Cryoelectron-microscopy structure of the enteropathogenic Escherichia coli type III secretion system EspA filament

Antimicrobial resistance spectrum and virulence characterization of Escherichia coli, Klebsiella pneumoniae and Proteus mirabilis isolated from asymptomatic and diarrheal rhesus monkeys

This study aims to deepen our understanding of the drug resistance and virulence characterization among gut bacteria in asymptomatic and diarrheal captive rhesus macaques (RMs). A total of 31 samples, including 8 asymptomatic RMs, 10 diarrheal RMs, and 1 dead RM, were collected from a breeding base in Sichuan, China, for bacterial isolation. As a result, Escherichia coli (n = 23), Klebsiella (n = 22), Proteus mirabilis (n = 10), Enterococcus (n = 10), Salmonella (n = 2), and Staphylococcus (n = 2) were isolated. All isolates were subjected to antimicrobial susceptibility testing and whole-genome sequencing, among which some E. coli, K. pneumoniae, and P. mirabilis were subjected to the Galleria mellonella and mice infection testing. The antimicrobial resistance rates of levofloxacin, enrofloxacin, and cefotaxime in diarrhea-associated isolates were higher than those of asymptomatic isolates. Consistent with the antimicrobial resistance phenotype, diarrheal isolates had a higher prevalence rate to qnrS1, blaTEM-1B and blaCTX-M-27 than asymptomatic isolates. Furthermore, compared with asymptomatic isolates, diarrheal isolates demonstrated a higher pathogenic potential against larvae and mice. Additionally, sequence types (STs) 14179-14181 in E. coli and ST 625 and ST 630-631 in Klebsiella aerogenes were firstly characterized. Our evidence underscores the considerable challenge posed by high rates of bacterial drug resistance in the effective treatment of diarrheal RMs.

A tuneable genetic switch for tight control of tac promoters in Escherichia coli boosts expression of synthetic injectisomes

Biosafety of engineered bacteria as living therapeutics requires a tight regulation to control the specific delivery of protein effectors, maintaining minimum leakiness in the uninduced (OFF) state and efficient expression in the induced (ON) state. Here, we report a three repressors (3R) genetic circuit that tightly regulates the expression of multiple tac promoters (Ptac) integrated in the chromosome of E. coli and drives the expression of a complex type III secretion system injectisome for therapeutic protein delivery. The 3R genetic switch is based on the tetracycline repressor (TetR), the non-inducible lambda repressor cI (ind-) and a mutant lac repressor (LacIW220F ) with higher activity. The 3R switch was optimized with different protein translation and degradation signals that control the levels of LacIW220F . We demonstrate the ability of an optimized switch to fully repress the strong leakiness of the Ptac promoters in the OFF state while triggering their efficient activation in the ON state with anhydrotetracycline (aTc), an inducer suitable for in vivo use. The implementation of the optimized 3R switch in the engineered synthetic injector E. coli (SIEC) strain boosts expression of injectisomes upon aTc induction, while maintaining a silent OFF state that preserves normal growth in the absence of the inducer. Since Ptac is a commonly used promoter, the 3R switch may have multiple applications for tight control of protein expression in E. coli. In addition, the modularity of the 3R switch may enable its tuning for the control of Ptac promoters with different inducers.

Structural Insights into Type III Secretion Systems of the Bacterial Flagellum and Injectisome

Two of the most fascinating bacterial nanomachines-the broadly disseminated rotary flagellum at the heart of cellular motility and the eukaryotic cell-puncturing injectisome essential to specific pathogenic species-utilize at their core a conserved export machinery called the type III secretion system (T3SS). The T3SS not only secretes the components that self-assemble into their extracellular appendages but also, in the case of the injectisome, subsequently directly translocates modulating effector proteins from the bacterial cell into the infected host. The injectisome is thought to have evolved from the flagellum as a minimal secretory system lacking motility, with the subsequent acquisition of additional components tailored to its specialized role in manipulating eukaryotic hosts for pathogenic advantage. Both nanomachines have long been the focus of intense interest, but advances in structural and functional understanding have taken a significant step forward since 2015, facilitated by the revolutionary advances in cryo-electron microscopy technologies. With several seminal structures of each nanomachine now captured, we review here the molecular similarities and differences that underlie their diverse functions.

Transmembrane substrates of type three secretion system injectisomes

The type three secretion system injectisome of Gram-negative bacterial pathogens injects virulence proteins, called effectors, into host cells. Effectors of mammalian pathogens carry out a range of functions enabling bacterial invasion, replication, immune suppression and transmission. The injectisome secretes two translocon proteins that insert into host cell membranes to form a translocon pore, through which effectors are delivered. A subset of effectors also integrate into infected cell membranes, enabling a unique range of biochemical functions. Both translocon proteins and transmembrane effectors avoid cytoplasmic aggregation and integration into the bacterial inner membrane. Translocated transmembrane effectors locate and integrate into the appropriate host membrane. In this review, we focus on transmembrane translocon proteins and effectors of bacterial pathogens of mammals. We discuss what is known about the mechanisms underlying their membrane integration, as well as the functions conferred by the position of injectisome effectors within membranes.

Enhanced protein translocation to mammalian cells by expression of EtgA transglycosylase in a synthetic injector E. coli strain

Background

Bacterial type III secretion systems (T3SSs) assemble a multiprotein complex termed the injectisome, which acts as a molecular syringe for translocation of specific effector proteins into the cytoplasm of host cells. The use of injectisomes for delivery of therapeutic proteins into mammalian cells is attractive for biomedical applications. With that aim, we previously generated a non-pathogenic Escherichia coli strain, called Synthetic Injector E. coli (SIEC), which assembles functional injectisomes from enteropathogenic E. coli (EPEC). The assembly of injectisomes in EPEC is assisted by the lytic transglycosylase EtgA, which degrades the peptidoglycan layer. As SIEC lacks EtgA, we investigated whether expression of this transglycosylase enhances the protein translocation capacity of the engineered bacterium.

Results

The etgA gene from EPEC was integrated into the SIEC chromosome under the control of the inducible tac promoter, generating the strain SIEC-eEtgA. The controlled expression of EtgA had no effect on the growth or viability of bacteria. Upon induction, injectisome assembly was ~ 30% greater in SIEC-eEtgA than in the parental strain, as determined by the level of T3SS translocon proteins, the hemolytic activity of the bacterial strain, and the impairment in flagellar motility. The functionality of SIEC-eEtgA injectisomes was evaluated in a derivative strain carrying a synthetic operon (eLEE5), which was capable of delivering Tir effector protein into the cytoplasm of HeLa cells triggering F-actin polymerization beneath the attached bacterium. Lastly, using β-lactamase as a reporter of T3SS-protein injection, we determined that the protein translocation capacity was ~ 65% higher in the SIEC-EtgA strain than in the parental SIEC strain.

Conclusions

We demonstrate that EtgA enhances the assembly of functional injectisomes in a synthetic injector E. coli strain, enabling the translocation of greater amounts of proteins into the cytoplasm of mammalian cells. Accordingly, EtgA expression may boost the protein translocation of SIEC strains programmed as living biotherapeutics.

Bacterial protein secretion systems: Game of types

Protein trafficking across the bacterial envelope is a process that contributes to the organisation and integrity of the cell. It is the foundation for establishing contact and exchange between the environment and the cytosol. It helps cells to communicate with one another, whether they establish symbiotic or competitive behaviours. It is instrumental for pathogenesis and for bacteria to subvert the host immune response. Understanding the formation of envelope conduits and the manifold strategies employed for moving macromolecules across these channels is a fascinating playground. The diversity of the nanomachines involved in this process logically resulted in an attempt to classify them, which is where the protein secretion system types emerged. As our knowledge grew, so did the number of types, and their rightful nomenclature started to be questioned. While this may seem a semantic or philosophical issue, it also reflects scientific rigour when it comes to assimilating findings into textbooks and science history. Here I give an overview on bacterial protein secretion systems, their history, their nomenclature and why it can be misleading for newcomers in the field. Note that I do not try to suggest a new nomenclature. Instead, I explore the reasons why naming could have escaped our control and I try to reiterate basic concepts that underlie protein trafficking cross membranes.

Virulence Factors of Enteric Pathogenic Escherichia coli: A Review

Escherichia coli are remarkably versatile microorganisms and important members of the normal intestinal microbiota of humans and animals. This harmless commensal organism can acquire a mixture of comprehensive mobile genetic elements that contain genes encoding virulence factors, becoming an emerging human pathogen capable of causing a broad spectrum of intestinal and extraintestinal diseases. Nine definite enteric E. coli pathotypes have been well characterized, causing diseases ranging from various gastrointestinal disorders to urinary tract infections. These pathotypes employ many virulence factors and effectors subverting the functions of host cells to mediate their virulence and pathogenesis. This review summarizes new developments in our understanding of diverse virulence factors associated with encoding genes used by different pathotypes of enteric pathogenic E. coli to cause intestinal and extraintestinal diseases in humans.

Plasmon-Enhanced Antibacterial Activity of Chiral Gold Nanoparticles and In Vivo Therapeutic Effect

d-cysteine (d-cys) has been demonstrated to possess an extraordinary antibacterial activity because of its unique steric configuration. However, inefficient antibacterial properties seriously hinder its wide applications. Here, cysteine-functionalized gold nanoparticles (d-/l-Au NPs) were prepared by loading d-/l-cysteine on the surface of gold nanoparticles for the effective inhibition of Escherichia coli (E. coli) in vitro and in vivo, and the effects on the intestinal microflora in mice were explored during the treatment of E. coli infection in the gut. We found that the antibacterial activity of d-/l-Au NPs was more than 2-3 times higher than pure d-cysteine, l-cysteine and Au NPs. Compared with l-Au NPs, d-Au NPs showed the stronger antibacterial activity, which was related to its unique steric configuration. Chiral Au NPs showed stronger destructive effects on cell membrane compared to other groups, which further leads to the leakage of the cytoplasm and bacterial cell death. The in vivo antibacterial experiment illustrated that d-Au NPs displayed impressive antibacterial activity in the treatment of E. coli-infected mice comparable to kanamycin, whereas they could not affect the balance of intestinal microflora. This work is of great significance in the development of an effective chiral antibacterial agent.

α-Helices in the Type III Secretion Effectors: A Prevalent Feature with Versatile Roles

Type III Secretion Systems (T3SSs) are multicomponent nanomachines located at the cell envelope of Gram-negative bacteria. Their main function is to transport bacterial proteins either extracellularly or directly into the eukaryotic host cell cytoplasm. Type III Secretion effectors (T3SEs), latest to be secreted T3S substrates, are destined to act at the eukaryotic host cell cytoplasm and occasionally at the nucleus, hijacking cellular processes through mimicking eukaryotic proteins. A broad range of functions is attributed to T3SEs, ranging from the manipulation of the host cell's metabolism for the benefit of the bacterium to bypassing the host's defense mechanisms. To perform this broad range of manipulations, T3SEs have evolved numerous novel folds that are compatible with some basic requirements: they should be able to easily unfold, pass through the narrow T3SS channel, and refold to an active form when on the other side. In this review, the various folds of T3SEs are presented with the emphasis placed on the functional and structural importance of α-helices and helical domains.