Construction and application of a luxABCDE reporter system for real-time monitoring of Enterococcus faecalis gene expression and growth

Advances in the Development of Bacterial Bioluminescence Imaging

Bioluminescence imaging (BLI) is a powerful method for visualizing biological processes and tracking cells. Engineered bioluminescent bacteria that utilize luciferase-catalyzed biochemical reactions to generate luminescence have become useful analytical tools for in vitro and in vivo bacterial imaging. Accordingly, this review initially introduces the development of engineered bioluminescent bacteria that use different luciferase-luciferin pairs as analytical tools and their applications for in vivo BLI, including real-time bacterial tracking of infection, probiotic investigation, tumor-targeted therapy, and drug screening. Applications of engineered bioluminescent bacteria as whole-cell biosensors for sensing biological changes in vitro and in vivo are then discussed. Finally, we review the optimizations and future directions of bioluminescent bacteria for imaging. This review aims to provide fundamental insights into bacterial BLI and highlight the potential development of this technique in the future.

Galleria mellonella-A Model for the Study of aPDT-Prospects and Drawbacks

Galleria mellonella is a promising in vivo model insect used for microbiological, medical, and pharmacological research. It provides a platform for testing the biocompatibility of various compounds and the kinetics of survival after an infection followed by subsequent treatment, and for the evaluation of various parameters during treatment, including the host-pathogen interaction. There are some similarities in the development of pathologies with mammals. However, a limitation is the lack of adaptive immune response. Antimicrobial photodynamic therapy (aPDT) is an alternative approach for combating microbial infections, including biofilm-associated ones. aPDT is effective against Gram-positive and Gram-negative bacteria, viruses, fungi, and parasites, regardless of whether they are resistant to conventional treatment. The main idea of this comprehensive review was to collect information on the use of G. mellonella in aPDT. It provides a collection of references published in the last 10 years from this area of research, complemented by some practical experiences of the authors of this review. Additionally, the review summarizes in brief information on the G. mellonella model, its advantages and methods used in the processing of material from these larvae, as well as basic knowledge of the principles of aPDT.

Inhibition of Quorum-Sensing Regulator from Pseudomonas aeruginosa Using a Flavone Derivative

Quorum sensing (QS) is a cell-to-cell communication process that controls bacterial collective behaviors. The QS network regulates and coordinates bacterial virulence factor expression, antibiotic resistance and biofilm formation. Therefore, inhibition of the QS system is an effective strategy to suppress the bacterial virulence. Herein, we identify a phosphate ester derivative of chrysin as a potent QS inhibitor of the human pathogen Pseudomonas aeruginosa (P. aeruginosa) using a designed luciferase reporter assay. In vitro biochemical analysis shows that the chrysin derivative binds to the bacterial QS regulator LasR and abrogates its DNA-binding capability. In particular, the derivative exhibits higher anti-virulence activity compared to the parent molecule. All the results reveal the potential application of flavone derivative as an anti-virulence compound to combat the infectious diseases caused by P. aeruginosa.

Monitoring Gene Expression during a Galleria mellonella Bacterial Infection

Galleria mellonella larvae are an alternative in vivo model that has been extensively used to study the virulence and pathogenicity of different bacteria due to its practicality and lack of ethical constraints. However, the larvae possess intrinsic autofluorescence that obstructs the use of fluorescent proteins to study bacterial infections, hence better methodologies are needed. Here, we report the construction of a promoter probe vector with bioluminescence expression as well as the optimization of a total bacterial RNA extraction protocol to enhance the monitoring of in vivo infections. By employing the vector to construct different gene promoter fusions, variable gene expression levels were efficiently measured in G. mellonella larvae at various time points during the course of infection and without much manipulation of the larvae. Additionally, our optimized RNA extraction protocol facilitates the study of transcriptional gene levels during an in vivo infection. The proposed methodologies will greatly benefit bacterial infection studies as they can contribute to a better understanding of the in vivo infection processes and pathogen–mammalian host interactions.

Immunotherapy with engineered bacteria by targeting the STING pathway for anti-tumor immunity

Synthetic biology is a powerful tool to create therapeutics which can be rationally designed to enable unique and combinatorial functionalities. Here we utilize non-pathogenic E coli Nissle as a versatile platform for the development of a living biotherapeutic for the treatment of cancer. The engineered bacterial strain, referred to as SYNB1891, targets STING-activation to phagocytic antigen-presenting cells (APCs) in the tumor and activates complementary innate immune pathways. SYNB1891 treatment results in efficacious antitumor immunity with the formation of immunological memory in murine tumor models and robust activation of human APCs. SYNB1891 is designed to meet manufacturability and regulatory requirements with built in biocontainment features which do not compromise its efficacy. This work provides a roadmap for the development of future therapeutics and demonstrates the transformative potential of synthetic biology for the treatment of human disease when drug development criteria are incorporated into the design process for a living medicine.

Candida albicans increases the pathogenicity of Staphylococcus aureus during polymicrobial infection of Galleria mellonella larvae

This study detailed the responses of Galleria mellonella larvae to disseminated infection caused by co-infection with Candida albicans and Staphylococcus aureus. Doses of C. albicans (1×10⁵ larva^-1) and S. aureus (1×10⁴ larva^-1) were non-lethal in mono-infection but when combined significantly (P<0.05) reduced larval survival at 24, 48 and 72 h relative to larvae receiving S. aureus (2×10⁴ larva^-1) alone. Co-infected larvae displayed a significantly higher density of S. aureus larva^-1 compared to larvae infected solely with S. aureus. Co-infection resulted in dissemination throughout the host and the appearance of large nodules. Co-infection of larvae with C. albicans and S. aureus (2×10⁴ larva^-1) resulted in an increase in the density of circulating haemocytes compared to that in larvae infected with only S. aureus. Proteomic analysis of co-infected larval haemolymph revealed increased abundance of proteins associated with immune responses to bacterial and fungal infection such as cecropin-A (+45.4-fold), recognition proteins [e.g. peptidoglycan-recognition protein LB (+14-fold)] and proteins associated with nodule formation [e.g. Hdd11 (+33.3-fold)]. A range of proteins were also decreased in abundance following co-infection, including apolipophorin (-62.4-fold), alpha-esterase 45 (-7.7-fold) and serine proteinase (-6.2-fold). Co-infection of larvae resulted in enhanced proliferation of S. aureus compared to mono-infection and an immune response showing many similarities to the innate immune response of mammals to infection. The utility of G. mellonella larvae for studying polymicrobial infection is highlighted.

Utilization of Galleria mellonella larvae to characterize the development of Staphylococcus aureus infection

Staphylococcus aureus is a human opportunistic pathogen that causes a wide range of superficial and systemic infections in susceptible patients. Here we describe how an inoculum of S. aureus activates the cellular and humoral response of Galleria mellonella larvae while growing and disseminating throughout the host, forming nodules and ultimately killing the host. An inoculum of S. aureus (2×10⁶ larva^{- 1} ) decreased larval viability at 24 (80±5.77 %), 48 (55.93±5.55 %) and 72 h (10.23±2.97 %) and was accompanied by significant proliferation and dissemination of S. aureus between 6 and 48 h and the formation of nodules in the host. The hemocyte (immune cell) densities increased between 4 and 24 h and hemocytes isolated from larvae after 24 h exposure to heat-killed S. aureus (2×10⁶ larva^{- 1} ) showed altered killing kinetics as compared to those from control larvae. Alterations in the humoral immune response of larvae 6 and 24 h post-infection were also determined by quantitative shotgun proteomics. The proteome of 6 h-infected larvae was enriched for antimicrobial proteins, proteins of the prophenoloxidase cascade and a range of peptidoglycan recognition proteins. By 24 h there was a significant increase in the abundance of a range of antimicrobial peptides with anti-staphylococcal activity and proteins associated with nodule formation. The results presented here indicate how S. aureus interacts with the larval immune response, induces the expression of a variety of immune-related peptides and also forms nodules which are a hallmark of soft tissue infections during human infection.

In vivo activity and low toxicity of the second-generation antimicrobial peptide DGL13K

Antimicrobial peptides have been evaluated as possible alternatives to traditional antibiotics. The translational potential of the antimicrobial peptide DGL13K was tested with focus on peptide toxicity and in vivo activity in two animal models. DGL13K was effective against Pseudomonas aeruginosa, Staphylococcus aureus and methicillin-resistant S. aureus with minimal bactericidal concentrations similar to the minimal inhibitory concentration. The peptide showed low toxicity to human red blood cells and HEK cells with median lethal dose around 1 mg/ml. The median lethal dose in greater wax moth larvae (Galleria mellonella) was about 125mg/kg while the peptide caused no skin toxicity in a mouse model. A novel high-throughput luminescence assay was used to test peptide activity in infected G. mellonella, thus reducing vertebrate animal use. DGL13K killed P. aeruginosa in both the G. mellonella model and a mouse burn wound infection model, with bacterial viability 3-10-fold lower than in untreated controls. Future experiments will focus on optimizing peptide delivery, dose and frequency to further improve the antibacterial effect.

Recent Advances in the Use of Galleria mellonella Model to Study Immune Responses against Human Pathogens

The use of invertebrates for in vivo studies in microbiology is well established in the scientific community. Larvae of Galleria mellonella are a widely used model for studying pathogenesis, the efficacy of new antimicrobial compounds, and immune responses. The immune system of G. mellonella larvae is structurally and functionally similar to the innate immune response of mammals, which makes this model suitable for such studies. In this review, cellular responses (hemocytes activity: phagocytosis, nodulation, and encapsulation) and humoral responses (reactions or soluble molecules released in the hemolymph as antimicrobial peptides, melanization, clotting, free radical production, and primary immunization) are discussed, highlighting the use of G. mellonella as a model of immune response to different human pathogenic microorganisms.

Rapid production and characterization of antimicrobial colicins using Escherichia coli-based cell-free protein synthesis

Colicins are antimicrobial proteins produced by Escherichia coli, which, upon secretion from the host, kill non-host E. coli strains by forming pores in the inner membrane and degrading internal cellular components such as DNA and RNA. Due to their unique cell-killing activities, colicins are considered viable alternatives to conventional antibiotics. Recombinant production of colicins requires co-production of immunity proteins to protect host cells; otherwise, the colicins are lethal to the host. In this study, we used cell-free protein synthesis (CFPS) to produce active colicins without the need for protein purification and co-production of immunity proteins. Cell-free synthesized colicins were active in killing model E. coli cells with different modes of cytotoxicity. Pore-forming colicins E1 and nuclease colicin E2 killed actively growing cells in a nutrient-rich medium, but the cytotoxicity of colicin Ia was low compared to E1 and E2. Moreover, colicin E1 effectively killed cells in a nutrient-free solution, while the activity of E2 was decreased compared to nutrient-rich conditions. Both colicins E1 and E2 decreased the level of persister cells (metabolically dormant cell populations that are insensitive to antibiotics) by up to six orders of magnitude compared to that of the rifampin pretreated persister cells. This study finds that colicins can eradicate non-growing cells including persisters, and that CFPS is a promising platform for rapid production and characterization of toxic proteins.

In-vivo monitoring of infectious diseases in living animals using bioluminescence imaging

Traditional methods of localizing and quantifying the presence of pathogenic microorganisms in living experimental animal models of infections have mostly relied on sacrificing the animals, dissociating the tissue and counting the number of colony forming units. However, the discovery of several varieties of the light producing enzyme, luciferase, and the genetic engineering of bacteria, fungi, parasites and mice to make them emit light, either after administration of the luciferase substrate, or in the case of the bacterial lux operon without any exogenous substrate, has provided a new alternative. Dedicated bioluminescence imaging (BLI) cameras can record the light emitted from living animals in real time allowing non-invasive, longitudinal monitoring of the anatomical location and growth of infectious microorganisms as measured by strength of the BLI signal. BLI technology has been used to follow bacterial infections in traumatic skin wounds and burns, osteomyelitis, infections in intestines, Mycobacterial infections, otitis media, lung infections, biofilm and endodontic infections and meningitis. Fungi that have been engineered to be bioluminescent have been used to study infections caused by yeasts (Candida) and by filamentous fungi. Parasitic infections caused by malaria, Leishmania, trypanosomes and toxoplasma have all been monitored by BLI. Viruses such as vaccinia, herpes simplex, hepatitis B and C and influenza, have been studied using BLI. This rapidly growing technology is expected to continue to provide much useful information, while drastically reducing the numbers of animals needed in experimental studies.

Adaptation of a Gaussia princeps Luciferase reporter system in Candida albicans for in vivo detection in the Galleria mellonella infection model

For the past 10 years, mini-host models and in particular the greater wax moth Galleria mellonella have tended to become a surrogate for murine models of fungal infection mainly due to cost, ethical constraints and ease of use. Thus, methods to better assess the fungal pathogenesis in G. mellonella need to be developed. In this study, we implemented the detection of Candida albicans cells expressing the Gaussia princeps luciferase in its cell wall in infected larvae of G. mellonella. We demonstrated that detection and quantification of luminescence in the pulp of infected larvae is a reliable method to perform drug efficacy and C. albicans virulence assays as compared to fungal burden assay. Since the linearity of the bioluminescent signal, as compared to the CFU counts, has a correlation of R(2) = 0.62 and that this method is twice faster and less labor intensive than classical fungal burden assays, it could be applied to large scale studies. We next visualized and followed C. albicans infection in living G. mellonella larvae using a non-toxic and water-soluble coelenterazine formulation and a CCD camera that is commonly used for chemoluminescence signal detection. This work allowed us to follow for the first time C. albicans course of infection in G. mellonella during 4 days.

Galleria mellonella as a model host for microbiological and toxin research

Mammals are widely used by microbiologists as a model host species to study infectious diseases of humans and domesticated livestock. These studies have been pivotal for our understanding of mechanisms of virulence and have allowed the development of diagnostics, pre-treatments and therapies for disease. However, over the past decade we have seen efforts to identify organisms which can be used as alternatives to mammals for these studies. The drivers for this are complex and multifactorial and include cost, ethical and scientific considerations. Galleria mellonella have been used as an alternative infection model since the 1980s and its utility for the study of bacterial disease and antimicrobial discovery was recently comprehensively reviewed. The wider applications of G. mellonella as a model host, including its susceptibility to 29 species of fungi, 7 viruses, 1 species of parasite and 16 biological toxins, are described in this perspective. In addition, the latest developments in the standardisation of G. mellonella larvae for research purposes has been reviewed.

The Basics of Bacteriuria: Strategies of Microbes for Persistence in Urine

Bacteriuria, the presence of bacteria in urine, is associated with asymptomatic, as well as symptomatic, urinary tract infection (UTI). Bacteriuria underpins some of the dynamics of microbial colonization of the urinary tract, and probably impacts the progression and persistence of infection in some individuals. Recent molecular discoveries in vitro have elucidated how some key bacterial traits can enable organisms to survive and grow in human urine as a means of microbial fitness adaptation for UTI. Several microbial characteristics that confer bacteruric potential have been identified including de novo synthesis of guanine, relative resistance to D-serine, and catabolism of malic acid. Microbial characteristics such as these are increasingly being defined through the use of synthetic human urine (SHU) in vitro as a model to mimic the in vivo environment that bacteria encounter in the bladder. There is considerable variation in the SHU model systems that have been used to study bacteriuria to date, and this influences the utility of these models. In this review, we discuss recent advances in our understanding of bacteruric potential with a focus on the specific mechanisms underlying traits that promote the growth of bacteria in urine. We also review the application of SHU in research studies modeling UTI and discuss the chemical makeup, and benefits and limitations that are encountered in utilizing SHU to study bacterial growth in urine in vitro.

Galleria mellonella infection models for the study of bacterial diseases and for antimicrobial drug testing

Galleria mellonella (greater wax moth or honeycomb moth) has been introduced as an alternative model to study microbial infections. G. mellonella larvae can be easily and inexpensively obtained in large numbers and are simple to use as they don't require special lab equipment. There are no ethical constraints and their short life cycle makes them ideal for large-scale studies. Although insects lack an adaptive immune response, their innate immune response shows remarkable similarities with the immune response in vertebrates. This review gives a current update of what is known about the immune system of G. mellonella and provides an extensive overview of how G. mellonella is used to study the virulence of Gram-positive and Gram-negative bacteria. In addition, the use of G. mellonella to evaluate the efficacy of antimicrobial agents and experimental phage therapy are also discussed. The review concludes with a critical assessment of the current limitatons of G. mellonella infection models.

Bioluminescence based biosensors for quantitative detection of enterococcal peptide-pheromone activity reveal inter-strain telesensing in vivo during polymicrobial systemic infection

Enterococcus faecalis is a significant threat in the nosocomial setting due to the emergence of isolates that are multi-antibiotic resistant, refractory to the available therapies and equipped with a variety of pathogenicity determinants. This bacterium uses quorum-sensing systems to regulate its physiological processes, including the expression of virulence traits, to adapt and proliferate within a host. Here, we describe the construction and application of two bioluminescence-based reporter systems for the direct detection of the quorum-sensing regulated expression of (i) the gelatinase biosynthesis-activating pheromone (GBAP) and (ii) the cytolysin small subunit (CylL_S) in natural samples. The two E. faecalis reporters conditionally expressed bioluminescence in the presence of GBAP and CylL_S both in the supernatants of liquid cultures and in an agar-overlay assay in as little as three hours, with a high level of sensitivity. Biosensors employed to investigate the interaction between the fsr and cyl systems revealed that fsr impeded CylL_S activity by 75%. Furthermore, we identified a clinical E. faecalis isolate that acted as a biological cheater, producing cytolysin only upon sensing CylL_S-producers in its environment. This isolate enhanced its virulence during polymicrobial systemic infection of Galleria mellonella.

Transcriptomic and functional analysis of NaCl-induced stress in Enterococcus faecalis

The robust physiology of Enterococcus faecalis facilitates tolerance to various stresses. We here report the transcriptional response of E. faecalis V583 to growth in the presence of 6.5% NaCl. Among the early responses observed was an immediate down-regulation of mscL, accompanied by an up-regulation of genes predicted to be involved in uptake of extracellular potassium and glycine betaine. The high NaCl concentration also induced expression of chaperons and cell envelope related traits, such as the enterococcal polysaccharide antigen (epa) locus. Functional genetic analysis revealed reduced salt stress resistance in both epaB and epaE mutants. The reduced salt resistance phenotype associated with the epaB mutant was restored by complementation, hence demonstrating a role of Epa in the physiological robustness of E. faecalis. Furthermore, we demonstrate that Epa confers increased resistance towards multiple cell envelope stress-inducing factors. Accordingly, these findings delineate a potential link between the robust nature of E. faecalis and its ability to perform as a human pathogen, and provide a new perspective on the mechanisms by which Epa contributes to virulence. Notably, the high NaCl concentration also resulted in strict repression of the gelE-sprE operon and impaired gelatinase activity. We demonstrate that NaCl antagonize the GBAP-pheromone dependent induction in a concentration dependent manner.

Non-invasive monitoring of Streptococcus pyogenes vaccine efficacy using biophotonic imaging

Streptococcus pyogenes infection of the nasopharynx represents a key step in the pathogenic cycle of this organism and a major focus for vaccine development, requiring robust models to facilitate the screening of potentially protective antigens. One antigen that may be an important target for vaccination is the chemokine protease, SpyCEP, which is cell surface-associated and plays a role in pathogenesis. Biophotonic imaging (BPI) can non-invasively characterize the spatial location and abundance of bioluminescent bacteria in vivo. We have developed a bioluminescent derivative of a pharyngeal S. pyogenes strain by transformation of an emm75 clinical isolate with the luxABCDE operon. Evaluation of isogenic recombinant strains in vitro and in vivo confirmed that bioluminescence conferred a growth deficit that manifests as a fitness cost during infection. Notwithstanding this, bioluminescence expression permitted non-invasive longitudinal quantitation of S. pyogenes within the murine nasopharynx albeit with a detection limit corresponding to approximately 10⁵ bacterial colony forming units (CFU) in this region. Vaccination of mice with heat killed streptococci, or with SpyCEP led to a specific IgG response in the serum. BPI demonstrated that both vaccine candidates reduced S. pyogenes bioluminescence emission over the course of nasopharyngeal infection. The work suggests the potential for BPI to be used in the non-invasive longitudinal evaluation of potential S. pyogenes vaccines.

In vivo assessment of growth and virulence gene expression during commensal and pathogenic lifestyles of luxABCDE-tagged Enterococcus faecalis strains in murine gastrointestinal and intravenous infection models

Cytolysin and gelatinase are prominent pathogenicity determinants associated with highly virulent Enterococcus faecalis strains. In an effort to explore the expression profiles of these virulence traits in vivo, we have employed E. faecalis variants expressing the luxABCDE cassette under the control of either the P16S, cytolysin, or gelatinase promoter for infections of Galleria mellonella caterpillars and mice. Systemic infection of G. mellonella with bioluminescence-tagged E. faecalis MMH594 revealed temporal regulation of both gelatinase and cytolysin promoters and demonstrated that these traits were induced in response to the host environment. Gavage of mice pretreated perorally with antibiotics resulted in efficient colonization of the murine gastrointestinal tract (GIT) in a strain-dependent manner, where the commensal baby isolate EF62 was more persistent than the nosocomial isolate MMH594. A highly significant correlation (R(2) > 0.94) was found between bioluminescence and the CFU counts in mouse fecal samples. Both strains showed similar preferences for growth and persistence in the ileum, cecum, and colon. Cytolysin expression was uniform in these compartments of the intestinal lumen. In spite of high numbers (10(9) CFU/g of intestinal matter) in the ileum, cecum, and colon, no evidence of translocation or systemic infection could be observed. In the murine intravenous infection model, cytolysin expression was readily detected in the liver, kidneys, and bladder. At 72 h postinfection, the highest bacterial loads were found in the liver, kidneys, and spleen, with organ-specific expression levels of cytolysin ~400- and ~900-fold higher in the spleen and heart, respectively, than in the liver and kidneys. Taken together, this system based on the bioluminescence imaging technology is established as a new, powerful method to monitor the differential regulation of E. faecalis virulence determinants and to study the spatiotemporal course of infection in living animals in real time.