Regulated programmed lysis of recombinant Salmonella in host tissues to release protective antigens and confer biological containment

Microbial messengers: nucleic acid delivery by bacteria

The demand for diverse nucleic acid delivery vectors, driven by recent biotechnological breakthroughs, offers opportunities for continuous improvements in efficiency, safety, and delivery capacity. With their enhanced safety and substantial cargo capacity, bacterial vectors offer significant potential across a variety of applications. In this review, we explore methods to engineer bacteria for nucleic acid delivery, including strategies such as engineering attenuated strains, lysis circuits, and conjugation machinery. Moreover, we explore pioneering techniques, such as manipulating nanoparticle (NP) coatings and outer membrane vesicles (OMVs), representing the next frontier in bacterial vector engineering. We foresee these advancements in bacteria-mediated nucleic acid delivery, through combining bacterial pathogenesis with engineering biology techniques, as a pivotal step forward in the evolution of nucleic acid delivery technologies.

Genetic engineering of Salmonella spp. for novel vaccine strategies and therapeutics

Salmonella enterica is a diverse species that infects both humans and animals. S. enterica subspecies enterica consists of more than 1,500 serovars. Unlike typhoidal Salmonella serovars which are human host-restricted, non-typhoidal Salmonella (NTS) serovars are associated with foodborne illnesses worldwide and are transmitted via the food chain. Additionally, NTS serovars can cause disease in livestock animals causing significant economic losses. Salmonella is a well-studied model organism that is easy to manipulate and evaluate in animal models of infection. Advances in genetic engineering approaches in recent years have led to the development of Salmonella vaccines for both humans and animals. In this review, we focus on current progress of recombinant live-attenuated Salmonella vaccines, their use as a source of antigens for parenteral vaccines, their use as live-vector vaccines to deliver foreign antigens, and their use as therapeutic cancer vaccines in humans. We also describe development of live-attenuated Salmonella vaccines and live-vector vaccines for use in animals.

A novel vaccine strategy using quick and easy conversion of bacterial pathogens to unnatural amino acid-auxotrophic suicide derivatives

We propose a novel strategy for quick and easy preparation of suicide live vaccine candidates against bacterial pathogens. This method requires only the transformation of one or more plasmids carrying genes encoding for two types of biological devices, an unnatural amino acid (uAA) incorporation system and toxin-antitoxin systems in which translation of the antitoxins requires the uAA incorporation. Escherichia coli BL21-AI laboratory strains carrying the plasmids were viable in the presence of the uAA, whereas the free toxins killed these strains after the removal of the uAA. The survival time after uAA removal could be controlled by the choice of the uAA incorporation system and toxin-antitoxin systems. Multilayered toxin-antitoxin systems suppressed escape frequency to less than 1 escape per 109 generations in the best case. This conditional suicide system also worked in Salmonella enterica and E. coli clinical isolates. The S. enterica vaccine strains were attenuated with a >105 fold lethal dose. Serum IgG response and protection against the parental pathogenic strain were confirmed. In addition, the live E. coli vaccine strain was significantly more immunogenic and provided greater protection than a formalin-inactivated vaccine. The live E. coli vaccine was not detected after inoculation, presumably because the uAA is not present in the host animals or the natural environment. These results suggest that this strategy provides a novel way to rapidly produce safe and highly immunogenic live bacterial vaccine candidates.

IMPORTANCE

Live vaccines are the oldest vaccines with a history of more than 200 years. Due to their strong immunogenicity, live vaccines are still an important category of vaccines today. However, the development of live vaccines has been challenging due to the difficulties in achieving a balance between safety and immunogenicity. In recent decades, the frequent emergence of various new and old pathogens at risk of causing pandemics has highlighted the need for rapid vaccine development processes. We have pioneered the use of uAAs to control gene expression and to conditionally kill host bacteria as a biological containment system. This report proposes a quick and easy conversion of bacterial pathogens into live vaccine candidates using this containment system. The balance between safety and immunogenicity can be modulated by the selection of the genetic devices used. Moreover, the uAA-auxotrophy can prevent the vaccine from infecting other individuals or establishing the environment.

Salmonella typhimurium Vaccine Candidate Delivering Infectious Bronchitis Virus S1 Protein to Induce Protection

Infectious bronchitis (IB) is a highly infectious viral disease of chickens which causes significant economic losses in the poultry industry worldwide. An effective vaccine against IB is urgently needed to provide both biosafety and high-efficiency immune protection. In this study, the S1 protein of the infectious bronchitis virus was delivered by a recombinant attenuated Salmonella typhimurium vector to form the vaccine candidate χ11246(pYA4545-S1). S. typhimurium χ11246 carried a sifA- mutation with regulated delayed systems, striking a balance between host safety and immunogenicity. Here, we demonstrated that S1 protein is highly expressed in HD11 cells. Immunization with χ11246(pYA4545-S1) induced the production of antibody and cytokine, leading to an effective immune response against IB. Oral immunization with χ11246(pYA4545-S1) provided 72%, 56%, and 56% protection in the lacrimal gland, trachea, and cloaca against infectious bronchitis virus infection, respectively. Furthermore, it significantly reduced histopathological lesions in chickens. Together, this study provides a new idea for the prevention of IB.

Vaccines to Control Salmonella in Poultry

This review is focused on describing and analyzing means by which Salmonella enterica serotype strains have been genetically modified with the purpose of developing safe, efficacious vaccines to present Salmonella-induced disease in poultry and to prevent Salmonella colonization of poultry to reduce transmission through the food chain in and on eggs and poultry meat. Emphasis is on use of recently developed means to generate defined deletion mutations to eliminate genetic sequences conferring antimicrobial resistance or residual elements that might lead to genetic instability. Problems associated with prior means to develop vaccines are discussed with presentation of various means by which these problems have been lessened, if not eliminated. Practical considerations are also discussed in hope of facilitating means to move lab-proven successful vaccination procedures and vaccine candidates to the marketplace to benefit the poultry industry.

Advances in Targeting Drug Biological Carriers for Enhancing Tumor Therapy Efficacy

Chemotherapy drugs continue to be the main component of oncology treatment research and have been proven to be the main treatment modality in tumor therapy. However, the poor delivery efficiency of cancer therapeutic drugs and their potential off-target toxicity significantly limit their effectiveness and extensive application. The recent integration of biological carriers and functional agents is expected to camouflage synthetic biomimetic nanoparticles for targeted delivery. The promising candidates, including but not limited to red blood cells and their membranes, platelets, tumor cell membrane, bacteria, immune cell membrane, and hybrid membrane are typical representatives of biological carriers because of their excellent biocompatibility and biodegradability. Biological carriers are widely used to deliver chemotherapy drugs to improve the effectiveness of drug delivery and therapeutic efficacy in vivo, and tremendous progress is made in this field. This review summarizes recent developments in biological vectors as targeted drug delivery systems based on microenvironmental stimuli-responsive release, thus highlighting the potential applications of target drug biological carriers. The review also discusses the possibility of clinical translation, as well as the exploitation trend of these target drug biological carriers.

Decoding the invasive nature of a tropical pathogen of concern: The invasive non-Typhoidal Salmonella strains causing host-restricted extraintestinal infections worldwide

Invasive-Non-Typhoidal Salmonella (iNTS) are the major cause of health concern in the low-income, under-developed nations in Africa and Asia that lack proper sanitation facilities. Around 5% of the NTS cases give rise to invasive, extraintestinal diseases leading to focal infections like osteomyelitis, meningitis, osteoarthritis, endocarditis and neonatal sepsis. iNTS serovars like S. Typhimurium, S. Enteritidis, S. Dublin, S. Choleraesuis show a greater propensity to become invasive than others which hints at the genetic basis of their emergence. The major risk factors attributing to the invasive diseases include immune-compromised individuals having co-infection with malaria or HIV, or suffering from malnutrition. The rampant use of antibiotics leading to the emergence of multi-drug resistant strains poses a great challenge in disease management. An extensive understanding of the iNTS pathogenesis and its epidemiology will open up avenues for the development of new vaccination and therapeutic strategies to restrict the spread of this neglected disease.

A Bacterial mRNA-Lysis-Mediated Cargo Release Vaccine System for Regulated Cytosolic Surveillance and Optimized Antigen Delivery

Engineered vector-based in vivo protein delivery platforms have made significant progress for both prophylactic and therapeutic applications. However, the lack of effective release strategies results in foreign cargo being trapped within the vector, restricting the provision of significant performance benefits and enhanced therapeutic results compared to traditional vaccines. Herein, the development of a Salmonella mRNA interferase regulation vector (SIRV) system is reported to overcome this challenge. The genetic circuits are engineered that (1) induce self-lysis to release foreign antigens into target cells and (2) activate the cytosolic surveillance cGAS-STING axis by releasing DNA into the cytoplasm. Delayed synthesis of the MazF interferase regulates differential mRNA cleavage, resulting in a 36-fold increase in the delivery of foreign antigens and modest activation of the inflammasome, which collectively contribute to the marked maturation of antigen-presenting cells (APCs). Bacteria delivering the protective antigen SaoA exhibits excellent immunogenicity and safety in mouse and pig models, significantly improving the survival rate of animals challenged with multiple serotypes of Streptococcus suis. Thus, the SIRV system enables the effective integration of various modular components and antigen cargos, allowing for the generation of an extensive range of intracellular protein delivery systems using multiple bacterial species in a highly efficient manner.

Engineered bacteria launch and control an oncolytic virus

The ability of bacteria and viruses to selectively replicate in tumors has led to synthetic engineering of new microbial therapies. Here we design a cooperative strategy whereby S. typhimurium bacteria transcribe and deliver the Senecavirus A RNA genome inside host cells, launching a potent oncolytic viral infection. Then, we engineer the virus to require a bacterially delivered protease in order to achieve virion maturation, demonstrating bacterial control over the virus. This work extends bacterially delivered therapeutics to viral genomes, and the governing of a viral population through engineered microbial interactions.

One-Sentence Summary

Bacteria are engineered to act as a synthetic "capsid" delivering Senecavirus A genome and controlling its spread.

Recombinant Attenuated Edwardsiella piscicida Vaccine Displaying Regulated Lysis to Confer Biological Containment and Protect Catfish against Edwardsiellosis

We implemented a unique strategy to construct a recombinant attenuated Edwardsiella vaccine (RAEV) with a biological containment phenotype that causes regulated bacterial cell wall lysis. This process ensures that the vaccine strain is not able to persist in the environment. The murA gene is responsible for the catalysis of one of the first steps in the biosynthesis of muramic acid, which is a crucial component of the bacterial cell wall. The regulated lysis phenotype was achieved by inserting the tightly regulated araC ParaBAD cassette in place of the chromosomal murA promoter. Strains with this mutation require growth media supplemented with arabinose in order to survive. Without arabinose, they are unable to synthesize the peptidoglycan cell wall. Following the colonization of fish lymphoid tissues, the murA protein is no longer synthesized due to the lack of arabinose. Lysis is subsequently achieved in vivo, thus preventing the generation of disease symptoms and the spread of the strain into the environment. Vaccine strain χ16016 with the genotype ΔPmurA180::TT araC ParaBADmurA is attenuated and shows a higher LD50 value than that of the wild-type strain. Studies have demonstrated that χ16016 induced TLR4, TLR5, TLR8, TLR9, NOD1 and NOD2-mediated NF-κB pathways and upregulated the gene expression of various cytokines, such as il-8, il-1β, tnf-a, il-6 and ifn-γ in catfish. We observed significant upregulation of the expression profiles of cd4, cd8 and mhc-II genes in different organs of vaccinated catfish. Vaccine strain χ16016 induced systemic and mucosal IgM titers and conferred significant protection to catfish against E. piscicida wild-type challenge. Our lysis RAEV is the first live attenuated vaccine candidate designed to be used in the aquaculture industry that displays this biological containment property.

Development of toxin-antitoxin self-destructive bacteria, aimed for salmonella vaccination

The most common source of foodborne Salmonella infection in humans is poultry eggs and meat, such that prevention of human infection is mostly achieved by vaccination of farm animals. While inactivated and attenuated vaccines are available, both present drawbacks. This study aimed to develop a novel vaccination strategy, which combines the effectiveness of live-attenuated and safety of inactivated vaccines by construction of inducible self-destructing bacteria utilizing toxin-antitoxin (TA) systems. Hok-Sok and CeaB-CeiB toxin-antitoxin systems were coupled with three induction systems aimed for activating cell killing upon lack of arabinose, anaerobic conditions or low concentration of metallic di-cations. The constructs were transformed into a pathogenic Salmonella enterica serovar Enteritidis strain and bacteria elimination was evaluated in vitro under specific activating conditions and in vivo following administration to chickens. Four constructs induced bacterial killing under the specified conditions, both in growth media and within macrophages. Cloacal swabs of all chicks orally administered transformed bacteria had no detectable levels of bacteria within 9 days of inoculation. By day ten, no bacteria were identified in the spleen and liver of most birds. Antibody immune response was raised toward TA carrying Salmonella which resembled response toward the wildtype bacteria. The constructs described in this study led to self-destruction of virulent Salmonella enteritidis both in vitro and in inoculated animals within a period which is sufficient for the induction of a protective immune response. This system may serve as a safe and effective live vaccine platform against Salmonella as well as other pathogenic bacteria.

Homologous Sequential Immunization Using Salmonella Oral Administration Followed by an Intranasal Boost with Ferritin-Based Nanoparticles Enhanced the Humoral Immune Response against H1N1 Influenza Virus

The influenza virus continues to pose a great threat to public health due to the frequent variations in RNA viruses. Vaccines targeting conserved epitopes, such as the extracellular domain of the transmembrane protein M2 (M2e), a nucleoprotein, and the stem region of hemagglutinin proteins, have been developed, but more efficient strategies, such as nanoparticle-based vaccines, are still urgently needed. However, the labor-intensive in vitro purification of nanoparticles is still necessary, which could hinder the application of nanoparticles in the veterinary field in the future. To overcome this limitation, we used regulated lysis Salmonella as an oral vector with which to deliver three copies of M2e (3M2e-H1N1)-ferritin nanoparticles in situ and evaluated the immune response. Then, sequential immunization using Salmonella-delivered nanoparticles followed by an intranasal boost with purified nanoparticles was performed to further improve the efficiency. Compared with 3M2e monomer administration, Salmonella-delivered in situ nanoparticles significantly increased the cellular immune response. Additionally, the results of sequential immunization showed that the intranasal boost with purified nanoparticles dramatically stimulated the activation of lung CD11b dendritic cells (DCs) and elevated the levels of effector memory T (TEM) cells in both spleen and lung tissues as well as those of CD4 and CD8 tissue-resident memory T (TRM) cells in the lungs. The increased production of mucosal IgG and IgA antibody titers was also observed, resulting in further improvements to protection against a virus challenge, compared with the pure oral immunization group. Salmonella-delivered in situ nanoparticles efficiently increased the cellular immune response, compared with the monomer, and sequential immunization further improved the systemic immune response, as shown by the activation of DCs, the production of TEM cells and TRM cells, and the mucosal immune response, thereby providing us with a novel strategy by which to apply nanoparticle-based vaccines in the future. IMPORTANCE Salmonella-delivered in situ nanoparticle platforms may provide novel nanoparticle vaccines for oral administration, which would be beneficial for veterinary applications. The combination of administering Salmonella-vectored, self-assembled nanoparticles and an intranasal boost with purified nanoparticles significantly increased the production of effector memory T cells and lung resident memory T cells, thereby providing partial protection against an influenza virus challenge. This novel strategy could open a novel avenue for the application of nanoparticle vaccines for veterinary purposes.

Biocontainment Techniques and Applications for Yeast Biotechnology

Biocontainment techniques for genetically modified yeasts (GMYs) are pivotal due to the importance of these organisms for biotechnological processes and also due to the design of new yeast strains by using synthetic biology tools and technologies. Due to the large genetic modifications that many yeast strains display, it is highly desirable to avoid the leakage of GMY cells into natural environments and, consequently, the spread of synthetic genes and circuits by horizontal or vertical gene transfer mechanisms within the microorganisms. Moreover, it is also desirable to avoid patented yeast gene technologies spreading outside the production facility. In this review, the different biocontainment technologies currently available for GMYs were evaluated. Interestingly, uniplex-type biocontainment approaches (UTBAs), which rely on nutrient auxotrophies induced by gene mutation or deletion or the expression of the simple kill switches apparatus, are still the major biocontainment approaches in use with GMY. While bacteria such as Escherichia coli account for advanced biocontainment technologies based on synthetic biology and multiplex-type biocontainment approaches (MTBAs), GMYs are distant from this scenario due to many reasons. Thus, a comparison of different UTBAs and MTBAs applied for GMY and genetically engineered microorganisms (GEMs) was made, indicating the major advances of biocontainment techniques for GMYs.

Protective immunity enhanced Salmonella vaccine vectors delivering Helicobacter pylori antigens reduce H. pylori stomach colonization in mice

Helicobacter pylori is a major cause of gastric mucosal inflammation, peptic ulcers, and gastric cancer. Emerging antimicrobial-resistant H. pylori has hampered the effective eradication of frequent chronic infections. Moreover, a safe vaccine is highly demanded due to the absence of effective vaccines against H. pylori. In this study, we employed a new innovative Protective Immunity Enhanced Salmonella Vaccine (PIESV) vector strain to deliver and express multiple H. pylori antigen genes. Immunization of mice with our vaccine delivering the HpaA, Hp-NAP, UreA and UreB antigens, provided sterile protection against H. pylori SS1 infection in 7 out of 10 tested mice. In comparison to the control groups that had received PBS or a PIESV carrying an empty vector, immunized mice exhibited specific and significant cellular recall responses and antigen-specific serum IgG1, IgG2c, total IgG and gastric IgA antibody titers. In conclusion, an improved S. Typhimurium-based live vaccine delivering four antigens shows promise as a safe and effective vaccine against H. pylori infection.

Intestinal Engineered Probiotics as Living Therapeutics: Chassis Selection, Colonization Enhancement, Gene Circuit Design, and Biocontainment

Intestinal probiotics are often used for the in situ treatment of diseases, such as metabolic disorders, tumors, and chronic inflammatory infections. Recently, there has been an increased emphasis on intelligent, customized treatments with a focus on long-term efficacy; however, traditional probiotic therapy has not kept up with this trend. The use of synthetic biology to construct gut-engineered probiotics as live therapeutics is a promising avenue in the treatment of specific diseases, such as phenylketonuria and inflammatory bowel disease. These studies generally involve a series of fundamental design issues: choosing an engineered chassis, improving the colonization ability of engineered probiotics, designing functional gene circuits, and ensuring the safety of engineered probiotics. In this review, we summarize the relevant past research, the progress of current research, and discuss the key issues that restrict the widespread application of intestinal engineered probiotic living therapeutics.

Attenuated Salmonella Typhimurium with truncated LPS and outer membrane-displayed RGD peptide for cancer therapy

Gram-negative, facultatively anaerobic bacteria Salmonella Typhimurium is a candidate agent or delivery vector for cancer therapy. Effective targeted therapies in addition to radiotherapy, chemotherapy and surgery have been urgently needed as an alternative or supplement. This study expected to further improve the tumor-targeting ability of Salmonella bacteria through genetic modifications. Based on an auxotrophic Salmonella bacterial strain (D2), we constructed Salmonella mutants with altered LPS length to facilitate displaying the RGD4C targeting peptide on the outer membrane surface of Salmonella. The expression of RGD4C peptide in fusion with OmpA was identified by outer membrane protein extraction and WB detection in different mutant strains. However, flow cytometry analysis following immunofluorescence staining demonstrated that the extracellular length of Salmonella LPS did affect the surface display of RGD4C peptide. The strain D2-RGD4C that synthesized intact LPS including lipid A, core oligosaccharides and O antigen polysaccharides could hardly display RGD4C peptide, showing the same fluorescence signal intensity as the strains not expressing RGD4C peptide. Among different strains, D2 ∆rfaJ-RGD4C that synthesized truncated LPS including lipid A and partial core oligosaccharides was capable of displaying RGD4C peptide most efficiently and showed the highest ability to target HUVECs expressing αV integrin and tumor tissue with abundant neovascularization. Animal experiments also demonstrated that this tumor-targeting attenuated Salmonella strain to simultaneously deliver endostatin and TRAIL, two agents with different anti-tumor activities, could significantly inhibit tumor growth and prolong mouse survival. Thus, our studies revealed that Salmonella could be genetically engineered to improve its tumor targeting via the truncation of LPS and surface display of targeting peptides, thereby eliciting superior anti-tumor effects through targeted delivery of drug molecules.

Oral Administration with Recombinant Attenuated Regulated Delayed Lysis Salmonella Vaccines Protecting against Staphylococcus aureus Kidney Abscess Formation

Abscess formation is one of the main symptoms of Staphylococcus aureus infection. It is very important to inhibit abscess formation for preventing S. aureus persistent infection. To find a feasible solution, the live oral vaccines delivering S. aureus antigens, rEsxAB and rHla_m, were constructed, which were based on the attenuated regulated delayed lysis Salmonella enterica subspecies Serovar Typhimurium strain χ11802, and the inhibiting effect on abscess formation was evaluated in mice kidneys. As the results showed, after oral administration, humoral immunity was induced via the mucosal route as the antigen-specific IgG in the serum and IgA in the intestinal mucus both showed significant increases. Meanwhile, the production of IFN-γ and IL-17 in the kidney tissue suggested that Th1/Th17-biased cellular immunity played a role in varying degrees. After challenged intravenously (i.v.) with S. aureus USA300, the χ11802(pYA3681−esxAB)-vaccinated group showed obvious inhibition in kidney abscess formation among the vaccinated group, as the kidney abscess incidence rate and the staphylococcal load significantly reduced, and the kidney pathological injury was improved significantly. In conclusion, this study provided experimental data and showed great potential for live oral vaccine development with the attenuated regulated delayed lysis Salmonella Typhimurium strains against S. aureus infection.

Engineering Bacteria and Bionic Bacterial Derivatives with Nanoparticles for Cancer Therapy

Natural bacteria are interesting subjects for cancer treatments owing to their unique autonomy-driven and hypoxic target properties. Genetically modified bacteria (such as bacteria with msbB gene and aroA gene modifications) can effectively cross sophisticated physiological barriers and transport antitumor agents into deep tumor tissues, and they have good biosafety. Additionally, bacteria can secrete cytokines (such as interleukin-224, interferon-gamma [IFN-γ], and interleukin-1β) and activate antitumor immune responses in the tumor microenvironment, resulting in tumor inhibition. All of these characteristics can be easily utilized to develop synergistic antitumor strategies by combining bacteria-based agents with other therapeutic approaches. Herein, representative studies of bacteria-instructed multimodal synergistic cancer therapy are introduced (e.g., photothermal therapy, chemoimmunotherapy, photodynamic therapy, and photocontrolled bacterial metabolite therapy), and their key advantages are systematically expounded. The current challenges and future prospects in advancing the development of bacteria-based micro/nanomedicines in the field of synthetic biology research are also emphasized, which will hopefully promote the development of related bacteria-based cancer therapies.

A triple-sugar regulated Salmonella vaccine protects against Clostridium perfringens-induced necrotic enteritis in broiler chickens

Gram-positive Clostridium perfringens type G, the causative agent of necrotic enteritis (NE), has gained more attention in the poultry industry due to governmental restrictions on the use of growth-promoting antibiotics in poultry feed. Our previous work has proved that regulated delayed lysis Salmonella vaccines delivering a plasmid encoding an operon fusion of the nontoxic C-terminal adhesive part of alpha toxin and a GST-NetB toxin fusion were able to elicit significant protective immunity in broilers against C. perfringens challenge. We recently improved our S. Typhimurium antigen delivery vaccine strain by integrating a rhamnose-regulated O-antigen synthesis gene enabling a triple-sugar regulation system to control virulence, antigen-synthesis and lysis in vivo traits. The strain also includes a ΔsifA mutation that was previously shown to increase the immunogenicity of and level of protective immunity induced by Salmonella vectored influenza and Eimeria antigens. The new antigen-delivery vaccine vector system confers on the vaccine strain a safe profile and improved protection against C. perfringens challenge. The strain with the triple-sugar regulation system delivering a regulated lysis plasmid pG8R220 encoding the PlcC and GST-NetB antigens protected chickens at a similar level observed in antibiotic-treated chickens. Feed conversion and growth performance were also similar to antibiotic-treated chickens. These studies made use of a severe C. perfringens challenge with lesion formation and mortality enhanced by pre-exposure to Eimeria maxima oocysts. The vaccine achieved effectiveness through three different immunization routes, oral, spray and in drinking water. The vaccine has a potential for application in commercial hatcher and broiler-rearing conditions.

Eukaryotic expression system complemented with expressivity of Semliki Forest Virus's RdRp and invasiveness of engineered Salmonella demonstrate promising potential for bacteria mediated gene therapy

This study describes an efficient eukaryotic expression system (pJHL204) built into the Salmonella delivery system to enhance the essential efficacy and effectiveness of conventional DNA therapy. The expression system utilizes RNA-dependent RNA polymerase activity (RdRp) of Semiliki Forest Virus attributing to dramatic antigen expression by cytoplasmic mRNA amplification. Functional characterization of the pJHL204 by in vitro and in vivo transfection studies revealed the improved expression of mRNA at least 150 folds than the RdRp mutant plasmid under in vitro conditions. Using green fluorescence protein (GFP) and mCherry as bait proteins this system was extensively characterized for plasmid delivery capacity, antigen expression, and safety using in vivo and in vitro models by employing flow cytometry, fluorescence microscopy, and immunohistochemical staining. Employment of Salmonella as a carrier significantly extends plasmid in vivo survivability and prolongs the effective duration until the elimination of the Salmonella carrier strain in the host. The strategy can be easily adapted for P2A connected multiple antigen delivery in a single vector system due to the significantly high cargo capacity of Salmonella. A mouse challenge study was carried out utilizing P2A connected H1N1 hemagglutinin (HA) and neuraminidase (NA) via the Salmonella carrier strain JOL2500 significantly reduced viral activity and protected mice against the H1N1 challenge and demonstrates potential to redefine in vivo DNA therapy as a reliable and safe system to treat human diseases using useful microbes like Salmonella.

Multivalent poultry vaccine development using Protein Glycan Coupling Technology

BACKGROUND

Poultry is the world's most popular animal-based food and global production has tripled in the past 20 years alone. Low-cost vaccines that can be combined to protect poultry against multiple infections are a current global imperative. Glycoconjugate vaccines, which consist of an immunogenic protein covalently coupled to glycan antigens of the targeted pathogen, have a proven track record in human vaccinology, but have yet to be used for livestock due to prohibitively high manufacturing costs. To overcome this, we use Protein Glycan Coupling Technology (PGCT), which enables the production of glycoconjugates in bacterial cells at considerably reduced costs, to generate a candidate glycan-based live vaccine intended to simultaneously protect against Campylobacter jejuni, avian pathogenic Escherichia coli (APEC) and Clostridium perfringens. Campylobacter is the most common cause of food poisoning, whereas colibacillosis and necrotic enteritis are widespread and devastating infectious diseases in poultry.

RESULTS

We demonstrate the functional transfer of C. jejuni protein glycosylation (pgl) locus into the genome of APEC χ7122 serotype O78:H9. The integration caused mild attenuation of the χ7122 strain following oral inoculation of chickens without impairing its ability to colonise the respiratory tract. We exploit the χ7122 pgl integrant as bacterial vectors delivering a glycoprotein decorated with the C. jejuni heptasaccharide glycan antigen. To this end we engineered χ7122 pgl to express glycosylated NetB toxoid from C. perfringens and tested its ability to reduce caecal colonisation of chickens by C. jejuni and protect against intra-air sac challenge with the homologous APEC strain.

CONCLUSIONS

We generated a candidate glycan-based multivalent live vaccine with the potential to induce protection against key avian and zoonotic pathogens (C. jejuni, APEC, C. perfringens). The live vaccine failed to significantly reduce Campylobacter colonisation under the conditions tested but was protective against homologous APEC challenge. Nevertheless, we present a strategy towards the production of low-cost "live-attenuated multivalent vaccine factories" with the ability to express glycoconjugates in poultry.

In Vivo Production of HN Protein Increases the Protection Rates of a Minicircle DNA Vaccine against Genotype VII Newcastle Disease Virus

The Cre-recombinase mediated in vivo minicircle DNA vaccine platform (CRIM) provided a novel option to replace a traditional DNA vaccine. To further improve the immune response of our CRIM vaccine, we designed a dual promoter expression plasmid named pYL87 which could synthesize short HN protein under a prokaryotic in vivo promoter P_pagC and full length HN protein of genotype VII Newcastle disease virus (NDV) under the previous eukaryotic CMV promoter at the same time. Making use of the self-lysed Salmonella strain as a delivery vesicle, chickens immunized with the pYL87 construction showed an increased serum haemagglutination inhibition antibody response, as well as an increased cell proliferation level and cellular IL-4 and IL-18 cytokines, compared with the previous CRIM vector pYL47. After the virus challenge, the pYL87 vector could provide 80% protection compared to 50% protection against genotype VII NDV in pYL47 immunized chickens, indicating a promising dual promoter strategy used in vaccine design.

Advances in the development of Salmonella-based vaccine strategies for protection against Salmonellosis in humans

Salmonella spp. are important human pathogens globally causing millions of cases of typhoid fever and non‐typhoidal salmonellosis annually. There are only a few vaccines licensed for use in humans which all target Salmonella enterica serovar Typhi. Vaccine development is hampered by antigenic diversity between the thousands of serovars capable of causing infection in humans. However, a number of attenuated candidate vaccine strains are currently being developed. As facultative intracellular pathogens with multiple systems for transporting effector proteins to host cells, attenuated Salmonella strains can also serve as ideal tools for the delivery of foreign antigens to create multivalent live carrier vaccines for simultaneous immunization against several unrelated pathogens. Further, the ease with which Salmonella can be genetically modified and the extensive knowledge of the virulence mechanisms of this pathogen means that this bacterium has often served as a model organism to test new approaches. In this review we focus on (1) recent advances in live attenuated Salmonella vaccine development, (2) improvements in expression of foreign antigens in carrier vaccines and (3) adaptation of attenuated strains as sources of purified antigens and vesicles that can be used for subunit and conjugate vaccines or together with attenuated vaccine strains in heterologous prime‐boosting immunization strategies. These advances have led to the development of new vaccines against Salmonella which have or will soon be tested in clinical trials.

Development of Antiviral Vaccine Utilizing Self-Destructing Salmonella for Antigen and DNA Vaccine Delivery

Vaccines are the most effective means to prevent infectious diseases, especially for viral infection. The key to an excellent antiviral vaccine is the ability to induce long-term protective immunity against a specific virus. Bacterial vaccine vectors have been used to impart protection against self, as well as heterologous antigens. One significant benefit of using live bacterial vaccine vectors is their ability to invade and colonize deep effector lymphoid tissues after mucosal delivery. The bacterium Salmonella is considered the best at this deep colonization. This is critically essential for inducing protective immunity. This chapter describes the methodology for developing genetically modified self-destructing Salmonella (GMS) vaccine delivery systems targeting influenza infection. Specifically, the methods covered include the procedures for the development of GMSs for protective antigen delivery to induce cellular immune responses and DNA vaccine delivery to induce systemic immunity against the influenza virus. These self-destructing GMS could be modified to provide effective biological containment for genetically engineered bacteria used for a diversity of purposes in addition to vaccines.

New technologies in developing recombinant-attenuated bacteria for cancer therapy

Cancer has always been a global problem, with more cases of cancer patients being diagnosed every year. Conventional cancer treatments, including radiotherapy, chemotherapy, and surgery, are still unable to bypass their obvious limitations, and developing effective targeted therapies is still required. More than one century ago, the doctor William B. Coley discovered that cancer patients had tumor regression by injection of Streptococcus bacteria. The studies of cancer therapy using bacterial microorganisms are now very widespread. In particular, the facultative anaerobic bacteria Salmonella typhimurium is widely investigated as it can selectively colonize different types of tumors, locally deliver various antitumor drugs, and inhibit tumor growth. The exciting antitumor efficacy and safety observed in animal tumor models prompted the well-known attenuated Salmonella bacterial strain VNP20009 to be tested in human clinical trials in the early 21st century. Regrettably, no patients showed significant therapeutic effects and even bacterial colonization in tumor tissue was undetectable in most patients. Salmonella bacteria are still considered as a promising agent or vehicle for cancer therapy. Recent efforts have been focused on the generation of attenuated bacterial strains with higher targeting for tumor tissue, and optimization of the delivery of therapeutic antitumor cargoes into the tumor microenvironment. This review will summarize new technologies or approaches that may improve bacteria-mediated cancer therapy.

Mucosal Delivery of a Self-destructing Salmonella-Based Vaccine Inducing Immunity Against Eimeria

A programmed self-destructive Salmonella vaccine delivery system was developed to facilitate efficient colonization in host tissues that allows release of the bacterial cell contents after lysis to stimulate mucosal, systemic, and cellular immunities against a diversity of pathogens. Adoption and modification of these technological improvements could form part of an integrated strategy for cost-effective control and prevention of infectious diseases, including those caused by parasitic pathogens. Avian coccidiosis is a common poultry disease caused by Eimeria. Coccidiosis has been controlled by medicating feed with anticoccidial drugs or administering vaccines containing low doses of virulent or attenuated Eimeria oocysts. Problems of drug resistance and nonuniform administration of these Eimeria resulting in variable immunity are prompting efforts to develop recombinant Eimeria vaccines. In this study, we designed, constructed, and evaluated a self-destructing recombinant attenuated Salmonella vaccine (RASV) lysis strain synthesizing the Eimeria tenella SO7 antigen. We showed that the RASV lysis strain χ11791(pYA5293) with a ΔsifA mutation enabling escape from the Salmonella-containing vesicle (or endosome) successfully colonized chicken lymphoid tissues and induced strong mucosal and cell-mediated immunities, which are critically important for protection against Eimeria challenge. The results from animal clinical trials show that this vaccine strain significantly increased food conversion efficiency and protection against weight gain depression after challenge with 105E. tenella oocysts with concomitant decreased oocyst output. More importantly, the programmed regulated lysis feature designed into this RASV strain promotes bacterial self-clearance from the host, lessening persistence of vaccine strains in vivo and survival if excreted, which is a critically important advantage in a vaccine for livestock animals. Our approach should provide a safe, cost-effective, and efficacious vaccine to control coccidiosis upon addition of additional protective Eimeria antigens. These improved RASVs can also be modified for use to control other parasitic diseases infecting other animal species.

Precision design of stable genetic circuits carried in highly-insulated E. coli genomic landing pads

Genetic circuits have many applications, from guiding living therapeutics to ordering process in a bioreactor, but to be useful they have to be genetically stable and not hinder the host. Encoding circuits in the genome reduces burden, but this decreases performance and can interfere with native transcription. We have designed genomic landing pads in Escherichia coli at high-expression sites, flanked by ultrastrong double terminators. DNA payloads >8 kb are targeted to the landing pads using phage integrases. One landing pad is dedicated to carrying a sensor array, and two are used to carry genetic circuits. NOT/NOR gates based on repressors are optimized for the genome and characterized in the landing pads. These data are used, in conjunction with design automation software (Cello 2.0), to design circuits that perform quantitatively as predicted. These circuits require fourfold less RNA polymerase than when carried on a plasmid and are stable for weeks in a recA+ strain without selection. This approach enables the design of synthetic regulatory networks to guide cells in environments or for applications where plasmid use is infeasible.

Controlling the Implementation of Transgenic Microbes: Are We Ready for What Synthetic Biology Has to Offer?

Synthetic biology has promised and delivered on an impressive array of applications based on genetically modified microorganisms. While novel biotechnology undoubtedly offers benefits, like all new technology, precautions should be considered during implementation to reduce the risk of both known and unknown adverse effects. To achieve containment of transgenic microorganisms, confidence to a near-scientific certainty that they cannot transfer their transgenic genes to other organisms, and that they cannot survive to propagate in unintended environments, is a priority. Here, we present an in-depth summary of biological containment systems for micro-organisms published to date, including the production of a genetic firewall through genome recoding and physical containment of microbes using auxotrophies, regulation of essential genes, and expression of toxic genes. The level of containment required to consider a transgenic organism suitable for deployment is discussed, as well as standards of practice for developing new containment systems.

Biosensing in Smart Engineered Probiotics

Engineered microbes are exciting alternatives to current diagnostics and therapeutics. Researchers have developed a wide range of genetic tools and parts to engineer probiotic and commensal microbes. Among these tools and parts, biosensors allow the microbes to sense and record or to sense and respond to chemical and environmental signals in the body, enabling them to report on health conditions of the animal host and/or deliver therapeutics in a controlled manner. This review focuses on how biosensing is applied to engineer "smart" microbes for in vivo diagnostic, therapeutic, and biocontainment goals. Hurdles that need to be overcome when transitioning from high-throughput in vitro systems to low-throughput in vivo animal models, new technologies that can be implemented to alleviate this experimental gap, and areas where future advancements can be made to maximize the utility of biosensing for medical applications are also discussed. As technologies for engineering microbes continue to be developed, these engineered organisms will be used to address many medical challenges.

Attenuated Salmonella Typhimurium expressing Salmonella Paratyphoid A O-antigen induces protective immune responses against two Salmonella strains

Salmonella enterica serovar Paratyphi A is the main causative agent of paratyphoid fever in many Asian countries. As paratyphoid is spread by the fecal-oral route, the most effective means of controlling S. Paratyphi A infection is through the availability of clean water supplies and working sanitation services. Because sanitation facilities improve slowly in these poor areas and antibiotic resistance is severe, the development of a safe and effective vaccine remains a priority for controlling the spread of paratyphoid disease. In this study, we investigated the strategy of heterologous O-antigenic O2 serotype (S. Paratyphi A characterized) conversion in S. Typhimurium to prevent paratyphoid infections. A series of S. Typhimurium mutants were constructed with replacement of abe, wzxB1 and wbaVB1 genes with respective prt-tyvA1, wzxA1 and wbaVA1, and the results showed that only three genes including prt, wbaVA1 and wzxA1 from S. Paratyphi A presence enable S. Typhimurium to sufficiently express O2 antigen polysaccharide. We also constructed a series of live attenuated S. Typhimurium vaccine candidates expressing heterologous O2 O-antigens, and a mouse model was used to evaluate the immunogenicity of live vaccines. ELISA data showed that vaccine candidates could induce a comparatively high level of S. Paratyphi A and/or S. Typhimurium LPS-specific IgG and IgA responses in murine model, and IgG2a levels were consistently higher than IgG1 levels. Moreover, the functional properties of serum antibodies were evaluated using in vitro C3 complement deposition and opsonophagocytic assays. Our work highlights the potential for developing S. Typhimurium live vaccines against S. Paratyphi A.

A Novel Cre Recombinase-Mediated In Vivo Minicircle DNA (CRIM) Vaccine Provides Partial Protection against Newcastle Disease Virus

Minicircle DNA (mcDNA), which contains only the necessary components for eukaryotic expression and is thus smaller than traditional plasmids, has been designed for application in genetic manipulation. In this study, we constructed a novel plasmid containing both the Cre recombinase under the phosphoglycerate kinase (PGK) promoter and recombinant lox66 and lox71 sites located outside the cytomegalovirus (CMV) expression cassette. The strictly controlled synthesis of Cre recombinase in vivo maintained the complete form of the plasmid in vitro, whereas the in vivo production of Cre transformed the parental plasmid to mcDNA after transfection. The newly designed Cre recombinase-mediated in vivo mcDNA platform, named CRIM, significantly increased the nuclear entry of mcDNA, followed by increased production of mRNA and protein, using enhanced green fluorescent protein (EGFP) as a model. Similar results were also observed in chickens when the vaccine was delivered by the regulated-delayed-lysis Salmonella strain χ11218, where significantly increased production of EGFP was observed in chicken livers. Then, we used the HN gene of genotype VII Newcastle disease virus as an antigen model to construct the traditional plasmid pYL43 and the novel mcDNA plasmid pYL47. After immunization, our CRIM vaccine provided significantly increased protection against challenge compared with that of the traditional plasmid, providing us with a novel mcDNA vaccine platform.IMPORTANCE Minicircle DNA (mcDNA) has been considered an attractive alternative to DNA vaccines; however, the relatively high cost and complicated process of purifying mcDNA dramatically restricts the application of mcDNA in the veterinary field. We designed a novel in vivo mcDNA platform in which the complete plasmid could spontaneously transform into mcDNA in vivo In combination with the regulated-delayed-lysis Salmonella strain, the newly designed mcDNA vaccine provides us with an elegant platform for veterinary vaccine development.

Construction of a novel DNA vaccine candidate targeting F gene of genotype VII Newcastle disease virus and chicken IL-18 delivered by Salmonella

AIMS

Genotype VII Newcastle disease (ND) is one of the most epidemic and serious infectious diseases in the poultry industry. A novel vaccine targeting VII Newcastle disease virus (NDV) is still proving elusive.

METHODS AND RESULTS

In this study, we constructed regulated delayed lysis Salmonella strains expressing either a fusion protein (F) alone under an eukaryotic CMV promoter or together with chicken IL-18 (chIL-18) as a molecular adjuvant under prokaryotic P_trc promoter, named pYL1 and pYL23 respectively. Oral immunization with recombinant strains induced NDV-specific serum IgG antibodies in both pYL1- and pYL23-immunized chickens. The presence of chIL-18 significantly increased lymphocyte proliferation in immunized chickens, as well as the percentages of CD3⁺ CD4⁺ and CD3⁺ CD8⁺ T cells in serum, even if a statistically significant difference did not exist. After a virulent challenge, pYL23 immunization provided about 80% protection at day 10 postinfection, compared with 60% of protection offered by pYL1 immunization and 100% protection in the inactivated vaccine group, indicating the enhanced immune response provided by chIL-18, which was also confirmed by histochemical analysis.

CONCLUSIONS

Recombinant lysis Salmonella-vectored DNA vaccine could provide us a novel potential option for controlling NDV infection.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study took use of a regulated delayed lysis Salmonella vector for the design of an orally administrated vaccine against NDV.

Salmonella-vectored vaccine delivering three Clostridium perfringens antigens protects poultry against necrotic enteritis

Necrotic enteritis is an economically important poultry disease caused by the bacterium Clostridium perfringens. There are currently no necrotic enteritis vaccines commercially available for use in broiler birds, the most important target population. Salmonella-vectored vaccines represent a convenient and effective option for controlling this disease. We used a single attenuated Salmonella vaccine strain, engineered to lyse within the host, to deliver up to three C. perfringens antigens. Two of the antigens were toxoids, based on C. perfringens α-toxin and NetB toxin. The third antigen was fructose-1,6-bisphosphate aldolase (Fba), a metabolic enzyme with an unknown role in virulence. Oral immunization with a single Salmonella vaccine strain producing either Fba, α-toxoid and NetB toxoid, or all three antigens, was immunogenic, inducing serum, cellular and mucosal responses against Salmonella and the vectored C. perfringens antigens. All three vaccine strains were partially protective against virulent C. perfringens challenge. The strains delivering Fba only or all three antigens provided the best protection. We also demonstrate that both toxins and Fba are present on the C. perfringens cell surface. The presence of Fba on the cell surface suggests that Fba may function as an adhesin.

Genetically engineered Salmonella Typhimurium: Recent advances in cancer therapy

Bacteria have been investigated as anti-tumor therapeutic agents for more than a century, since Coley first observed successful curing of a patient with inoperable cancer by injection of streptococcal organisms. Previous studies have demonstrated that some obligate or facultative anaerobes can selectively accumulate and proliferate within tumors and suppress their growth. Developments in molecular biology as well as the complete genome sequencing of many bacterial species have increased the applicability of bacterial organisms for cancer treatment. In particular, the facultative anaerobe Salmonella Typhimurium has been widely studied and genetically engineered to improve its tumor-targeting ability as well as to reduce bacterial virulence. Moreover, the effectiveness of engineered attenuated S. Typhimurium strains employed as live delivery vectors of various anti-tumor therapeutic agents or combined with other therapies has been evaluated in a large number of animal experiments. The well-known S. Typhimurium mutant VNP20009 and its derivative strain TAPET-CD have even been applied in human clinical trials. However, Salmonella-mediated cancer therapies have not achieved the expected success, except in animal experiments. Many problems remain to be solved to exploit more promising strategies for combatting cancer with Salmonella bacteria. Here, we summarize the promising studies regarding cancer therapy mediated by Salmonella bacteria and highlight the main mechanisms of Salmonella anti-tumor activities.

Identification of Genes Involved in Biogenesis of Outer Membrane Vesicles (OMVs) in Salmonella enterica Serovar Typhi

Outer membrane vesicles (OMVs) are nano-sized proteoliposomes discharged from the cell envelope of Gram-negative bacteria. OMVs normally contain toxins, enzymes and other factors, and are used as vehicles in a process that has been considered a generalized, evolutionarily conserved delivery system among bacteria. Furthermore, OMVs can be used in biotechnological applications that require delivery of biomolecules, such as vaccines, remarking the importance of their study. Although it is known that Salmonella enterica serovar Typhi (S. Typhi), the etiological agent of typhoid fever in humans, delivers toxins (e.g., HlyE) via OMVs, there are no reports identifying genetic determinants of the OMV biogenesis in this serovar. In the present work, and with the aim to identify genes participating in OMV biogenesis in S. Typhi, we screened 15,000 random insertion mutants for increased HlyE secretion. We found 9 S. Typhi genes (generically called zzz genes) determining an increased HlyE secretion that were also involved in OMV biogenesis. The genes corresponded to ompA, nlpI, and tolR (envelope stability), rfaE and waaC (LPS synthesis), yipP (envC), mrcB (synthesis and remodeling of peptidoglycan), degS (stress sensor serine endopeptidase) and hns (global transcriptional regulator). We found that S. Typhi Δzzz mutants were prone to secrete periplasmic, functional proteins with a relatively good envelope integrity. In addition, we showed that zzz genes participate in OMV biogenesis, modulating different properties such as OMV size distribution, OMV yield and OMV protein cargo.

Local application of bacteria improves safety of Salmonella -mediated tumor therapy and retains advantages of systemic infection

Cancer is a devastating disease and a large socio-economic burden. Novel therapeutic solutions are on the rise, although a cure remains elusive. Application of microorganisms represents an ancient therapeutic strategy, lately revoked and refined via simultaneous attenuation and amelioration of pathogenic properties. Salmonella Typhimurium has prevailed in preclinical development. Yet, using virulent strains for systemic treatment might cause severe side effects. In the present study, we highlight a modified strain based on Salmonella Typhimurium UK-1 expressing hexa-acylated Lipid A. We corroborate improved anti-tumor properties of this strain and investigate to which extent an intra-tumoral (i.t.) route of infection could help improve safety and retain advantages of systemic intravenous (i.v.) application. Our results show that i.t. infection exhibits therapeutic efficacy against CT26 and F1.A11 tumors similar to a systemic route of inoculation. Moreover, i.t. application allows extensive dose titration without compromising tumor colonization. Adverse colonization of healthy organs was generally reduced via i.t. infection and accompanied by less body weight loss of the murine host. Despite local application, adjuvanticity remained, and a CT26-specific CD8⁺ T cell response was effectively stimulated. Most interestingly, also secondary tumors could be targeted with this strategy, thereby extending the unique tumor targeting ability of Salmonella. The i.t. route of inoculation may reap the benefits of systemic infection and aid in safety assurance while directing potency of an oncolytic vector to where it is most needed, namely the primary tumor.

Salmonella Vaccines: Conduits for Protective Antigens

Vaccines afford a better and more cost-effective approach to combatting infectious diseases than continued reliance on antibiotics or antiviral or antiparasite drugs in the current era of increasing incidences of diseases caused by drug-resistant pathogens. Recombinant attenuated Salmonella vaccines (RASVs) have been significantly improved to exhibit the same or better attributes than wild-type parental strains to colonize internal lymphoid tissues and persist there to serve as factories to continuously synthesize and deliver rAgs. Encoded by codon-optimized pathogen genes, Ags are selected to induce protective immunity to infection by that pathogen. After immunization through a mucosal surface, the RASV attributes maximize their abilities to elicit mucosal and systemic Ab responses and cell-mediated immune responses. This article summarizes many of the numerous innovative technologies and discoveries that have resulted in RASV platforms that will enable development of safe efficacious RASVs to protect animals and humans against a diversity of infectious disease agents.

Therapy of solid tumors using probiotic Symbioflor-2: restraints and potential

To date, virulent bacteria remain the basis of most bacteria mediated cancer therapies. For clinical application attenuation is required. However, this might result in a drastically lowered therapeutic capacity. Herein we argue that the E. coli probiotic Symbioflor-2, with a history of safe application may constitute a viable tumor therapeutic candidate. We demonstrate that Symbioflor-2 displays a highly specific tumor targeting ability as determined in murine CT26 and RenCa tumor models. The excellent specificity was ascribed to reduced levels of adverse colonization. A high safety standard was demonstrated in WT and Rag1^−/− mice. Thus, Symbioflor-2 may represent an ideal tumor targeting delivery system for therapeutic molecules. Moreover, Symbioflor-2 was capable of inducing CT26 tumor clearance as result of an adjuvant effect on tumor specific CD8⁺ T cells analogous to the Salmonella variant SL7207. However, lower therapeutic efficacy against RenCa tumors suggested a generally reduced therapeutic potency for probiotics. Interestingly, concurrent depletion of Gr-1⁺ or Ly6G⁺ cells installed therapeutic efficacy equal to SL7207, thus highlighting the role of innate effector cells in restraining the anti-tumor effects of Symbioflor-2. Collectively, our findings argue for a strategy of safe strain application and a more sustainable use of bacteria as a delivery system for therapeutic molecules.

Rational Design of Evolutionarily Stable Microbial Kill Switches

The evolutionary stability of synthetic genetic circuits is key to both the understanding and application of genetic control elements. One useful but challenging situation is a switch between life and death depending on environment. Here are presented "essentializer" and "cryodeath" circuits, which act as kill switches in Escherichia coli. The essentializer element induces cell death upon the loss of a bi-stable cI/Cro memory switch. Cryodeath makes use of a cold-inducible promoter to express a toxin. We employ rational design and a toxin/antitoxin titering approach to produce and screen a small library of potential constructs, in order to select for constructs that are evolutionarily stable. Both kill switches were shown to maintain functionality in vitro for at least 140 generations. Additionally, cryodeath was shown to control the growth environment of a population, with an escape frequency of less than 1 in 10⁵ after 10 days of growth in the mammalian gut.

Two Novel Salmonella Bivalent Vaccines Confer Dual Protection against Two Salmonella Serovars in Mice

Non-typhoidal Salmonella includes thousands of serovars that are leading causes of foodborne diarrheal illness worldwide. In this study, we constructed three bivalent vaccines for preventing both Salmonella Typhimurium and Salmonella Newport infections by using the aspartate semialdehyde dehydrogenase (Asd)-based balanced-lethal vector-host system. The constructed Asd⁺ plasmid pCZ11 carrying a subset of the Salmonella Newport O-antigen gene cluster including the wzx-wbaR-wbaL-wbaQ-wzy-wbaW-wbaZ genes was introduced into three Salmonella Typhimurium mutants: SLT19 (Δasd) with a smooth LPS phenotype, SLT20 (Δasd ΔrfbN) with a rough LPS phenotype, and SLT22 (Δasd ΔrfbN ΔpagL::T araC P_BADrfbN) with a smooth LPS phenotype when grown with arabinose. Immunoblotting demonstrated that SLT19 harboring pCZ11 [termed SLT19 (pCZ11)] co-expressed the homologous and heterologous O-antigens; SLT20 (pCZ11) exclusively expressed the heterologous O-antigen; and when arabinose was available, SLT22 (pCZ11) expressed both types of O-antigens, while in the absence of arabinose, SLT22 (pCZ11) expressed only the heterologous O-antigen. Exclusive expression of the heterologous O-antigen in Salmonella Typhimurium decreased the swimming ability of the bacterium and its susceptibility to polymyxin B. Next, the crp gene was deleted from the three recombinant strains for attenuation purposes, generating the three bivalent vaccine strains SLT25 (pCZ11), SLT26 (pCZ11), and SLT27 (pCZ11), respectively. Groups of BALB/c mice (12 mice/group) were orally immunized with 10⁹ CFU of each vaccine strain twice at an interval of 4 weeks. Compared with a mock immunization, immunization with all three vaccine strains induced significant serum IgG responses against both Salmonella Typhimurium and Salmonella Newport LPS. The bacterial loads in the mouse tissues were significantly lower in the three vaccine-strain-immunized groups than in the mock group after either Salmonella Typhimurium or Salmonella Newport lethal challenge. All of the mice in the three vaccine-immunized groups survived the lethal Salmonella Typhimurium challenge. In contrast, SLT26 (pCZ11) and SLT27 (pCZ11) conferred full protection against lethal Salmonella Newport challenge, but SLT25 (pCZ11) provided only 50% heterologous protection. Thus, we developed two novel Salmonella bivalent vaccines, SLT26 (pCZ11) and SLT27 (pCZ11), suggesting that the delivery of a heterologous O-antigen in attenuated Salmonella strains is a prospective approach for developing Salmonella vaccines with broad serovar coverage.

Recombinant attenuated Salmonella Typhimurium with heterologous expression of the Salmonella Choleraesuis O-polysaccharide: high immunogenicity and protection

Non-typhoidal Salmonella are associated with gastrointestinal disease worldwide and invasive disease in Africa. We constructed novel bivalent vaccines through the recombinant expression of heterologous O-antigens from Salmonella Choleraesuis in Salmonella Typhimurium. A recombinant Asd⁺ plasmid pCZ1 with the cloned Salmonella Choleraesuis O-antigen gene cluster was introduced into three constructed Salmonella Typhimurium Δasd mutants: SLT11 (ΔrfbP), SLT12 (ΔrmlB-rfbP) and SLT16 (ΔrfbP ∆pagL::TT araCP_BADrfbP). Immunoblotting demonstrated that SLT11 (pCZ1) and SLT12 (pCZ1) efficiently expressed the heterologous O-antigen. In the presence of arabinose, SLT16 (pCZ1) expressed both the homologous and heterologous O-antigens, whereas in the absence of arabinose, SLT16 (pCZ1) mainly expressed the heterologous O-antigen. We deleted the crp/cya genes in SLT12 (pCZ1) and SLT16 (pCZ1) for attenuation purposes, generating the recombinant vaccine strains SLT17 (pCZ1) and SLT18 (pCZ1). Immunization with either SLT17 (pCZ1) or SLT18 (pCZ1) induced specific IgG against the heterologous O-antigen, which mediated significant killing of Salmonella Choleraesuis and provided full protection against a lethal homologous challenge in mice. Furthermore, SLT17 (pCZ1) or SLT18 (pCZ1) immunization resulted in 83% or 50% heterologous protection against Salmonella Choleraesuis challenge, respectively. Our study demonstrates that heterologous O-antigen expression is a promising strategy for the development of multivalent Salmonella vaccines.

Engineered Probiotics: Applications and Biological Containment

Bioengineered probiotics represent the next generation of whole cell-mediated biotherapeutics. Advances in synthetic biology, genome engineering, and DNA sequencing and synthesis have enabled scientists to design and develop probiotics with increased stress tolerance and the ability to target specific pathogens and their associated toxins, as well as to mediate targeted delivery of vaccines, drugs, and immunomodulators directly to host cells. Herein, we review the most significant advances in the development of this field. We discuss the critical issue of biological containment and consider the role of synthetic biology in the design and construction of the probiotics of the future.

Synthetic biology platform technologies for antimicrobial applications

The growing prevalence of antibiotic resistance calls for new approaches in the development of antimicrobial therapeutics. Likewise, improved diagnostic measures are essential in guiding the application of targeted therapies and preventing the evolution of therapeutic resistance. Discovery platforms are also needed to form new treatment strategies and identify novel antimicrobial agents. By applying engineering principles to molecular biology, synthetic biologists have developed platforms that improve upon, supplement, and will perhaps supplant traditional broad-spectrum antibiotics. Efforts in engineering bacteriophages and synthetic probiotics demonstrate targeted antimicrobial approaches that can be fine-tuned using synthetic biology-derived principles. Further, the development of paper-based, cell-free expression systems holds promise in promoting the clinical translation of molecular biology tools for diagnostic purposes. In this review, we highlight emerging synthetic biology platform technologies that are geared toward the generation of new antimicrobial therapies, diagnostics, and discovery channels.

Genomic Recoding Broadly Obstructs the Propagation of Horizontally Transferred Genetic Elements

Horizontally transferred genetic elements such as viruses and conjugative plasmids move DNA between organisms, increasing genetic diversity but destabilizing engineered biological systems. Here, we used a genomically recoded Escherichia coli strain lacking UAG stop codons and the recognition protein release factor 1 to study how an alternative genetic code influences horizontally transferred genetic element propagation. The alternative genetic code conferred resistance to multiple viruses (λ, M13, P1, MS2) at titers up to 10(11) PFU/ml and impaired conjugative plasmids (F and RK2) up to 10(5)-fold. By recoding UAG codons to UAA in viruses and plasmids, we restored viral infectivity and conjugative function. Propagating viruses on a mixed community of cells with standard and alternative genetic codes reduced viral titer, and over time viruses adapted to the alternative genetic code. This work demonstrates that altering the genetic code broadly obstructs the propagation of horizontally transferred genetic elements and supports the use of genomic recoding as a strategy to stabilize engineered biological systems.

Heterologous Pseudomonas aeruginosa O-antigen delivery using a Salmonella enterica serovar Typhimurium wecA mutant strain

There is a broad interest in adapting live vaccine strains (LVS) for use as recombinant vaccines that can deliver heterologous antigens. The Salmonella enterica serovar Typhimurium SL1344 ΔwecA LVS contains a mutation in wecA that abrogates production of Enterobacterial common antigen. This ΔwecA strain is attenuated in vivo, persistently colonizes the host, and protects against both wild type and cross-Salmonella serovar lethal challenge in a murine model of salmonellosis. Given these characteristics, we hypothesized that the SL1344 ΔwecA strain could be used as a carrier for heterologous antigen expression. To test this hypothesis, SL1344 ΔwecA was engineered to express the Pseudomonas aeruginosa O11 O-antigen gene cluster. Intraperitoneal (IP) but not oral immunization of BALB/c mice with the heterologous expression strain protected against lethal P. aeruginosa intranasal (IN) challenge. Furthermore, IP immunization resulted in P. aeruginosa O11-specific Ig and IgG antibody production. Functional analysis of sera collected from the IP immunized mice showed antibody-mediated agglutination and opsonophagocytic activity against P. aeruginosa. En masse, these results indicate that the S. Typhimurium SL1344 ΔwecA strain expressing the P. aeruginosa O11 O-antigen gene cluster is able to induce a humoral immune response and to protect against lethal P. aeruginosa challenge. As such, the S. Typhimurium SL1344 ΔwecA LVS can likely serve as a vehicle for expression of a wide variety of heterologous antigens as a means to create recombinant vaccines.

Bacteria in Cancer Therapy: Renaissance of an Old Concept

The rising incidence of cancer cases worldwide generates an urgent need of novel treatment options. Applying bacteria may represent a valuable therapeutic variant that is intensively investigated nowadays. Interestingly, the idea to apply bacteria wittingly or unwittingly dates back to ancient times and was revived in the 19th century mainly by the pioneer William Coley. This review summarizes and compares the results of the past 150 years in bacteria mediated tumor therapy from preclinical to clinical studies. Lessons we have learned from the past provide a solid foundation on which to base future efforts. In this regard, several perspectives are discussed by which bacteria in addition to their intrinsic antitumor effect can be used as vector systems that shuttle therapeutic compounds into the tumor. Strategic solutions like these provide a sound and more apt exploitation of bacteria that may overcome limitations of conventional therapies.

Protection Against Necrotic Enteritis in Broiler Chickens by Regulated Delayed Lysis Salmonella Vaccines

Necrotic enteritis (NE), caused by Gram-positive Clostridium perfringens type A strains, has gained more attention in the broiler industry due to governmental restrictions affecting the use of growth-promoting antibiotics in feed. To date, there is only one commercial NE vaccine available, based on the C. perfringens alpha toxin. However, recent work has suggested that the NetB toxin, not alpha toxin, is the most critical virulence factor for causing NE. These findings notwithstanding, it is clear from prior research that immune responses against both toxins can provide some protection against NE. In this study, we delivered a carboxyl-terminal fragment of alpha toxin and a GST-NetB fusion protein using a novel attenuated Salmonella vaccine strain designed to lyse after 6-10 rounds of replication in the chicken host. We immunized birds with vaccine strains producing each protein individually, a mixture of the two strains, or with a single vaccine strain that produced both proteins. Immunization with strains producing either of the single proteins was not protective, but immunization with a mixture of the two or with a single strain producing both proteins resulted in protective immunity. The vaccine strain synthesizing both PlcC and GST-NetB was able to elicit strong production of intestinal IgA, IgY, and IgM antibodies and significantly protect broilers against C. perfringens challenge against both mild and severe challenges. Although not part of our experimental plan, the broiler chicks we obtained for these studies were apparently contaminated during transit from the hatchery with group D Salmonella. Despite this drawback, the vaccines worked well, indicating applicability to real-world conditions.

Plasmids as Tools for Containment

Active containment systems are a major tool for reducing the uncertainty associated with the introduction of monocultures, genetically engineered or not, into target habitats for a large number of biotechnological applications (e.g., bioremediation, bioleaching, biopesticides, biofuels, biotransformations, live vaccines, etc.). While biological containment reduces the survival of the introduced organism outside the target habitat and/or upon completion of the projected task, gene containment strategies reduce the lateral spread of the key genetic determinants to indigenous microorganisms. In fundamental research, suicide circuits become relevant tools to address the role of gene transfer, mainly plasmid transfer, in evolution and how this transfer contributes to genome plasticity and to the rapid adaptation of microbial communities to environmental changes. Many lethal functions and regulatory circuits have been used and combined to design efficient containment systems. As many new genomes are being sequenced, novel lethal genes and regulatory elements are available, e.g., new toxin-antitoxin modules, and they could be used to increase further the current containment efficiencies and to expand containment to other organisms. Although the current containment systems can increase the predictability of genetically modified organisms in the environment, containment will never be absolute, due to the existence of mutations that lead to the appearance of surviving subpopulations. In this sense, orthogonal systems (xenobiology) appear to be the solution for setting a functional genetic firewall that will allow absolute containment of recombinant organisms.