the antibacterial activity of colistin significantly. In addition, the furanone C-30/colistin combination can not only inhibit the formation of bacterial biofilm but also has a better eradication effect on preformed mature biofilms. The result of scanning electron microscopy (SEM) demonstrated that the furanone C-30/colistin combination led to a significant reduction in the number of cells in biofilms. Furthermore, furanone C-30 at 50 μg/ml did not cause any additional toxicity to RAW264.7 cells according to a cytotoxicity assay. In in vivo infection experiments, the furanone C-30/colistin combination increased the survival rate of infected Galleria mellonella larvae as well as decreased the microbial load in a mouse thigh infection model. The synergistic effect of the furanone C-30/colistin combination against colistin-resistant GNB is encouraging, and this work may shed light on a new therapeutic approach to combat colistin-resistant pathogens.

IMPORTANCE Colistin is among the few antibiotics effective against multidrug-resistant Gram-negative bacteria (GNB) clinical isolates. However, colistin-resistant GNB strains have emerged in recent years. Therefore, the combination of colistin and nonantibacterial drugs has attracted much attention. In this study, the furanone C-30/colistin combination showed good antibacterial and antibiofilm activity in vitro and in vivo. In addition, increased membrane permeability leads to the synergistic effect of the furanone C-30/colistin combination. Because of the low cytotoxicity of furanone C-30, this combination has good application prospects in clinical anti-infective therapy. This finding might shed light on the discovery of combination therapy for infections caused by colistin-resistant GNB pathogens.

Support vector machine-based prediction of pore-forming toxins (PFT) using distributed representation of reduced alphabets

Bacterial virulence can be attributed to a wide variety of factors including toxins that harm the host. Pore-forming toxins are one class of toxins that confer virulence to the bacteria and are one of the promising targets for therapeutic intervention. In this work, we develop a sequence-based machine learning framework for the prediction of pore-forming toxins. For this, we have used distributed representation of the protein sequence encoded by reduced alphabet schemes based on conformational similarity and hydropathy index as input features to Support Vector Machines (SVMs). The choice of conformational similarity and hydropathy indices is based on the functional mechanism of pore-forming toxins. Our methodology achieves about 81% accuracy indicating that conformational similarity, an indicator of the flexibility of amino acids, along with hydrophobic index can capture the intrinsic features of pore-forming toxins that distinguish it from other types of transporter proteins. Increased understanding of the mechanisms of pore-forming toxins can further contribute to the use of such "mechanism-informed" features that may increase the prediction accuracy further.

Cysteamine Inhibits Glycine Utilisation and Disrupts Virulence in Pseudomonas aeruginosa

Pseudomonas aeruginosa is a major opportunistic human pathogen which employs a myriad of virulence factors. In people with cystic fibrosis (CF) P. aeruginosa frequently colonises the lungs and becomes a chronic infection that evolves to become less virulent over time, but often adapts to favour persistence in the host with alginate-producing mucoid, slow-growing, and antibiotic resistant phenotypes emerging. Cysteamine is an endogenous aminothiol which has been shown to prevent biofilm formation, reduce phenazine production, and potentiate antibiotic activity against P. aeruginosa, and has been investigated in clinical trials as an adjunct therapy for pulmonary exacerbations of CF. Here we demonstrate (for the first time in a prokaryote) that cysteamine prevents glycine utilisation by P. aeruginosa in common with previously reported activity blocking the glycine cleavage system in human cells. Despite the clear inhibition of glycine metabolism, cysteamine also inhibits hydrogen cyanide (HCN) production by P. aeruginosa, suggesting a direct interference in the regulation of virulence factor synthesis. Cysteamine impaired chemotaxis, lowered pyocyanin, pyoverdine and exopolysaccharide production, and reduced the toxicity of P. aeruginosa secreted factors in a Galleria mellonella infection model. Thus, cysteamine has additional potent anti-virulence properties targeting P. aeruginosa, further supporting its therapeutic potential in CF and other infections.

Indole decreases the virulence of pathogenic vibrios belonging to the Harveyi clade

AIM

Indole is a signaling molecule secreted by over 85 species of bacteria, including several Vibrio species, and it has been reported to affect different bacterial phenotypes such as biofilm formation, motility, and virulence. In this study, we aimed at investigating the inter-strain variability of the effect of indole in 12 different strains belonging to the Harveyi clade of vibrios.

METHODS AND RESULTS

Indole reduced the virulence of all strains towards gnotobiotic brine shrimp larvae. The survival rate of brine shrimp larvae challenged with vibrios pretreated with indole was increased by 1.3-fold to 1.8-fold. Additionally, indole significantly decreased the biofilm formation in all of the strains, decreased the swimming motility in eight of the strains, and decreased swarming motility in five of the strains. When cultured in the presence of exogenous indole, the mRNA level of the pirA and pirB toxin genes were down-regulated to 65% and 46%, and to 62% and 55% in the AHPND-causing strains Vibrio parahaemolyticus M0904 and Vibrio campbellii S01, respectively.

CONCLUSIONS

These data indicate that indole has a significant impact on the virulence of different strains belonging to the Harveyi clade of vibrios.

SIGNIFICANCE AND IMPACT OF THE STUDY

Our results suggest that indole signaling is a valid target for the development of novel therapeutics in order to control infections caused by Harveyi clade vibrios in aquaculture.

Xylopic acid-amodiaquine and xylopic acid-artesunate combinations are effective in managing malaria in Plasmodium berghei-infected mice

Background

Evidence of Plasmodium resistance to some of the current anti-malarial agents makes it imperative to search for newer and effective drugs to combat malaria. Therefore, this study evaluated whether the co-administrations of xylopic acid-amodiaquine and xylopic acid-artesunate combinations will produce a synergistic anti-malarial effect.

Methods

Antiplasmodial effect of xylopic acid (XA: 3, 10, 30, 100, 150 mg kg⁻¹), artesunate (ART: 1, 2, 4, 8, 16 mg kg⁻¹), and amodiaquine (AQ: 1.25, 2.5, 5, 10, 20 mg kg⁻¹) were evaluated in Plasmodium berghei (strain ANKA)-infected mice to determine respective ED₅₀s. Artemether/lumefantrine was used as the positive control. XA/ART and XA/AQ were subsequently administered in a fixed-dose combination of their ED₅₀s (1:1) and the combination fractions of their ED₅₀s (1/2, 1/4, 1/8, 1/16, and 1/32) to determine the experimental ED₅₀s (Z_exp). An isobologram was constructed to determine the nature of the interaction between XA/ART, and XA/AQ combinations by comparing Z_exp with the theoretical ED₅₀ (Z_add). Bodyweight and 30-day survival post-treatment were additionally recorded.

Results

ED₅₀s for XA, ART, and AQ were 9.0 ± 3.2, 1.61 ± 0.6, and 3.1 ± 0.8 mg kg⁻¹, respectively. The Z_add, Z_exp, and interaction index for XA/ART co-administration was 5.3 ± 2.61, 1.98 ± 0.25, and 0.37, respectively while that of XA/AQ were 6.05 ± 2.0, 1.69 ± 0.42, and 0.28, respectively. The Z_exp for both combination therapies lay significantly (p < 0.001) below the additive isoboles showing XA acts synergistically with both ART and AQ in clearing the parasites. High doses of XA/ART combination significantly (p < 0.05) increased the survival days of infected mice with a mean hazard ratio of 0.40 while all the XA/AQ combination doses showed a significant (p < 0.05) increase in the survival days of infected mice with a mean hazard ratio of 0.27 similar to AL. Both XA/ART and XA/AQ combined treatments significantly (p < 0.05) reduced weight loss.

Conclusion

Xylopic acid co-administration with either artesunate or amodiaquine produces a synergistic anti-plasmodial effect in mice infected with P. berghei.

Antivirulence DsbA inhibitors attenuate Salmonella enterica serovar Typhimurium fitness without detectable resistance

Inhibition of the DiSulfide Bond (DSB) oxidative protein folding machinery, a major facilitator of virulence in Gram‐negative bacteria, represents a promising antivirulence strategy. We previously developed small molecule inhibitors of DsbA from Escherichia coli K‐12 (EcDsbA) and showed that they attenuate virulence of Gram‐negative pathogens by directly inhibiting multiple diverse DsbA homologues. Here we tested the evolutionary robustness of DsbA inhibitors as antivirulence antimicrobials against Salmonella enterica serovar Typhimurium under pathophysiological conditions in vitro. We show that phenylthiophene DsbA inhibitors slow S. Typhimurium growth in minimal media, phenocopying S. Typhimurium isogenic dsbA null mutants. Through passaging experiments, we found that DsbA inhibitor resistance was not induced under conditions that rapidly induced resistance to ciprofloxacin, an antibiotic commonly used to treat Salmonella infections. Furthermore, no mutations were identified in the dsbA gene of inhibitor‐treated S. Typhimurium, and S. Typhimurium virulence remained susceptible to DsbA inhibitors. Our work demonstrates that under in vitro pathophysiological conditions, DsbA inhibitors can have both antivirulence and antibiotic action. Importantly, our finding that DsbA inhibitors appear to be evolutionarily robust offers promise for their further development as next‐generation antimicrobials against Gram‐negative pathogens.

Putative determinants of virulence in Melissococcus plutonius, the bacterial agent causing European foulbrood in honey bees

Melissococcus plutonius

is a bacterial pathogen that causes epidemic outbreaks of European foulbrood (EFB) in honey bee populations. The pathogenicity of a bacterium depends on its virulence, and understanding the mechanisms influencing virulence may allow for improved disease control and containment. Using a standardized in vitro assay, we demonstrate that virulence varies greatly among sixteen M. plutonius isolates from five European countries. Additionally, we explore the causes of this variation. In this study, virulence was independent of the multilocus sequence type of the tested pathogen, and was not affected by experimental co-infection with Paenibacillus alvei, a bacterium often associated with EFB outbreaks. Virulence in vitro was correlated with the growth dynamics of M. plutonius isolates in artificial medium, and with the presence of a plasmid carrying a gene coding for the putative toxin melissotoxin A. Our results suggest that some M. plutonius strains showed an increased virulence due to the acquisition of a toxin-carrying mobile genetic element. We discuss whether strains with increased virulence play a role in recent EFB outbreaks.

QseC Inhibition as a Novel Antivirulence Strategy for the Prevention of Acute Hepatopancreatic Necrosis Disease (AHPND)-Causing Vibrio parahaemolyticus

Acute hepatopancreatic necrosis disease (AHPND) caused by Vibrio parahaemolyticus resulted in great economic losses in global shrimp aquaculture. There is an urgent need for development of novel strategies to combat AHPND-causing V. parahaemolyticus (Vp _AHPND), given that one of the greatest challenges currently is the widespread use of antibiotics and subsequent emergence of multidrug-resistant bacteria. Here, we proposed a broad-spectrum antivirulence approach targeting a conserved histidine kinase, QseC, which has been demonstrated to activate virulence expression in several Gram-negative pathogens. Our results showed that QseC mediated the catecholamine stimulated effects on growth and flagellar motility of Vp _AHPND. Transcriptome analysis revealed that QseC was involved in the global regulation of the virulence of Vp _AHPND as the ΔqseC mutant exhibited a decreased expression of genes related to type IV pilin, flagellar motility, and biofilm formation, while an overexpression of type VI secretion system and cell wall biosynthesis. Subsequently, the bacterial catecholamine receptor antagonist LED209 not only neutralized the stimulatory effects of host catecholamines on the growth and motility of Vp _AHPND in vitro, but also attenuated the virulence of Vp _AHPND towards brine shrimp larvae and white shrimp in vivo. Additionally, LED209 presented no interference with pathogen growth, nor the toxicity to the experimental animals. These results suggest that QseC can be an attractive antivirulence therapy target, and LED209 is a promising candidate for development of broad-spectrum antivirulence agents. This is the first study that demonstrated the role of QseC in the global regulation of Vp _AHPND infection and demonstrated the antivirulence potential of LED209, which provides insight into the use of an antivirulence approach for targeting not only Vp _AHPND, but also a much larger collection of pathogenic bacteria.

Tolerance of liver fluke infection varies between breeds and producers in beef cattle

Liver flukes (Fasciola spp.) are important helminth parasites of livestock globally and cause substantial reductions in health and productivity of beef cattle. Attempts to control fluke have been thwarted by the difficulty of vaccine design, the evolution of flukicide resistance and the need to control the intermediate snail host. Mechanisms to reduce the impact of parasites on animal performance have typically focused on promoting host resistance - defined as the ability of the host to kill and remove the parasite from its system - and such strategies include improving protein nutrition or selective breeding for resistance. Organisms, however, have another broad mechanism for mitigating the impact of parasites: they can show tolerance, defined as the ability to maintain health or performance under increasing parasite burden. Tolerance has been studied in the plant literature for over a century, but there are very few empirical studies of parasite tolerance in livestock. In this study, we used data collected from >90 000 beef cattle to estimate the impact of the severity of liver fluke infection on performance and variation in tolerance of fluke. Severity of liver fluke infection was estimated using liver "fibrosis score" on a scale of 0-3 and performance estimated as (1) age at slaughter and (2) daily dead weight gain. Animals with higher fibrosis scores were slaughtered around 2 weeks later than animals with no fluke and gained around 10 g less weight per day. There was also considerable variation in these effects of fibrosis score, such that animals from different producers and breeds varied in their tolerance of fluke infection. While breeds did not vary in the association between fibrosis and age at slaughter, there was considerable variation among producers: high fibrosis score delayed slaughter by up to 50 days in some producers, but not at all in others. Meanwhile, there was support for variation in the slope of daily dead weight gain on fibrosis score among both breeds and producers, with some unaffected by high fluke scores and some breeds and producers experiencing a 20 g/day lower weight gain under high fluke scores. Our results point to the potential for both environmental and genetic variation in tolerance of liver fluke in cattle, paving the way for quantitative genetic and nutritional research into the feasibility of promoting tolerance as a disease mitigation strategy.

Host-shift as the cause of emerging infectious diseases: Experimental approaches using Drosophila-virus interactions

Host shifts, when a cross-species transmission of a pathogen can lead to successful infections, are the main cause of emerging infectious diseases, such as COVID-19. A complex challenge faced by the scientific community is to address the factors that determine whether the cross-species transmissions will result in spillover or sustained onwards infections. Here we review recent literature and present a perspective on current approaches we are using to understand the mechanisms underlying host shifts. We highlight the usefulness of the interactions between Drosophila species and viruses as an ideal study model. Additionally, we discuss how cross-infection experiments - when pathogens from a natural reservoir are intentionally injected in novel host species- can test the effect cross-species transmissions may have on the fitness of virus and host, and how the host phylogeny may influence this response. We also discuss experiments evaluating how cooccurrence with other viruses or the presence of the endosymbiont bacteria Wolbachia may affect the performance of new viruses in a novel host. Finally, we discuss the need of surveys of virus diversity in natural populations using next-generation sequencing technologies. In the long term, these approaches can contribute to a better understanding of the basic biology of host shifts.

Controlling foodborne pathogens with natural antimicrobials by biological control and antivirulence strategies

Foodborne diseases represent a global health threat besides the great economic losses encountered by the food industry. These hazards necessitate the implementation of food preservation methods to control foodborne pathogens, the causal agents of human illnesses. Until now, most control methods rely on inhibiting the microbial growth or eliminating the pathogens by applying lethal treatments. Natural antimicrobials, which inhibit microbial growth, include traditional chemicals, naturally occurring antimicrobials, or biological preservation (e.g. beneficial microbes, bacteriocins, or bacteriophages). Although having great antimicrobial effectiveness, challenges due to the adaptation of foodborne pathogens to such control methods are becoming apparent. Such adaptation enables the survival of the pathogens in foods or food-contact environments. This imperative concern inspires contemporary research and food industry sector to develop technologies which do not target microbial growth but disarming microbial virulence factors. These technologies, referred to as "antivirulence", render the microbe non-capable of causing the disease with very limited or no opportunities for the pathogenic microorganisms to develop resistance. For the sake of safer and fresh-like foods, with no effect on the sensory properties of foods, a combination of two or more natural antimicrobials or with other stressors, is now widespread, to preserve foods. This review introduces and critically describes the traditional versus the emerging uses of natural antimicrobials for controlling foodborne pathogens in foods. Development of biological control strategies using natural antimicrobials proved to be effective in inhibiting microbial growth in foods and allowing improved food safety. In the meanwhile, discovery of new antivirulence agents could be a transformative strategy in food preservation in the far future.

Combining antibiotics with antivirulence compounds can have synergistic effects and reverse selection for antibiotic resistance in Pseudomonas aeruginosa

Antibiotics are losing efficacy due to the rapid evolution and spread of resistance. Treatments targeting bacterial virulence factors have been considered as alternatives because they target virulence instead of pathogen viability, and should therefore exert weaker selection for resistance than conventional antibiotics. However, antivirulence treatments rarely clear infections, which compromises their clinical applications. Here, we explore the potential of combining antivirulence drugs with antibiotics against the opportunistic human pathogen Pseudomonas aeruginosa. We combined two antivirulence compounds (gallium, a siderophore quencher, and furanone C-30, a quorum sensing [QS] inhibitor) together with four clinically relevant antibiotics (ciprofloxacin, colistin, meropenem, tobramycin) in 9×9 drug concentration matrices. We found that drug-interaction patterns were concentration dependent, with promising levels of synergies occurring at intermediate drug concentrations for certain drug pairs. We then tested whether antivirulence compounds are potent adjuvants, especially when treating antibiotic resistant (AtbR) clones. We found that the addition of antivirulence compounds to antibiotics could restore growth inhibition for most AtbR clones, and even abrogate or reverse selection for resistance in five drug combination cases. Molecular analyses suggest that selection against resistant clones occurs when resistance mechanisms involve restoration of protein synthesis, but not when efflux pumps are up-regulated. Altogether, our work provides a first systematic analysis of antivirulence-antibiotic combinatorial treatments and suggests that such combinations have the potential to be both effective in treating infections and in limiting the spread of antibiotic resistance.

Inhibition of biofilm and virulence properties of Pseudomonas aeruginosa by sub-inhibitory concentrations of aminoglycosides

Aminoglycosides are a commonly used class of antibiotics; however, their application has been discontinued due to the emergence of multi-drug resistance bacterial strains. In the present study, the subinhibitory concentrations (sub-MIC) of several aminoglycosides were determined and tested as an antibiofilm and for their anti-virulence properties against Pseudomonas aeruginosa PAO1, which is an opportunistic foodborne pathogen. P. aeruginosa PAO1 exhibits multiple mechanisms of resistance, including the formation of biofilm and production of several virulence factors, against aminoglycoside antibiotics. The sub-MIC of these antibiotics exhibited biofilm inhibition of P. aeruginosa in alkaline TSB (pH 7.9). Moreover, various concentrations of these aminoglycosides also eradicate the mature biofilm of P. aeruginosa. In the presence of sub-MIC of aminoglycosides, the morphological changes of P. aeruginosa were found to change from rod-shaped to the filamentous, elongated, and streptococcal forms. Similar growth conditions and sub-MIC of aminoglycosides were also found to attenuate several virulence properties of P. aeruginosa PAO1. Molecular docking studies demonstrate that these aminoglycosides possess strong binding properties with the LasR protein, which is a well-characterized quorum-sensing receptor of P. aeruginosa. The present study suggests a new approach to revitalize aminoglycosides as antibiofilm and antivirulence drugs to treat infections caused by pathogenic bacteria.

Probiotic Consortia: Reshaping the Rhizospheric Microbiome and Its Role in Suppressing Root-Rot Disease of Panax notoginseng

Root-rot disease caused by Fusarium oxysporum is a growing problem in agriculture for commercial cultivation of Panax notoginseng. Diverse microbes colonize plant roots, and numerous earlier studies have characterized the rhizospheric microbiome of P. notoginseng; nevertheless, the function of probiotic consortia on the rhizospheric microbiome against the root-rot disease remain elusive. We have compared and described the rhizospheric microbiome of lightly and severely diseased P. notoginseng as well as the interactions of the probiotic consortia and rhizospheric microbiome, and their function to alleviate the plant diseases were explored by inoculating probiotic consortia in bulk soil. From the perspective of microbial diversity, the rhizospheric dominant bacterial and fungal genera were utterly different between lightly and severely diseased plants. Through inoculating assembled probiotic consortia to diseased plant roots, we found that the application of probiotic consortia reshaped the rhizosphere microbiome, increasing the relative abundance of bacteria and fungi, while the relative abundance of potential pathogens was decreased significantly. We developed a microcosm system that provides a preliminary ecological framework for constructing an active probiotic community to reshape soil microbiota and restrain the disease. Microbial community structure differs between lightly and seriously diseased plants. The application of probiotic consortia changes the imbalance of micro-ecology to a state of relative health, reducing plant mortality. Plant disease suppression may be achieved by seeking and applying antagonistic microbes based on their direct inhibitory capability or by restructuring the soil microbiome structure and function.

Proliferation and benevolence-A framework for dissecting the mechanisms of microbial virulence and health promotion

Key topics in the study of host–microbe interactions—such as the prevention of drug resistance and the exploitation of beneficial effects of bacteria—would benefit from concerted efforts with both mechanistic and evolutionary approaches. But due to differences in intellectual traditions, insights gained in one field rarely benefit the other. Here, we develop a conceptual and analytical framework for the integrated study of host–microbe interactions. This framework partitions the health effects of microbes and the effector molecules they produce into components with different evolutionary implications. It thereby facilitates the prediction of evolutionary responses to inhibition and exploitation of specific molecular mechanisms.

Harnessing bacterial interactions to manage infections: a review on the opportunistic pathogen Pseudomonas aeruginosa as a case example

During infections, bacterial pathogens can engage in a variety of interactions with each other, ranging from the cooperative sharing of resources to deadly warfare. This is especially relevant in opportunistic infections, where different strains and species often co-infect the same patient and interact in the host. Here, we review the relevance of these social interactions during opportunistic infections using the human pathogen Pseudomonas aeruginosa as a case example. In particular, we discuss different types of pathogen-pathogen interactions, involving both cooperation and competition, and elaborate on how they impact virulence in multi-strain and multi-species infections. We then review evolutionary dynamics within pathogen populations during chronic infections. We particuarly discuss how local adaptation through niche separation, evolutionary successions and antagonistic co-evolution between pathogens can alter virulence and the damage inflicted on the host. Finally, we outline how studying bacterial social dynamics could be used to manage infections. We show that a deeper appreciation of bacterial evolution and ecology in the clinical context is important for understanding microbial infections and can inspire novel treatment strategies.

Vanillin inhibits PqsR-mediated virulence in Pseudomonas aeruginosa

Vanillin, a natural phenolic aldehyde from vanilla bean, has been reported to inhibit pqs quorum sensing in Pseudomonas aeruginosa, with potential applications in combinatorial antimicrobial therapy against biofilm infections.

Probiotic Consortia: Reshaping the Rhizospheric Microbiome and Its Role in Suppressing Root-Rot Disease of Panax notoginseng

Root-rot disease caused by Fusarium oxysporum is a growing problem in agriculture for commercial cultivation of Panax notoginseng. Diverse microbes colonize plant roots, and numerous earlier studies have characterized the rhizospheric microbiome of P. notoginseng; nevertheless, the function of probiotic consortia on the rhizospheric microbiome against the root-rot disease remain elusive. We have compared and described the rhizospheric microbiome of lightly and severely diseased P. notoginseng as well as the interactions of the probiotic consortia and rhizospheric microbiome, and their function to alleviate the plant diseases were explored by inoculating probiotic consortia in bulk soil. From the perspective of microbial diversity, the rhizospheric dominant bacterial and fungal genera were utterly different between lightly and severely diseased plants. Through inoculating assembled probiotic consortia to diseased plant roots, we found that the application of probiotic consortia reshaped the rhizosphere microbiome, increasing the relative abundance of bacteria and fungi, while the relative abundance of potential pathogens was decreased significantly. We developed a microcosm system that provides a preliminary ecological framework for constructing an active probiotic community to reshape soil microbiota and restrain the disease. Microbial community structure differs between lightly and seriously diseased plants. The application of probiotic consortia changes the imbalance of micro-ecology to a state of relative health, reducing plant mortality. Plant disease suppression may be achieved by seeking and applying antagonistic microbes based on their direct inhibitory capability or by restructuring the soil microbiome structure and function.

BabA and LPS inhibitors against Helicobacter pylori: pectins and pectin-like rhamnogalacturonans as adhesion blockers

The first step in the development of Helicobacter pylori pathogenicity is receptor-mediated adhesion to gastric epithelium. Adhesins of H. pylori not only enable colonisation of the epithelium, with BabA interacting with Lewis^b, but also interaction of lipopolysaccharide (LPS) with galectin-3 contributes to attachment of H. pylori to the host cells. Anti-adhesive compounds against H. pylori have been described, but specific analytical assays for pinpointing the interaction with BabA are limited. LPS-galectin-3 inhibitors have not been described until now. A sandwich ELISA with recombinant BabA₅₄₇-6K was developed to investigate the interaction of BabA with Lewis^b-HSA. Isothermal titration calorimetry gave thermodynamic information on the interaction between BabA, Lewis^b-HSA and anti-adhesive compounds. A highly esterified rhamnogalacturonan from Abelmoschus esculentus inhibited the adhesion of H. pylori to adherent gastric adenocarcinoma (AGS) cells (IC₅₀ 550 μg/mL) and interacted with BabA (IC₅₀ 17 μg/mL). Pectins with similar rhamnogalacturonan structure showed weak anti-adhesive activity. Highly branched rhamnogalacturonans with low uronic acid content and high degree of esterification are potent BabA inhibitors. BabA represents a promising target for the development of anti-adhesive drugs against H. pylori. The rhamnogalacturonan influenced also the binding affinity of H. pylori to recombinant galectin-3 in a concentration-dependent manner with an IC₅₀ of 222 μg/mL. Similar effects were obtained with pectin from apple fruits, while pectins from other sources were inactive.

Down-regulation of TUFM impairs host cell interaction and virulence by Paracoccidioides brasiliensis

The genus Paracoccidioides consist of dimorphic fungi geographically limited to the subtropical regions of Latin America, which are responsible for causing deep systemic mycosis in humans. However, the molecular mechanisms by which Paracoccidioides spp. causes the disease remain poorly understood. Paracoccidioides spp. harbor genes that encode proteins involved in host cell interaction and mitochondrial function, which together are required for pathogenicity and mediate virulence. Previously, we identified TufM (previously known as EF-Tu) in Paracoccidioides brasiliensis (PbTufM) and suggested that it may be involved in the pathogenicity of this fungus. In this study, we examined the effects of downregulating PbTUFM using a silenced strain with a 55% reduction in PbTUFM expression obtained by antisense-RNA (aRNA) technology. Silencing PbTUFM yielded phenotypic differences, such as altered translation elongation, respiratory defects, increased sensitivity of yeast cells to reactive oxygen stress, survival after macrophage phagocytosis, and reduced interaction with pneumocytes. These results were associated with reduced virulence in Galleria mellonella and murine infection models, emphasizing the importance of PbTufM in the full virulence of P. brasiliensis and its potential as a target for antifungal agents against paracoccidioidomycosis.

In silico Selection and Experimental Validation of FDA-Approved Drugs as Anti-quorum Sensing Agents

The emergence of antibiotic resistant bacterial pathogens is increasing at an unprecedented pace, calling for the development of new therapeutic options. Small molecules interfering with virulence processes rather than growth hold promise as an alternative to conventional antibiotics. Anti-virulence agents are expected to decrease bacterial virulence and to pose reduced selective pressure for the emergence of resistance. In the opportunistic pathogen Pseudomonas aeruginosa the expression of key virulence traits is controlled by quorum sensing (QS), an intercellular communication process that coordinates gene expression at the population level. Hence, QS inhibitors represent promising anti-virulence agents against P. aeruginosa. Virtual screenings allow fast and cost-effective selection of target ligands among vast libraries of molecules, thus accelerating the time and limiting the cost of conventional drug-discovery processes, while the drug-repurposing approach is based on the identification of off-target activity of FDA-approved drugs, likely endowed with low cytotoxicity and favorable pharmacological properties. This study aims at combining the advantages of virtual screening and drug-repurposing approaches to identify new QS inhibitors targeting the pqs QS system of P. aeruginosa. An in silico library of 1,467 FDA-approved drugs has been screened by molecular docking, and 5 hits showing the highest predicted binding affinity for the pqs QS receptor PqsR (also known as MvfR) have been selected. In vitro experiments have been performed by engineering ad hoc biosensor strains, which were used to verify the ability of hit compounds to decrease PqsR activity in P. aeruginosa. Phenotypic analyses confirmed the impact of the most promising hit, the antipsychotic drug pimozide, on the expression of P. aeruginosa PqsR-controlled virulence traits. Overall, this study highlights the potential of virtual screening campaigns of FDA-approved drugs to rapidly select new inhibitors of important bacterial functions.

Fisetin, a 3,7,3',4'-Tetrahydroxyflavone Inhibits the PI3K/Akt/mTOR and MAPK Pathways and Ameliorates Psoriasis Pathology in 2D and 3D Organotypic Human Inflammatory Skin Models

Psoriasis is a chronic immune-mediated skin disease that involves the interaction of immune and skin cells, and is characterized by cytokine-driven epidermal hyperplasia, deviant differentiation, inflammation, and angiogenesis. Because the available treatments for psoriasis have significant limitations, dietary products are potential natural sources of therapeutic molecules, which can repair the molecular defects associated with psoriasis and could possibly be developed for its management. Fisetin (3,7,3',4'-tetrahydroxyflavone), a phytochemical naturally found in pigmented fruits and vegetables, has demonstrated proapoptotic and antioxidant effects in several malignancies. This study utilized biochemical, cellular, pharmacological, and tissue engineering tools to characterize the effects of fisetin on normal human epidermal keratinocytes (NHEKs), peripheral blood mononuclear cells (PBMC), and CD4+ T lymphocytes in 2D and 3D psoriasis-like disease models. Fisetin treatment of NHEKs dose- and time-dependently induced differentiation and inhibited interleukin-22-induced proliferation, as well as activation of the PI3K/Akt/mTOR pathway. Fisetin treatment of TNF-α stimulated NHEKs also significantly inhibited the activation of p38 and JNK, but had enhanced effect on ERK1/2 (MAPK). In addition, fisetin treatment significantly decreased the secretion of Th1/Th-17 pro-inflammatory cytokines, particularly IFN-γ and IL-17A by 12-O-tetradecanolylphorbol 13-acetate (TPA)-stimulated NHEKs and anti-CD3/CD28-activated human PBMCs. Furthermore, we established the in vivo relevance of fisetin functions, using a 3D full-thickness human skin model of psoriasis (FTRHSP) that closely mimics in vivo human psoriatic skin lesions. Herein, fisetin significantly ameliorated psoriasis-like disease features, and decreased the production of IL-17 by CD4+ T lymphocytes co-cultured with FTRHSP. Collectively, our data identify the prodifferentiative, antiproliferative, and anti-inflammatory effects of fisetin, via modulation of the PI3K-Akt-mTOR and p38/JNK pathways and the production of cytokines in 2D and 3D human skin models of psoriasis. These results suggest that fisetin has a great potential to be developed as an effective and inexpensive agent for the treatment of psoriasis and other related inflammatory skin disorders.

Disease Tolerance: Linking Sickness Behaviours to Metabolism Helps Mitigate Malaria

Malaria-infected mice exhibit a range of sickness behaviours, and experience metabolic shifts and physiological pathologies that result in reduced energy expenditure. Treating sick mice with glucose increases disease tolerance by improving the physiological and behavioural symptoms of malaria infection without affecting parasite loads.

Recent Advances in Anti-virulence Therapeutic Strategies With a Focus on Dismantling Bacterial Membrane Microdomains, Toxin Neutralization, Quorum-Sensing Interference and Biofilm Inhibition

Antimicrobial resistance constitutes one of the major challenges facing humanity in the Twenty-First century. The spread of resistant pathogens has been such that the possibility of returning to a pre-antibiotic era is real. In this scenario, innovative therapeutic strategies must be employed to restrict resistance. Among the innovative proposed strategies, anti-virulence therapy has been envisioned as a promising alternative for effective control of the emergence and spread of resistant pathogens. This review presents some of the anti-virulence strategies that are currently being developed, it will cover strategies focused on quench pathogen quorum sensing (QS) systems, disassemble of bacterial functional membrane microdomains (FMMs), disruption of biofilm formation and bacterial toxin neutralization.

How Individual Variation in Host Tolerance Affects Competence to Transmit Parasites

Tolerance, or the maintenance of host health or fitness at a given parasite burden, has often been studied in evolutionary and medical contexts, particularly with respect to effects on the evolution of parasite virulence and individual patient outcomes. These bodies of work have provided insight about tolerance for evolutionary phenomena (e.g., virulence) and individual health (e.g., recovering from an infection). However, due to the specific motivations of that work, few studies have considered the ecological ramifications of variation in tolerance, namely, how variation in forms of tolerance could mediate parasite movement through populations and even community-level disease dynamics. Tolerance is most commonly regarded as the relationship between host fitness and parasite burden. However, few if any studies have actually quantified host fitness, instead utilizing proxies of fitness as the response variables to be regressed against parasite burden. Here, we address how attention to the effects of parasite burden on traits that are relevant to host competence (i.e., the ability to amplify parasites to levels transmissible to other hosts/vectors) will enhance our understanding of disease dynamics in nature. We also provide several forms of guidance for how to overcome the challenges of quantifying tolerance in wild organisms.

Interference With Quorum-Sensing Signal Biosynthesis as a Promising Therapeutic Strategy Against Multidrug-Resistant Pathogens

Faced with the global health threat of increasing resistance to antibiotics, researchers are exploring interventions that target bacterial virulence factors. Quorum sensing is a particularly attractive target because several bacterial virulence factors are controlled by this mechanism. Furthermore, attacking the quorum-sensing signaling network is less likely to select for resistant strains than using conventional antibiotics. Strategies that focus on the inhibition of quorum-sensing signal production are especially attractive because the enzymes involved are expressed in bacterial cells but are not present in their mammalian counterparts. We review here various approaches that are being taken to interfere with quorum-sensing signal production via the inhibition of autoinducer-2 synthesis, PQS synthesis, peptide autoinducer synthesis, and N-acyl-homoserine lactone synthesis. We expect these approaches will lead to the discovery of new quorum-sensing inhibitors that can help to stem the tide of antibiotic resistance.

Resistance and Tolerance to Cryptococcal Infection: An Intricate Balance That Controls the Development of Disease

Cryptococcus neoformans is a ubiquitous environmental yeast and a leading cause of invasive fungal infection in humans. The most recent estimate of global disease burden includes over 200,000 cases of cryptococcal meningitis each year. Cryptococcus neoformans expresses several virulence factors that may have originally evolved to protect against environmental threats, and human infection may be an unintended consequence of these acquired defenses. Traditionally, C. neoformans has been viewed as a purely opportunistic pathogen that targets severely immune compromised hosts; however, during the past decade the spectrum of susceptible individuals has grown considerably. In addition, the closely related strain Cryptococcus gattii has recently emerged in North America and preferentially targets individuals with intact immunity. In parallel to the changing epidemiology of cryptococcosis, an increasing role for host immunity in the pathogenesis of severe disease has been elucidated. Initially, the HIV/AIDS epidemic revealed the capacity of C. neoformans to cause host damage in the absence of adaptive immunity. Subsequently, the development and clinical implementation of highly active antiretroviral treatment (HAART) led to recognition of an immune reconstitution inflammatory syndrome (IRIS) in a subset of HIV+ individuals, demonstrating the pathological role of host immunity in disease. A post-infectious inflammatory syndrome (PIIRS) characterized by abnormal T cell-macrophage activation has also been documented in HIV-negative individuals following antifungal therapy. These novel clinical conditions illustrate the highly complex host-pathogen relationship that underlies severe cryptococcal disease and the intricate balance between tolerance and resistance that is necessary for effective resolution. In this article, we will review current knowledge of the interactions between cryptococci and mammalian hosts that result in a tolerant phenotype. Future investigations in this area have potential for translation into improved therapies for affected individuals.

An ecosystem framework for understanding and treating disease

Pathogens and cancers are pervasive health risks in the human population. I argue that if we are to better understand disease and its treatment, then we need to take an ecological perspective of disease itself. I generalize and extend an emerging framework that views disease as an ecosystem and many of its components as interacting in a community. I develop the framework for biological etiological agents (BEAs) that multiply within humans—focusing on bacterial pathogens and cancers—but the framework could be extended to include other host and parasite species. I begin by describing why we need an ecosystem framework to understand disease, and the main components and interactions in bacterial and cancer disease ecosystems. Focus is then given to the BEA and how it may proceed through characteristic states, including emergence, growth, spread and regression. The framework is then applied to therapeutic interventions. Central to success is preventing BEA evasion, the best known being antibiotic resistance and chemotherapeutic resistance in cancers. With risks of evasion in mind, I propose six measures that either introduce new components into the disease ecosystem or manipulate existing ones. An ecosystem framework promises to enhance our understanding of disease, BEA and host (co)evolution, and how we can improve therapeutic outcomes.

Probing the evolutionary robustness of two repurposed drugs targeting iron uptake in Pseudomonas aeruginosa

LAY SUMMARY

We probed the evolutionary robustness of two antivirulence drugs, gallium and flucytosine, targeting the iron-scavenging pyoverdine in the opportunistic pathogen Pseudomonas aeruginosa. Using an experimental evolution approach in human serum, we showed that antivirulence treatments are not evolutionarily robust per se, but vary in their propensity to select for resistance.

BACKGROUND AND OBJECTIVES

Treatments that inhibit the expression or functioning of bacterial virulence factors hold great promise to be both effective and exert weaker selection for resistance than conventional antibiotics. However, the evolutionary robustness argument, based on the idea that antivirulence treatments disarm rather than kill pathogens, is controversial. Here, we probe the evolutionary robustness of two repurposed drugs, gallium and flucytosine, targeting the iron-scavenging pyoverdine of the opportunistic human pathogen Pseudomonas aeruginosa.

METHODOLOGY

We subjected replicated cultures of bacteria to two concentrations of each drug for 20 consecutive days in human serum as an ex vivo infection model. We screened evolved populations and clones for resistance phenotypes, including the restoration of growth and pyoverdine production, and the evolution of iron uptake by-passing mechanisms. We whole-genome sequenced evolved clones to identify the genetic basis of resistance.

RESULTS

We found that mutants resistant against antivirulence treatments readily arose, but their selective spreading varied between treatments. Flucytosine resistance quickly spread in all populations due to disruptive mutations in upp, a gene encoding an enzyme required for flucytosine activation. Conversely, resistance against gallium arose only sporadically, and was based on mutations in transcriptional regulators, upregulating pyocyanin production, a redox-active molecule promoting siderophore-independent iron acquisition. The spread of gallium resistance was presumably hampered because pyocyanin-mediated iron delivery benefits resistant and susceptible cells alike.

CONCLUSIONS AND IMPLICATIONS

Our work highlights that antivirulence treatments are not evolutionarily robust per se. Instead, evolutionary robustness is a relative measure, with specific treatments occupying different positions on a continuous scale.

Molecules and Mechanisms Underlying the Antimicrobial Activity of Escapin, an l-Amino Acid Oxidase from the Ink of Sea Hares

Many marine animals use chemicals to defend themselves and their eggs from predators. Beyond their ecologically relevant functions, these chemicals may also have properties that make them beneficial for humans, including biomedical and industrial applications. For example, some chemical defenses are also powerful antimicrobial or antitumor agents with relevance to human health and disease. One such chemical defense, escapin, an l-amino acid oxidase in the defensive ink of the sea hare Aplysia californica, and related proteins have been investigated for their biomedical properties. This review details our current understanding of escapin's antimicrobial activity, including the array of molecules generated by escapin's oxidation of its major substrates, l-lysine and l-arginine, and mechanisms underlying these molecules' bactericidal and bacteriostatic effects on planktonic cells and the prevention of formation and removal of bacterial biofilms. Models of escapin's effects are presented, and future directions are proposed.

Parasite Tolerance and Host Competence in Avian Host Defense to West Nile Virus

Competence, or the propensity of a host to transmit parasites, is partly underlain by host strategies to cope with infection (e.g., resistance and tolerance). Resistance represents the ability of hosts to prevent or clear infections, whereas tolerance captures the ability of individuals to cope with a given parasite burden. Here, we investigated (1) whether one easy-to-measure form of tolerance described well the dynamic relationships between host health and parasite burden, and (2) whether individual resistance and tolerance to West Nile virus (WNV) were predictable from single cytokine measures. We exposed house sparrows (HOSP) to WNV and measured subsequent changes in host performance, viral burden, and cytokine expression. We then used two novel approaches (one complex, one simpler) to estimate tolerance within-individual HOSP using four separate host performance traits. We lastly investigated changes in the expression of pro-inflammatory cytokine interferon-γ (IFN-γ) and anti-inflammatory cytokine interleukin-10 (IL-10). Both approaches to estimating tolerance were equivalent among WNV-infected HOSP; thus, an easy-to-measure tolerance estimation may be successfully applied in field studies. Constitutive expression of IFN-γ and IL-10 were predictive of resistance and tolerance to WNV, implicating these cytokines as viable biomarkers of host competence to WNV.

Assessing the public health impact of tolerance-based therapies with mathematical models

Disease tolerance is a defense strategy against infections that aims at maintaining host health even at high pathogen replication or load. Tolerance mechanisms are currently intensively studied with the long-term goal of exploiting them therapeutically. Because tolerance-based treatment imposes less selective pressure on the pathogen it has been hypothesised to be “evolution-proof”. However, the primary public health goal is to reduce the incidence and mortality associated with a disease. From this perspective, tolerance-based treatment bears the risk of increasing the prevalence of the disease, which may lead to increased mortality. We assessed the promise of tolerance-based treatment strategies using mathematical models. Conventional treatment was implemented as an increased recovery rate, while tolerance-based treatment was assumed to reduce the disease-related mortality of infected hosts without affecting recovery. We investigated the endemic phase of two types of infections: acute and chronic. Additionally, we considered the effect of pathogen resistance against conventional treatment. We show that, for low coverage of tolerance-based treatment, chronic infections can cause even more deaths than without treatment. Overall, we found that conventional treatment always outperforms tolerance-based treatment, even when we allow the emergence of pathogen resistance. Our results cast doubt on the potential benefit of tolerance-based over conventional treatment. Any clinical application of tolerance-based treatment of infectious diseases has to consider the associated detrimental epidemiological feedback.

Conventional therapies improve patient health by eliminating the pathogen, or, at least, reducing its burden. Recently, alternative therapies that exploit host tolerance mechanisms have received attention from the medical community as a promising strategy. These treatments aim at reducing the level of illness due to the infection, rather than eliminating the pathogen directly. Using a mathematical model, we show that although these treatments are beneficial at the individual level, they can have undesired public health consequences. In particular we show that tolerance-based treatment gives more time for the disease to spread in the population, which in turn increase its prevalence. Moreover, in the case of a low coverage of the treatment of a chronic infection, the overall mortality can increase.