Targeting bacterial pathogenesis by inhibiting virulence-associated Type III and Type IV secretion systems

Engineering the T6SS of Pseudomonas for targeted delivery of antibacterial and antifungal effectors

BACKGROUND

Bacteria employ diverse molecular systems, such as the type VI secretion system (T6SS) to outcompete other microorganisms and adapt to ecological niches. The T6SS is a versatile nanomachine capable of delivering toxic effectors into neighboring cells, providing advantages in bacterial interactions. In recent years, T6SSs have been proposed as promising tools for engineering selective antimicrobial platforms.

RESULTS

In this study, we successfully engineered Pseudomonas putida KT2440 to heterologously express and release T6SS effectors. The expression of Tse1, an effector from Pseudomonas chlororaphis, induced sporulation in plant-beneficial Bacillus strains via a T6SS-dependent mechanism, particularly when Tse1 was paired with a PAAR protein. Similarly, the engineered strain effectively inhibited Aeromonas hydrophila growth using the phospholipase toxin TplE from Pseudomonas aeruginosa. Furthermore, antifungal activity was achieved by coexpressing Tfe2, an effector from Serratia marcescens, with VgrGs, resulting in increased reactive oxygen species levels and cellular damage in Botrytis cinerea. Importantly, the T6SS was also employed to deliver non-T6SS effectors such as chitosanase, demonstrating its versatility in degrading fungal cell walls.

CONCLUSIONS

Our findings demonstrate that the T6SS can be engineered to deliver both canonical and noncanonical effectors, providing a robust platform for targeted antibacterial and antifungal applications. The modularity of the system enables precise pairing of effectors with structural components such as VgrG and PAAR proteins, optimizing delivery efficiency. These engineered systems provide new opportunities for the development of biocontrol strategies in agriculture, microbiome modulation, and potential therapeutic applications. Future advancements in bioinformatics and protein engineering will further increase the specificity and functionality of T6SS-based delivery systems, offering innovative tools for managing microbial ecosystems and addressing global challenges in health and agriculture.

Fighting Antimicrobial Resistance: Innovative Drugs in Antibacterial Research

In the fight against bacterial infections, particularly those caused by multi-resistant pathogens known as "superbugs", the need for new antibacterials is undoubted in scientific communities and is by now also widely perceived by the general population. However, the antibacterial research landscape has changed considerably over the past years. With few exceptions, the majority of big pharma companies has left the field and thus, the decline in R&D on antibacterials severely impacts the drug pipeline. In recent years, antibacterial research has increasingly relied on smaller companies or academic research institutions, which mostly have only limited financial resources, to carry a drug discovery and development process from the beginning and through to the beginning of clinical phases. This review formulates the requirements for an antibacterial in regard of targeted pathogens, resistance mechanisms and drug discovery. Strategies are shown for the discovery of new antibacterial structures originating from natural sources, by chemical synthesis and more recently from artificial intelligence approaches. This is complemented by principles for the computer-aided design of antibacterials and the refinement of a lead structure. The second part of the article comprises a compilation of antibacterial molecules classified according to bacterial target structures, e.g. cell wall synthesis, protein synthesis, as well as more recently emerging target classes, e.g. fatty acid synthesis, proteases and membrane proteins. Aspects of the origin, the antibacterial spectrum, resistance and the current development status of the presented drug molecules are highlighted.

A widespread plant defense compound disarms bacterial type III injectisome assembly

Numerous gram-negative bacterial pathogens employ the type III secretion system (T3SS), a multiprotein injectisome, to deliver virulence proteins into host cells and cause diseases. We uncover erucamide as a previously unknown phytoalexin of both dicots and monocots that blocks the T3SS function of multiple bacterial pathogens. Genetically impairing erucamide accumulation or exogenous application in Arabidopsis highlighted erucamide's role in antibacterial immunity. Erucamide binds Hypersensitive response and conserved C (HrcC), a key T3SS component, to block injectisome assembly. Analyses of erucamide analogs and HrcC mutants indicated that the erucamide-HrcC binding is required for inhibiting T3SS in vitro and antibacterial resistance in plants, suggesting an essential role of erucamide-HrcC binding in disease resistance. This work reveals a plant chemical defense that targets major virulence machinery in bacterial pathogens.

Interplay of virulence factors shapes ecology and treatment outcomes in polymicrobial infections

Polymicrobial infections, caused by a community of multiple micro-organisms, are often associated with increased infection severity and poorer patient outcomes. The design of improved antimicrobial treatment strategies for PMIs can be supported by an understanding of their ecological and evolutionary dynamics. Bacterial species present in polymicrobial infections can produce virulence factors to inhibit host immune responses, such as neutrophil recruitment and phagocytosis. The presence of virulence factors can indirectly affect other bacterial species acting as a type of host-mediated interspecies interaction. The aim of this study was to assess how bacterial virulence factors targeting neutrophil function influence ecology and treatment outcomes of PMIs. An agent-based model was constructed which describes a dual-species bacterial population in the presence of neutrophils and a bacteriostatic drug. Our analysis has revealed unforeseen dynamics of the interplay of multiple virulence factors acting as interspecies interaction. We found that the distribution of two phagocytosis-inhibiting virulence factors amongst species can impact whether they have a mutually protective effect for both species. The addition of a virulence factor inhibiting neutrophil recruitment was found to reduce the protective effect of phagocytosis-inhibiting virulence factors. Furthermore we demonstrate the importance of virulence strength of a species relative to other virulent species to determine the fate of a species. We conclude that virulence factors are an important driver of population dynamics in polymicrobial infections, and may be a relevant therapeutic target for treatment of polymicrobial infections.

Male-specific bacteriophages and their potential on combating the spreading of T4SS-bearing antimicrobial resistance plasmids

Antimicrobial resistance (AMR) has been recognized as an important health crisis in the twenty first century. Type IV secretion systems (T4SSs) play key roles in the dissemination of AMR plasmids. Novel strategies that combat AMR problem by targeting T4SS sprung up in recent years. Here, we focus on the strategy of male-specific phages that could target and kill bacteria carrying conjugative AMR plasmids encoding T4SSs. We reviewed the recent advances in male-specific phages, including anti-conjugation mechanisms, clinical isolation and identification methods, classification and characteristics, in vitro and in vivo anti-conjugation efficacy and improving strategies. Male-specific phages constitute exciting candidates for developing sustainable anti-resistance biocontrol applications.

The phylogenomic landscape of extended-spectrum β-lactamase producing Citrobacter species isolated from surface water

BACKGROUND

Citrobacter species are Gram-negative opportunistic pathogens commonly reported in nosocomial-acquired infections. This study characterised four Citrobacter species that were isolated from surface water in the North West Province, South Africa.

RESULTS

Phenotypic antimicrobial susceptibility profiles of the isolates demonstrated their ability to produce the extended-spectrum β-lactamase (ESBL). Whole genomes were sequenced to profile antibiotic resistance and virulence genes, as well as mobile genetic elements. In silico taxonomic identification was conducted by using multi-locus sequence typing and average nucleotide identity. A pangenome was used to determine the phylogenomic landscape of the Citrobacter species by using 109 publicly available genomes. The strains S21 and S23 were identified as C. braakii, while strains S24 and S25 were C. murliniae and C. portucalensis, respectively. Comparative genomics and sequenced genomes of the ESBL-producing isolates consisted of n = 91; 83% Citrobacter species in which bla-CMY-101 (n = 19; 32,2%) and bla-CMY-59 (n = 12; 38,7%) were prevalent in C. braakii, and C. portucalensis strains, respectively. Macrolide (acrAB-TolC, and mdtG) and aminoglycoside (acrD) efflux pumps genes were identified in the four sequenced Citrobacter spp. isolates. The quinolone resistance gene, qnrB13, was exclusive to the C. portucalensis S25 strain. In silico analysis detected plasmid replicon types IncHI1A, IncP, and Col(VCM04) in C. murliniae S24 and C. portucalensis S25, respectively. These potentially facilitate the T4SS secretion system in Citrobacter species. In this study, the C. braakii genomes could be distinguished from C. murliniae and C. portucalensis on the basis of gene encoding for cell surface localisation of the CPS (vexC) and identification of genes involved in capsule polymer synthesis (tviB and tviE). A cluster for the salmochelin siderophore system (iro-BCDEN) was found in C. murliniae S24. This is important when it comes to the pathogenicity pathway that confers an advantage in colonisation.

CONCLUSIONS

The emerging and genomic landscapes of these ESBL-producing Citrobacter species are of significant concern due to their dissemination potential in freshwater systems. The presence of these ESBL and multidrug-resistant (MDR) pathogens in aquatic environments is of One Health importance, since they potentially impact the clinical domain, that is, in terms of human health and the agricultural domain, that is, in terms of animal health and food production as well as the environmental domain.

Primary architecture and energy requirements of Type III and Type IV secretion systems

Many pathogens use Type III and Type IV protein secretion systems to secrete virulence factors from the bacterial cytosol into host cells. These systems operate through a one-step mechanism. The secreted substrates (protein or nucleo-protein complexes in the case of Type IV conjugative systems) are guided to the base of the secretion channel, where they are directly delivered into the host cell in an ATP-dependent unfolded state. Despite the numerous disparities between these secretion systems, here we have focused on the structural and functional similarities between both systems. In particular, on the structural similarity shared by one of the main ATPases (EscN and VirD4 in Type III and Type IV secretion systems, respectively). Interestingly, these ATPases also exhibit a structural resemblance to F1-ATPases, which suggests a common mechanism for substrate secretion. The correlation between structure and function of essential components in both systems can provide significant insights into the molecular mechanisms involved. This approach is of great interest in the pursuit of identifying inhibitors that can effectively target these systems.

Click Triazole as a Linker for Pretargeting Strategies: Synthesis, Docking Investigations, Fluorescence Diagnosis, and Antibacterial Action Studies

In this study, three compounds A1, A2, and A3 and fluorescent probes T1, T2, T3, and T4 were designed and synthesized. ¹H NMR, ¹³C NMR, and MS characterization and elemental analysis were used to confirm A1-A3 and T1-T4. A1-A3 and T1-T4 formed diagnostic molecules by "click" reactions. A1-A3 and T1-T4 did not significantly increase cell death at concentrations of 80 μmol/L. Preliminary screening of the compounds for antibacterial activity revealed that A2 has better antibacterial activity against Agrobacterium tumefaciens. The synthesized compounds and fluorescent probes can be targeted and combined in the physiological condition to form diagnostic molecules for fluorescence detection of Agrobacterium tumefaciens. The binding sites of A1-A3 were deduced theoretically using the AutoDock Vina software docking tool. Further study of the mechanism of the antibacterial action of these compounds is likely to identify new agents against resistant bacterial strains.