Current and future prospects for CRISPR-based tools in bacteria

Role of CRISPR-Cas systems and anti-CRISPR proteins in bacterial antibiotic resistance

The emergence and development of antibiotic resistance in bacteria is a serious threat to global public health. Antibiotic resistance genes (ARGs) are often located on mobile genetic elements (MGEs). They can be transferred among bacteria by horizontal gene transfer (HGT), leading to the spread of drug-resistant strains and antibiotic treatment failure. CRISPR (clustered regularly interspaced short palindromic repeats)-Cas (CRISPR-associated genes) is one of the many strategies bacteria have developed under long-term selection pressure to restrict the HGT. CRISPR-Cas systems exist in about half of bacterial genomes and play a significant role in limiting the spread of antibiotic resistance. On the other hand, bacteriophages and other MGEs encode a wide range of anti-CRISPR proteins (Acrs) to counteract the immunity of the CRISPR-Cas system. The Acrs could decrease the CRISPR-Cas system's activity against phages and facilitate the acquisition of ARGs and virulence traits for bacteria. This review aimed to assess the relationship between the CRISPR-Cas systems and Acrs with bacterial antibiotic resistance. We also highlighted the CRISPR technology and Acrs to control and prevent antibacterial resistance. The CRISPR-Cas system can target nucleic acid sequences with high accuracy and reliability; therefore, it has become a novel gene editing and gene therapy tool to prevent the spread of antibiotic resistance. CRISPR-based approaches may pave the way for developing smart antibiotics, which could eliminate multidrug-resistant (MDR) bacteria and distinguish between pathogenic and beneficial microorganisms. Additionally, the engineered anti-CRISPR gene-containing phages in combination with antibiotics could be used as a cutting-edge treatment approach to reduce antibiotic resistance.

Recent Advances in Metabolic Engineering for the Biosynthesis of Phosphoenol Pyruvate-Oxaloacetate-Pyruvate-Derived Amino Acids

The phosphoenol pyruvate-oxaloacetate-pyruvate-derived amino acids (POP-AAs) comprise native intermediates in cellular metabolism, within which the phosphoenol pyruvate-oxaloacetate-pyruvate (POP) node is the switch point among the major metabolic pathways existing in most living organisms. POP-AAs have widespread applications in the nutrition, food, and pharmaceutical industries. These amino acids have been predominantly produced in Escherichia coli and Corynebacterium glutamicum through microbial fermentation. With the rapid increase in market requirements, along with the global food shortage situation, the industrial production capacity of these two bacteria has encountered two bottlenecks: low product conversion efficiency and high cost of raw materials. Aiming to push forward the update and upgrade of engineered strains with higher yield and productivity, this paper presents a comprehensive summarization of the fundamental strategy of metabolic engineering techniques around phosphoenol pyruvate-oxaloacetate-pyruvate node for POP-AA production, including L-tryptophan, L-tyrosine, L-phenylalanine, L-valine, L-lysine, L-threonine, and L-isoleucine. Novel heterologous routes and regulation methods regarding the carbon flux redistribution in the POP node and the formation of amino acids should be taken into consideration to improve POP-AA production to approach maximum theoretical values. Furthermore, an outlook for future strategies of low-cost feedstock and energy utilization for developing amino acid overproducers is proposed.

An adapted method for Cas9-mediated editing reveals the species-specific role of β-glucoside utilization driving competition between Klebsiella species

Cas9-based gene editing tools have revolutionized genetics, enabling the fast and precise manipulation of diverse bacterial species. However, widely applicable genetic tools for non-model gut bacteria are unavailable. Here, we present a two-plasmid Cas9-based system designed for gene deletion and knock-in complementation in three members of the Klebsiella oxytoca species complex (KoSC), which we applied to study the genetic factors underlying the role of these bacteria in competition against Klebsiella pneumoniae. Firstly, the system allowed efficient and precise full-length gene deletion via enhanced lambda Red expression. Furthermore, we tested the efficiency of two independent, functionally validated complementation strategies. Ultimately, the insertion of universal "bookmark" targets during gene deletion subsequently allows the most optimal genetic complementation in K. oxytoca, Klebsiella michiganensis, and Klebsiella grimontii. This approach offers a significant advantage by enabling the use of a single high-efficiency "bookmark" for complementing other loci or strains, eliminating the need for site-specific design. We revealed that the carbohydrate permease CasA is critical in ex vivo assays for K. pneumoniae inhibition by K. oxytoca but is neither sufficient nor required for K. michiganensis and K. grimontii. Thus, the adaptation of state-of-the-art genetic tools to KoSC allows the identification of species-specific functions in microbial competition.

IMPORTANCE

Cas9-based gene editing tools have revolutionized bacterial genetics, yet, their application to non-model gut bacteria is frequently hampered by various limitations. We utilized a two-plasmid Cas9-based system designed for gene deletion in Klebsiella pneumoniae and demonstrate after optimization its utility for gene editing in three members of the Klebsiella oxytoca species complex (KoSC) namely K. oxytoca, Klebsiella michiganensis, and Klebsiella grimontii. We then adapted a recently developed protocol for functional complementation based on universal "bookmark" targets applicable to all tested species. In summary, species-specific adaptation of state-of-the-art genetic tools allows efficient gene deletion and complementation in type strains as well as natural isolates of KoSC members to study microbial interactions.

Rewiring Metabolic Flux in Corynebacterium glutamicum Using a CRISPR/dCpf1-Based Bifunctional Regulation System

Corynebacterium glutamicum, a microorganism classified as generally recognized as safe for use in the industrial production of food raw materials and additives, has encountered challenges in achieving widespread adoption and popularization as microbial cell factories. These obstacles arise from the intricate nature of manipulating metabolic flux through conventional methods, such as gene knockout and enzyme overexpression. To address this challenge, we developed a CRISPR/dCpf1-based bifunctional regulation system to bidirectionally regulate the expression of multiple genes in C. glutamicum. Specifically, through fusing various transcription factors to the C-terminus of dCpf1, the resulting dCpf1-SoxS exhibited both CRISPR interference (CRISPRi) and CRISPR activation (CRISPRa) capabilities in C. glutamicum by altering the binding sites of crRNAs. The bifunctional regulation system was used to fine-tune metabolic flux from shikimic acid (SA) and l-serine biosynthesis, resulting in 27-fold and 10-fold increases in SA and l-serine production, respectively, compared to the original strain. These findings highlight the potential of the CRISPR/dCpf1-based bifunctional regulation system in effectively enhancing the yield of target products in C. glutamicum.

Improved prediction of bacterial CRISPRi guide efficiency from depletion screens through mixed-effect machine learning and data integration

CRISPR interference (CRISPRi) is the leading technique to silence gene expression in bacteria; however, design rules remain poorly defined. We develop a best-in-class prediction algorithm for guide silencing efficiency by systematically investigating factors influencing guide depletion in genome-wide essentiality screens, with the surprising discovery that gene-specific features substantially impact prediction. We develop a mixed-effect random forest regression model that provides better estimates of guide efficiency. We further apply methods from explainable AI to extract interpretable design rules from the model. This study provides a blueprint for predictive models for CRISPR technologies where only indirect measurements of guide activity are available.

Genetic Engineering of Acidithiobacillus ferridurans Using CRISPR Systems To Mitigate Toxic Release in Biomining

Biomining processes utilize microorganisms, such as Acidithiobacillus, to extract valuable metals by producing sulfuric acid and ferric ions that dissolve sulfidic minerals. However, excessive production of these compounds can result in metal structure corrosion and groundwater contamination. Synthetic biology offers a promising solution to improve Acidithiobacillus strains for sustainable, eco-friendly, and cost-effective biomining, but genetic engineering of these slow-growing microorganisms is challenging with current inefficient and time-consuming methods. To address this, we established a CRISPR-dCas9 system for gene knockdown in A. ferridurans JAGS, successfully downregulating the transcriptional levels of two genes involved in sulfur oxidation. More importantly, we constructed an all-in-one CRISPR-Cas9 system for fast and efficient genome editing in A. ferridurans JAGS, achieving seamless gene deletion (HdrB3), promoter substitution (Prus to Ptac), and exogenous gene insertion (GFP). Additionally, we created a HdrB-Rus double-edited strain and performed biomining experiments to extract Ni from pyrrhotite tailings. The engineered strain demonstrated a similar Ni recovery rate to wild-type A. ferridurans JAGS but with significantly lower production of iron ions and sulfuric acid in leachate. These high-efficient CRISPR systems provide a powerful tool for studying gene functions and creating useful recombinants for synthetic biology-assisted biomining applications in the future.

CRISPR-Cas-Based Antimicrobials: Design, Challenges, and Bacterial Mechanisms of Resistance

The emergence of antibiotic-resistant bacterial strains is a source of public health concern across the globe. As the discovery of new conventional antibiotics has stalled significantly over the past decade, there is an urgency to develop novel approaches to address drug resistance in infectious diseases. The use of a CRISPR-Cas-based system for the precise elimination of targeted bacterial populations holds promise as an innovative approach for new antimicrobial agent design. The CRISPR-Cas targeting system is celebrated for its high versatility and specificity, offering an excellent opportunity to fight antibiotic resistance in pathogens by selectively inactivating genes involved in antibiotic resistance, biofilm formation, pathogenicity, virulence, or bacterial viability. The CRISPR-Cas strategy can enact antimicrobial effects by two approaches: inactivation of chromosomal genes or curing of plasmids encoding antibiotic resistance. In this Review, we provide an overview of the main CRISPR-Cas systems utilized for the creation of these antimicrobials, as well as highlighting promising studies in the field. We also offer a detailed discussion about the most commonly used mechanisms for CRISPR-Cas delivery: bacteriophages, nanoparticles, and conjugative plasmids. Lastly, we address possible mechanisms of interference that should be considered during the intelligent design of these novel approaches.

The Microbial Connection to Sustainable Agriculture

Microorganisms are an important element in modeling sustainable agriculture. Their role in soil fertility and health is crucial in maintaining plants' growth, development, and yield. Further, microorganisms impact agriculture negatively through disease and emerging diseases. Deciphering the extensive functionality and structural diversity within the plant-soil microbiome is necessary to effectively deploy these organisms in sustainable agriculture. Although both the plant and soil microbiome have been studied over the decades, the efficiency of translating the laboratory and greenhouse findings to the field is largely dependent on the ability of the inoculants or beneficial microorganisms to colonize the soil and maintain stability in the ecosystem. Further, the plant and its environment are two variables that influence the plant and soil microbiome's diversity and structure. Thus, in recent years, researchers have looked into microbiome engineering that would enable them to modify the microbial communities in order to increase the efficiency and effectiveness of the inoculants. The engineering of environments is believed to support resistance to biotic and abiotic stressors, plant fitness, and productivity. Population characterization is crucial in microbiome manipulation, as well as in the identification of potential biofertilizers and biocontrol agents. Next-generation sequencing approaches that identify both culturable and non-culturable microbes associated with the soil and plant microbiome have expanded our knowledge in this area. Additionally, genome editing and multidisciplinary omics methods have provided scientists with a framework to engineer dependable and sustainable microbial communities that support high yield, disease resistance, nutrient cycling, and management of stressors. In this review, we present an overview of the role of beneficial microbes in sustainable agriculture, microbiome engineering, translation of this technology to the field, and the main approaches used by laboratories worldwide to study the plant-soil microbiome. These initiatives are important to the advancement of green technologies in agriculture.

Genome engineering via gene editing technologies in microalgae

CRISPR-Cas has revolutionized genetic modification with its comparative simplicity and accuracy, and it can be used even at the genomic level. Microalgae are excellent feedstocks for biofuels and nutraceuticals because they contain high levels of fatty acids, carotenoids, and other metabolites; however, genome engineering for microalgae is not yet as developed as for other model organisms. Microalgal engineering at the genetic and metabolic levels is relatively well established, and a few genomic resources are available. Their genomic information was used for a "safe harbor" site for stable transgene expression in microalgae. This review proposes further genome engineering schemes including the construction of sgRNA libraries, pan-genomic and epigenomic resources, and mini-genomes, which can together be developed into synthetic biology for carbon-based engineering in microalgae. Acetyl-CoA is at the center of carbon metabolic pathways and is further reviewed for the production of molecules including terpenoids in microalgae.

CRISPR-Cas engineering in food science and sustainable agriculture: recent advancements and applications

The developments in the food supply chain to support the growing population of the world is one of today's most pressing issues, and to achieve this goal improvements should be performed in both crops and microbes. For this purpose, novel approaches such as genome editing (GE) methods have upgraded the biological sciences for genome manipulation and, among such methods, clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated proteins (Cas) are the main exciting innovations since the Green Revolution. CRISPR/Cas systems can be a potent tool for the food industry, improvement of agricultural crops and even for protecting food-grade bacteria from foreign genetic invasive elements. This review introduces the history and mechanism of the CRISPR-Cas system as a genome editing tool and its applications in the vaccination of starter cultures, production of antimicrobials and bioactive compounds, and genome editing of microorganisms.

Systematically attenuating DNA targeting enables CRISPR-driven editing in bacteria

Bacterial genome editing commonly relies on chromosomal cleavage with Cas nucleases to counter-select against unedited cells. However, editing normally requires efficient recombination and high transformation efficiencies, which are unavailable in most strains. Here, we show that systematically attenuating DNA targeting activity enables RecA-mediated repair in different bacteria, allowing chromosomal cleavage to drive genome editing. Attenuation can be achieved by altering the format or expression strength of guide (g)RNAs; using nucleases with reduced cleavage activity; or engineering attenuated gRNAs (atgRNAs) with disruptive hairpins, perturbed nuclease-binding scaffolds, non-canonical PAMs, or guide mismatches. These modifications greatly increase cell counts and even improve the efficiency of different types of edits for Cas9 and Cas12a in Escherichia coli and Klebsiella oxytoca. We further apply atgRNAs to restore ampicillin sensitivity in Klebsiella pneumoniae, establishing a resistance marker for genetic studies. Attenuating DNA targeting thus offers a counterintuitive means to achieve CRISPR-driven editing across bacteria.

The economics and policy of genome editing in crop improvement

In this review article we analyze the economics of genome editing and its potential long-term effect on crop improvement and agriculture. We describe the emergence of genome editing as a novel platform for crop improvement, distinct from the existing platforms of plant breeding and genetic engineering. We review key technical characteristics of genome editing and describe how it enables faster trait development, lower research and development costs, and the development of novel traits not possible through previous crop improvement methods. Given these fundamental technical and economic advantages, we describe how genome editing can greatly increase the productivity and broaden the scope of crop improvement with potential outsized economic effects. We further discuss how the global regulatory policy environment, which is still emerging, can shape the ultimate path of genome editing innovation, its effect on crop improvement, and its overall socioeconomic benefits to society.

Development and application of CRISPR-based genetic tools in Bacillus species and Bacillus phages

Recently, the clustered regularly interspaced short palindromic repeats (CRISPR) system has been developed into a precise and efficient genome editing tool. Since its discovery as an adaptive immune system in prokaryotes, it has been applied in many different research fields including biotechnology and medical sciences. The high demand for rapid, highly efficient and versatile genetic tools to thrive in bacteria-based cell factories accelerates this process. This review mainly focuses on significant advancements of the CRISPR system in Bacillus subtilis, including the achievements in gene editing, and on problems still remaining. Next, we comprehensively summarize this genetic tool's up-to-date development and utilization in other Bacillus species, including B. licheniformis, B. methanolicus, B. anthracis, B. cereus, B. smithii and B. thuringiensis. Furthermore, we describe the current application of CRISPR tools in phages to increase Bacillus hosts' resistance to virulent phages and phage genetic modification. Finally, we suggest potential strategies to further improve this advanced technique and provide insights into future directions of CRISPR technologies for rendering Bacillus species cell factories more effective and more powerful.

Auxotrophic Lactobacillus Expressing Porcine Rotavirus VP4 Constructed Using CRISPR-Cas9D10A System Induces Effective Immunity in Mice

Porcine rotavirus (PoRV) mainly causes acute diarrhea in piglets under eight weeks of age and has potentially high morbidity and mortality rates. As vaccine carriers for oral immunization, lactic acid bacteria (LAB) are an ideal strategy for blocking PoRV infections. However, the difficulty in knocking out specific genes, inserting foreign genes, and the residues of antibiotic selection markers are major challenges for the oral vaccination of LAB. In this study, the target gene, alanine racemase (alr), in the genome of Lactobacillus casei strain W56 (L. casei W56) was knocked out to construct an auxotrophic L. casei strain (L. casei Δalr W56) using the CRISPR-Cas9D10A gene editing system. A recombinant strain (pPG-alr-VP4/Δalr W56) was constructed using an electrotransformed complementary plasmid. Expression of the alr-VP4 fusion protein from pPG-alr-VP4/Δalr W56 was detected using Western blotting. Mice orally immunized with pPG-alr-VP4/Δalr W56 exhibited high levels of serum IgG and mucosal secretory immunoglobulin A (SIgA), which exhibited neutralizing effects against PoRV. Cytokines levels in serum detected using ELISA, indicated that the recombinant strain induced an immune response dominated by Th2 cells. Our data suggest that pPG-alr-VP4/Δalr W56, an antibiotic-resistance-free LAB, provides a safer vaccine strategy against PoRV infection.

CRISPR-Cas9 Toolkit for Genome Editing in an Autotrophic CO2-Fixing Methanogenic Archaeon

The CRISPR-Cas9 system is a robust genome editing tool that is widely applied in eukaryotes and bacteria. However, use of this technique has only been developed for one species of Archaea, a domain of life ranking in parallel with Eukarya and Bacteria. In this study, we applied the CRISPR-Cas9 genome editing technique to Methanococcus maripaludis, an autotrophic and hydrogenotrophic methanogenic archaeon with a remarkably polyploid genome comprising up to ~55 chromosomal copies per cell. An editing plasmid was designed that encodes small guide RNA (sgRNA), Cas9 protein and an ~1-kb repair template (donor). Highly efficient (75% to 100%) and precise genome editing was achieved following one-step transformation. Significantly, the Cas9-based system efficiently deleted one or two genes and a large DNA fragment (~9 kb) and even synchronously deleted 13 genes located at three loci in all chromosomal copies of M. maripaludis. Moreover, precise in situ genome modifications, such as gene tagging and multiple- and even single-nucleotide mutagenesis, were also introduced with high efficiency. Further, as a proof of concept, precise mutagenesis at the nucleotide level allowed the engineering of both transcriptional and translational activities. Mutations were introduced into an archaeal promoter BRE (transcription factor B [TFB] recognition element), a terminator U-tract region, and a gene coding region. Stop codon introduction into a gene through single-nucleotide substitution shut down its expression, providing an alternative strategy for gene inactivation. In conclusion, the robust CRISPR-Cas9 genetic toolkit developed in this investigation greatly facilitates the application of M. maripaludis as a model system in the study of archaeal biology and biotechnology development, particularly CO2-based biotechnologies. IMPORTANCE Archaea are prokaryotes with intriguing biological characteristics. They possess bacterial cell structures but eukaryotic homologous information processing machinery and eukaryotic featured proteins. Archaea also display excellent adaptability to extreme environments and play pivotal roles in ecological processes, thus exhibiting valuable biotechnological potential. However, the in-depth understanding and practical application of archaea are much lagging, because only a minority of pure cultures are available, and even worse, very few can be genetically manipulated. This work developed CRISPR-Cas9-based genome editing technology in Methanococcus maripaludis, a CO2-fixing methanogenic archaeon. The CRISPR-Cas9 approach developed in this study provides an elegant and efficient genome editing toolkit that can be applied in the knockout of single or multiple genes, in situ gene tagging, multiple- or single-nucleotide mutagenesis, and inactivation of gene expression by introduction of stop codons. The successful development of the CRISPR-Cas9 toolkit will facilitate the application of M. maripaludis in archaeal biology research and biotechnology development, particularly CO2-derived biotechnologies.

CRISPR-Cas Technology: Emerging Applications in Clinical Microbiology and Infectious Diseases

Through the years, many promising tools for gene editing have been developed including zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), CRISPR-associated protein 9 (Cas9), and homing endonucleases (HEs). These novel technologies are now leading new scientific advancements and practical applications at an inimitable speed. While most work has been performed in eukaryotes, CRISPR systems also enable tools to understand and engineer bacteria. The increase in the number of multi-drug resistant strains highlights a necessity for more innovative approaches to the diagnosis and treatment of infections. CRISPR has given scientists a glimmer of hope in this area that can provide a novel tool to fight against antimicrobial resistance. This system can provide useful information about the functions of genes and aid us to find potential targets for antimicrobials. This paper discusses the emerging use of CRISPR-Cas systems in the fields of clinical microbiology and infectious diseases with a particular emphasis on future prospects.

Development of CRISPR-Cas9 knock-in tools for free fatty acid production using the fast-growing cyanobacterial strain Synechococcus elongatus UTEX 2973

Synechococcus elongatus UTEX 2973 has one of the fastest measured doubling time of cyanobacteria making it an important candidate for metabolic engineering. Traditional genetic engineering methods, which rely on homologous recombination, however, are inefficient, labor-intensive, and time-consuming due to the oligoploidy or polyploidy nature of cyanobacteria and the reliance on unique antibiotic resistance markers. CRISPR-Cas9 has emerged as an effective and versatile editing platform in a wide variety of organisms, but its application for cyanobacterial engineering is limited by the inherent toxicity of Cas9 resulting in poor transformation efficiencies. Here, we demonstrated that a single-plasmid CRISPR-Cas9 system, pCRISPOmyces-2, can effectively knock-in a truncated thioesterase gene from Escherichia coli to generate free fatty acid (FFA) producing mutants of Syn2973. To do so, three parameters were evaluated on the effect of generating recipient colonies after conjugation with pCRISPOmyces-2-based plasmids: 1) a modified conjugation protocol termed streaked conjugation, 2) the deletion of the gene encoding RecJ exonuclease, and 3) single guide RNA (sgRNA) sequence. With the use of the streaked conjugation protocol and a ΔrecJ mutant strain of Syn2973, the conjugation efficiency for the pCRISPomyces-2 plasmid could be improved by 750-fold over the wildtype (WT) for a conjugation efficiency of 2.0 × 10^-6 transconjugants/recipient cell. While deletion of the RecJ exonuclease alone increased the conjugation efficiency by 150-fold over the WT, FFA generation was impaired in FFA-producing mutants with the ΔrecJ background, and the large number of poor FFA-producing isolates indicated the potential increase in spontaneous mutation rates. The sgRNA sequence was found to be critical in achieving the desired CRISPR-Cas9-mediated knock-in mutation as the sgRNA impacts conjugation efficiency, likelihood of homogenous recombinants, and free fatty acid production in engineered strains.

Streptomyces: host for refactoring of diverse bioactive secondary metabolites

Microbial secondary metabolites are intensively explored due to their demands in pharmaceutical, agricultural and food industries. Streptomyces are one of the largest sources of secondary metabolites having diverse applications. In particular, the abundance of secondary metabolites encoding biosynthetic gene clusters and presence of wobble position in Streptomyces strains make it potential candidate as a native or heterologous host for secondary metabolite production including several cryptic gene clusters expression. Here, we have discussed the developments in Streptomyces strains genome mining, its exploration as a suitable host and application of synthetic biology for refactoring genetic systems for developing chassis for enhanced as well as novel secondary metabolites with reduced genome and cleaned background.

Exploiting heterologous and endogenous CRISPR-Cas systems for genome editing in the probiotic Clostridium butyricum

Clostridium butyricum has been widely used as a probiotic for humans and food animals. However, the mechanisms of beneficial effects of C. butyricum on the host remain poorly understood, largely due to the lack of high-throughput genome engineering tools. Here, we report the exploitation of heterologous Type II CRISPR-Cas9 system and endogenous Type I-B CRISPR-Cas system in probiotic C. butyricum for seamless genome engineering. Although successful genome editing was achieved in C. butyricum when CRISPR-Cas9 system was employed, the expression of toxic cas9 gene result in really poor transformation, spurring us to develop an easy-applicable and high-efficient genome editing tool. Therefore, the endogenous Type I-B CRISPR-Cas machinery located on the megaplasmid of C. butyricum was co-opted for genome editing. In vivo plasmid interference assays identified that ACA and TAA were functional protospacer adjacent motif sequences needed for site-specific CRISPR attacking. Using the customized endogenous CRISPR-Cas system, we successfully deleted spo0A and aldh genes in C. butyricum, yielding an efficiency of up to 100%. Moreover, the conjugation efficiency of endogenous CRISPR-Cas system was dramatically enhanced due to the precluding expression of cas9. Altogether, the two approaches developed herein remarkably expand the existing genetic toolbox available for investigation of C. butyricum.

The History of Colistin Resistance Mechanisms in Bacteria: Progress and Challenges

Since 2015, the discovery of colistin resistance genes has been limited to the characterization of new mobile colistin resistance (mcr) gene variants. However, given the complexity of the mechanisms involved, there are many colistin-resistant bacterial strains whose mechanism remains unknown and whose exploitation requires complementary technologies. In this review, through the history of colistin, we underline the methods used over the last decades, both old and recent, to facilitate the discovery of the main colistin resistance mechanisms and how new technological approaches may help to improve the rapid and efficient exploration of new target genes. To accomplish this, a systematic search was carried out via PubMed and Google Scholar on published data concerning polymyxin resistance from 1950 to 2020 using terms most related to colistin. This review first explores the history of the discovery of the mechanisms of action and resistance to colistin, based on the technologies deployed. Then we focus on the most advanced technologies used, such as MALDI-TOF-MS, high throughput sequencing or the genetic toolbox. Finally, we outline promising new approaches, such as omics tools and CRISPR-Cas9, as well as the challenges they face. Much has been achieved since the discovery of polymyxins, through several innovative technologies. Nevertheless, colistin resistance mechanisms remains very complex.

CRISPR-Cas9: A Powerful Tool to Efficiently Engineer Saccharomyces cerevisiae

Saccharomyces cerevisiae has been for a long time a common model for fundamental biological studies and a popular biotechnological engineering platform to produce chemicals, fuels, and pharmaceuticals due to its peculiar characteristics. Both lines of research require an effective editing of the native genetic elements or the inclusion of heterologous pathways into the yeast genome. Although S. cerevisiae is a well-known host with several molecular biology tools available, a more precise tool is still needed. The clustered, regularly interspaced, short palindromic repeats-associated Cas9 (CRISPR-Cas9) system is a current, widespread genome editing tool. The implementation of a reprogrammable, precise, and specific method, such as CRISPR-Cas9, to edit the S. cerevisiae genome has revolutionized laboratory practices. Herein, we describe and discuss some applications of the CRISPR-Cas9 system in S. cerevisiae from simple gene knockouts to more complex processes such as artificial heterologous pathway integration, transcriptional regulation, or tolerance engineering.

Mercury cycling in freshwater systems - An updated conceptual model

The widely accepted conceptual model of mercury (Hg) cycling in freshwater lakes (atmospheric deposition and runoff of inorganic Hg, methylation in bottom sediments and subsequent bioaccumulation and biomagnification in biota) is practically accepted as common knowledge. There is mounting evidence that the dominant processes that regulate inputs, transformations, and bioavailability of Hg in many lakes may be missing from this picture, and the fixation on the temperate stratified lake archetype is impeding our exploration of understudied, but potentially important sources of methylmercury to freshwater lakes. In this review, the importance of understudied biogeochemical processes and sites of methylmercury production are highlighted, including the complexity of redox transformations of Hg within the lake system itself, the complex assemblage of microbes found in biofilms and periphyton (two vastly understudied important sources of methylmercury in many freshwater ecosystems), and the critical role of autochthonous and allochthonous dissolved organic matter which mediates the net supply of methylmercury from the cellular to catchment scale. A conceptual model of lake Hg in contrasting lakes and catchments is presented, highlighting the importance of the autochthonous and allochthonous supply of dissolved organic matter, bioavailable inorganic mercury and methylmercury and providing a framework for future convergent research at the lab and field scales to establish more mechanistic process-based relationships within and among critical compartments that regulate methylmercury concentrations in freshwater ecosystems.

CRISPR-Cas9 Genome Editing Tool for the Production of Industrial Biopharmaceuticals

A broad range of cell lines with characteristic features are used as bio-factories to produce recombinant proteins for basic research and therapeutic purposes. Genetic engineering strategies have been used to manipulate the genome of mammalian cells, insects, and yeasts for heterologous expression. One reason is that the glycosylation pattern of the expression hosts differs somehow from mammalian cells, which may cause immunogenic reactions upon administration in humans. CRISPR-Cas9 is a simple, efficient, and versatile genome engineering tool that can be programmed to precisely make double-stranded breaks at the desired loci. Compared to the classical genome editing methods, a CRISPR-Cas9 system is an ideal tool, providing the opportunity to integrate or delete genes from the target organisms. Besides broadened applications, limited studies have used CRISPR-Cas9 for editing the endogenous pathways in expression systems for biopharmaceutical applications. In the present review, we discuss the use of CRISPR-Cas9 in expression systems to improve host cell lines, increase product yield, and humanize glycosylation pathways by targeting intrinsic genes.

Francisella novicida CRISPR-Cas Systems Can Functionally Complement Each Other in DNA Defense while Providing Target Flexibility

CRISPR-Cas systems are prokaryotic adaptive immune systems that facilitate protection of bacteria and archaea against infection by external mobile genetic elements. The model pathogen Francisella novicida encodes a CRISPR-Cas12a (FnoCas12a) system and a CRISPR-Cas9 (FnoCas9) system, the latter of which has an additional and noncanonical function in bacterial virulence. Here, we investigated and compared the functional roles of the FnoCas12a and FnoCas9 systems in transformation inhibition and bacterial virulence. Unlike FnoCas9, FnoCas12a was not required for F. novicida virulence. However, both systems were highly effective at plasmid restriction and acted independently of each other. We further identified a critical protospacer-adjacent motif (PAM) necessary for transformation inhibition by FnoCas12a, demonstrating a greater flexibility for target identification by FnoCas12a than previously appreciated and a specificity that is distinct from that of FnoCas9. The effectors of the two systems exhibited different patterns of expression at the mRNA level, suggesting that they may confer distinct benefits to the bacterium in diverse environments. These data suggest that due to the differences between the two CRISPR-Cas systems, together they may provide F. novicida with a more comprehensive defense against foreign nucleic acids. Finally, we demonstrated that the FnoCas12a and FnoCas9 machineries could be simultaneously engineered to restrict the same nonnative target, thereby expanding the toolset for prokaryotic genome manipulation.IMPORTANCE CRISPR-Cas9 and CRISPR-Cas12a systems have been widely commandeered for genome engineering. However, they originate in prokaryotes, where they function as adaptive immune systems. The details of this activity and relationship between these systems within native host organisms have been minimally explored. The human pathogen Francisella novicida contains both of these systems, with the Cas9 system also exhibiting a second activity, modulating virulence through transcriptional regulation. We compared and contrasted the ability of these two systems to control virulence and restrict DNA within their native host bacterium, highlighting differences and similarities in these two functions. Collectively, our results indicate that these two distinct and reprogrammable endogenous systems provide F. novicida with a more comprehensive defense against mobile genetic elements.

Targeting of cholera toxin A (ctxA) gene by zinc finger nuclease: pitfalls of using gene editing tools in prokaryotes

Background and purpose:

The study was launched to use zinc finger nuclease (ZFN) technology to disrupt the cholera toxin gene (ctxA) for inhibiting CT toxin production in Vibrio cholera (V. cholera).

Experimental approach:

An engineered ZFN was designed to target the catalytic site of the ctxA gene. The coding sequence of ZFN was cloned to pKD46, pTZ57R T/A vector, and E2-crimson plasmid and transformed to Escherichia coli (E. coli) Top10 and V. cholera. The efficiency of ZFN was evaluated by colony counting.

Findings/Results:

No expression was observed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis and western blotting in transformed E. coli. The ctxA gene sequencing did not show any mutation. Polymerase chain reaction on pKD46-ZFN plasmid had negative results. Transformation of E. coli Top10 with T/A vectors containing whole ZFN sequence led to 7 colonies all of which contained bacteria with self-ligated vector. Transformation with left array ZFN led to 24 colonies of which 6 contained bacteria with self-ligated vector and 18 of them contained bacteria with vector/left array. Transformation of V. cholera with E2-crimson vectors containing whole ZFN did not produce any colonies. Transformation with left array vectors led to 17 colonies containing bacteria with vector/left array. Left array protein band was captured using western blot assay.

Conclusions and implications:

ZFN might have off target on bacterial genome causing lethal double-strand DNA break due to lack of non-homologous end joining (NHEJ) mechanism. It is recommended to develop ZFNs against bacterial genes, engineered packaging host with NHEJ repair system is essential.

Efficient Multiplex Gene Repression by CRISPR-dCpf1 in Corynebacterium glutamicum

Corynebacterium glutamicum is an important workhorse for industrial production of diversiform bioproducts. Multiplex control of metabolic pathway genes is crucial for maximizing biosynthesis of desired products. However, few tools for simultaneously regulating multiple genes in C. glutamicum have been reported. Here, a CRISPR-dCpf1-based multiplex gene repression system was developed for C. glutamicum. This system successfully repressed two fluorescent reporter genes simultaneously by expressing a dCpf1 (E1006A, D917A) and a designed single crRNA array. To demonstrate applications of this CRISPR-dCpf1 system in metabolic engineering, we applied this system to repress four genes involved in lysine biosynthesis (gltA, pck, pgi, and hom) with a single array, which increased the lysine titer and yield for over 4.0-fold. Quantitative PCR demonstrated that transcription of all the four endogenous target genes were repressed by over 90%. Thus, the CRISPR-dCpf1 system is a simple and effective technique for multiplex gene repression in C. glutamicum and holds promise for metabolic engineering of C. glutamicum to produce valuable chemicals and fuels.

Reversal of mcr-1-Mediated Colistin Resistance in Escherichia coli by CRISPR-Cas9 System

Purpose

The plasmid-borne mobilized colistin resistance gene (mcr-1) was discovered in 2015. Subsequently, the rapid horizontal transfer of mcr-1 gene to diverse bacterial species poses a serious threat to public health, which urgently needs the introduction of novel antimicrobial strategies. Therefore, the purpose of this study is to sensitize bacteria to colistin and reduce the propagation of mcr-1 gene by curing mcr-1-harboring plasmid in Escherichia coli (E. coli) using the CRISPR-Cas9 system.

Methods

Two sgRNAs specific to mcr-1 gene were designed and cloned into plasmid pCas9. The recombinant plasmid pCas9-mcr was transformed into E. coli carrying pUC19-mcr-1 or pHNSHP45, separately. The elimination efficiency in strains was evaluated by PCR and quantitative real-time PCR (qPCR). The antimicrobial susceptibility test was performed using the broth microdilution method.

Results

In this study, we constructed the high copy number plasmid pUC19-mcr-1 and recombinant plasmid pCas9-m1 or pCas9-m2, which contain 20 nt or 30 nt sgRNA sequences targeted to mcr-1, respectively. PCR and qPCR results showed that mcr-1-harboring plasmids could be efficiently eliminated, and there was no significant correlation between sgRNA lengths and curing efficiency. However, when comparing restructured high copy number plasmid (pUC19-mcr-1) to natural resistance plasmid (pHNSHP45) in eliminating efficiency, we found that the content of plasmid backbone had an influence on efficiency. Furthermore, the conjugation assays verified that the engineered CRISPR-Cas9 system in bacteria or in bacteria genome can protect the recipient from plasmid-borne mcr-1 transfer via conjugation. Additionally, sequence analysis showed that three different types of defects in CRISPR-Cas9 system lead to escape mutants.

Conclusion

We presented a method that only one plasmid-mediated CRISPR-Cas9 system can be used to efficiently resensitize E. coli to colistin. Moreover, this system provided a great potentiality to counteract the propagation of mcr-1 among bacterial pathogens.

A matter of background: DNA repair pathways as a possible cause for the sparse distribution of CRISPR-Cas systems in bacteria

The absence of CRISPR-Cas systems in more than half of the sequenced bacterial genomes is intriguing, because their role in adaptive immunity and their frequent transfer between species should have made them almost ubiquitous, as is the case in Archaea. Here, we investigate the possibility that the success of CRISPR-Cas acquisition by horizontal gene transfer is affected by the interactions of these systems with the host genetic background and especially with components of double-strand break repair systems (DSB-RS). We first described the distribution of systems specialized in the repair of double-strand breaks in Bacteria: homologous recombination and non-homologous end joining. This allowed us to show that such systems are more often positively or negatively correlated with the frequency of CRISPR-Cas systems than random genes of similar frequency. The detailed analysis of these co-occurrence patterns shows that our method identifies previously known cases of mechanistic interactions between these systems. It also reveals other positive and negative patterns of co-occurrence between DSB-RS and CRISPR-Cas systems. Notably, it shows that the patterns of distribution of CRISPR-Cas systems in Proteobacteria are strongly dependent on the epistatic groups including RecBCD and AddAB. Our results suggest that the genetic background plays an important role in the success of adaptive immunity in different bacterial clades and provide insights to guide further experimental research on the interactions between CRISPR-Cas and DSB-RS. This article is part of a discussion meeting issue 'The ecology and evolution of prokaryotic CRISPR-Cas adaptive immune systems'.

Improved sgRNA design in bacteria via genome-wide activity profiling

CRISPR/Cas9 is a promising tool in prokaryotic genome engineering, but its success is limited by the widely varying on-target activity of single guide RNAs (sgRNAs). Based on the association of CRISPR/Cas9-induced DNA cleavage with cellular lethality, we systematically profiled sgRNA activity by co-expressing a genome-scale library (∼70 000 sgRNAs) with Cas9 or its specificity-improved mutant in Escherichia coli. Based on this large-scale dataset, we constructed a comprehensive and high-density sgRNA activity map, which enables selecting highly active sgRNAs for any locus across the genome in this model organism. We also identified ‘resistant’ genomic loci with respect to CRISPR/Cas9 activity, notwithstanding the highly accessible DNA in bacterial cells. Moreover, we found that previous sgRNA activity prediction models that were trained on mammalian cell datasets were inadequate when coping with our results, highlighting the key limitations and biases of previous models. We hence developed an integrated algorithm to accurately predict highly effective sgRNAs, aiming to facilitate CRISPR/Cas9-based genome engineering, screenings and antimicrobials design in bacteria. We also isolated the important sgRNA features that contribute to DNA cleavage and characterized their key differences among wild type Cas9 and its mutant, shedding light on the biophysical mechanisms of the CRISPR/Cas9 system.

The Continuing Threat of Methicillin-Resistant Staphylococcus aureus

Staphylococcus aureus has been an exceptionally successful pathogen, which is still relevant in modern age-medicine due to its adaptability and tenacity. This bacterium may be a causative agent in a plethora of infections, owing to its abundance (in the environment and in the normal flora) and the variety of virulence factors that it possesses. Methicillin-resistant S. aureus (MRSA) strains—first described in 1961—are characterized by an altered penicillin-binding protein (PBP2a/c) and resistance to all penicillins, cephalosporins, and carbapenems, which makes the β-lactam armamentarium clinically ineffective. The acquisition of additional resistance determinants further complicates their eradication; therefore, MRSA can be considered as the first representative of multidrug-resistant bacteria. Based on 230 references, the aim of this review is to recap the history, the emergence, and clinical features of various MRSA infections (hospital-, community-, and livestock-associated), and to summarize the current advances regarding MRSA screening, typing, and therapeutic options (including lipoglycopeptides, oxazolidinones, anti-MRSA cephalosporins, novel pleuromutilin-, tetracycline- and quinolone-derivatives, daptomycin, fusidic acid, in addition to drug candidates in the development phase), both for an audience of clinical microbiologists and infectious disease specialists.

CRISPR tracking reveals global spreading of antimicrobial resistance genes by Staphylococcus of canine origin

The close contact between pets and their owners is a potential source for microorganisms and genetic material exchange. Staphylococcus species considered as harmless inhabitants of animals' and humans' microbiota can act as reservoirs of antimicrobial resistance genes to more virulent species, thereby increasing their potential to resist drug therapy. This process could be inhibited by the antiplasmid immunity conferred by CRISPR systems. On the other hand, CRISPR spacer sequences can be explored as molecular clocks to track the history of genetic invasion suffered by a bacterial strain. To understand better the role of domestic dogs in human health as an antimicrobial resistance genes source, we analyzed 129 genomes of Staphylococcus strains of canine origin for the presence of CRISPR systems. Only 8% of the strains were positive for CRISPR, which is consistent with Staphylococcus role as gene reservoirs. The plasmidial origin or some spacers confirms the unsuccessful attempt of plasmid exchange in strains carrying CRISPRs. Some of these systems are within a staphylococcal cassette chromosome mec (SCCmec), sharing 98% of identity between their harboring strains. These CRISPRs' spacers reveal that this SCCmec was transferred between canine S. pseudintermedius strains, then to S. schleiferi and to Staphylococcus strains isolated from human beings. Our findings shows genetic evidence for the global spreading of pathogenic bacteria and the antimicrobial resistance genes carried by them and reinforce that, in the age of antimicrobial resistance, it is imperative that drug therapies consider the integrated nature of the relationship between pets and humans.

Nonconventional Therapeutics against Staphylococcus aureus

Staphylococcus aureus is one of the most important human pathogens that is responsible for a variety of diseases ranging from skin and soft tissue infections to endocarditis and sepsis. In recent decades, the treatment of staphylococcal infections has become increasingly difficult as the prevalence of multi-drug resistant strains continues to rise. With increasing mortality rates and medical costs associated with drug resistant strains, there is an urgent need for alternative therapeutic options. Many innovative strategies for alternative drug development are being pursued, including disruption of biofilms, inhibition of virulence factor production, bacteriophage-derived antimicrobials, anti-staphylococcal vaccines, and light-based therapies. While many compounds and methods still need further study to determine their feasibility, some are quickly approaching clinical application and may be available in the near future.

Incorporation of a Synthetic Amino Acid into dCas9 Improves Control of Gene Silencing

The CRISPR-Cas9 nuclease has been repurposed as a tool for gene repression (CRISPRi). This catalytically dead Cas9 (dCas9) variant inhibits transcription by blocking either initiation or elongation by the RNA polymerase complex. Conditional control of dCas9-mediated repression has been achieved with inducible promoters that regulate the expression of the dcas9 gene. However, as dCas9-mediated gene silencing is very efficient, even slightly leaky dcas9 expression leads to significant background levels of repression of the target gene. In this study, we report on the development of optimized control of dCas9-mediated silencing through additional regulation at the translation level. We have introduced the TAG stop codon in the dcas9 gene in order to insert a synthetic amino acid, l-biphenylalanine (BipA), at a permissive site in the dCas9 protein. In the absence of BipA, a nonfunctional, truncated dCas9 is produced, but when BipA is present, the TAG codon is translated resulting in a functional, full-length dCas9 protein. This synthetic, BipA-containing dCas9 variant (dCas9-BipA) could still fully repress gene transcription. Comparison of silencing mediated by dCas9 to dCas9-BipA revealed a 14-fold reduction in background repression by the latter system. The here developed proof-of-principle system thus reduces unwanted background levels of gene silencing, allowing for tight and timed control of target gene expression.

Reversal of carbapenem-resistance in Shewanella algae by CRISPR/Cas9 genome editing

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Emergence of carbapenem-resistant S. algae is a severe problem.

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Re-sensitization of S. algae to carbapenem by CRISPR/Cas9 genome editing.

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The bla_OXA-55-like gene is essential for carbapenem resistance in S. algae.

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One-plasmid genome editing system for CRISPR/Cas9 genome editing in S. algae.

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CRISPR/Cas9 genome editing is a promising approach to validate the gene function.

Antibiotic resistance in pathogens is a growing threat to human health. Of particular concern is resistance to carbapenem, which is an antimicrobial agent listed as critically important by the World Health Organization. With the global spread of carbapenem-resistant organisms, there is an urgent need for new treatment options. Shewanella algae is an emerging pathogen found in marine environments throughout the world that has increasing resistance to carbapenem. The organism is also a possible antibiotic resistance reservoir in humans and in its natural habitat. The development of CRISPR/Cas9-based methods has enabled precise genetic manipulation. A number of attempts have been made to knock out resistance genes in various organisms. The study used a single plasmid containing CRISPR/Cas9 and recE/recT recombinase to reverse an antibiotic-resistant phenotype in S. algae and showed bla_OXA-55-like gene is essential for the carbapenem resistance. This result demonstrates a potential validation strategy for functional genome annotation in S. algae.

Recent advances in the treatment of pathogenic infections using antibiotics and nano-drug delivery vehicles

The worldwide misuse of antibiotics and the subsequent rise of multidrug-resistant pathogenic bacteria have prompted a paradigm shift in the established view of antibiotic and bacterial-human relations. The clinical failures of conventional antibiotic therapies are associated with lengthy detection methods, poor penetration at infection sites, disruption of indigenous microflora and high potential for mutational resistance. One of the most promising strategies to improve the efficacy of antibiotics is to complex them with micro or nano delivery materials. Such materials/vehicles can shield antibiotics from enzyme deactivation, increasing the therapeutic effectiveness of the drug. Alternatively, drug-free nanomaterials that do not kill the pathogen but target virulent factors such as adhesins, toxins, or secretory systems can be used to minimize resistance and infection severity. The main objective of this review is to examine the potential of the aforementioned materials in the detection and treatment of antibiotic-resistant pathogenic organisms.

Investigation of direct repeats, spacers and proteins associated with clustered regularly interspaced short palindromic repeat (CRISPR) system of Vibrio parahaemolyticus

Vibrio parahaemolyticus, a ubiquitous bacterium of the marine environment is an important food-borne pathogen responsible for gastroenteritis worldwide. In this study, we aimed to investigate the occurrence and diversity of the CRISPR-Cas system in V. parahaemolyticus genomes using a bioinformatics approach. The CRISPR-Cas system functions as an adaptive immune system in prokaryotes that provides immunity against foreign genetic elements. In total, 570 genomes V. parahaemolyticus genomes were analyzed of which 200 confirmed for the presence of CRISPR-Cas system. The CRISPR-Cas loci were further analyzed for their repeats, spacers and associated Cas proteins. Among the 200 V. parahaemolyticus strains analyzed, 16 (8%) strains possessed the CRISPR-Cas system of complete subtype I-F, while the remaining 184 (92%) harbored the minimalistic type, a subtype I-F variant. Orphan CRISPR repeats and Cas genes were found in one strain each. The CRISPR-associated direct repeat had an unit length of 28 bases. The number of repeat units in each array ranged from 3 to 5 or 5-41 depending on whether they belonged to the minimalistic or complete subtype-IF CRISPR-Cas system, respectively. Of the 768 spacers analyzed in this study, 295 were found to be unique to V. parahaemolyticus. Homology analysis of the conserved spacers revealed matches to plasmids, phages and gut viruses and self chromosomes. Among the CRISPR-associated proteins, Cas5 and Cas7 proteins were found to be conserved. However, variations were seen in the Cas6 protein, which could be grouped into four different types based on their protein length as well as amino acid composition. We present here the diversity and main features of the CRISPR-Cas system in V. parahaemolyticus, which could provide valuable insights in elucidating the role and mechanism of CRISPR/Cas elements in this pathogen.

CRISPR-Cpf1-Assisted Multiplex Genome Editing and Transcriptional Repression in Streptomyces

Streptomyces has a strong capability for producing a large number of bioactive natural products and remains invaluable as a source for the discovery of novel drug leads. Although the Streptococcus pyogenes CRISPR-Cas9-assisted genome editing tool has been developed for rapid genetic engineering in Streptomyces, it has a number of limitations, including the toxicity of SpCas9 expression in some important industrial Streptomyces strains and the need for complex expression constructs when targeting multiple genomic loci. To address these problems, in this study, we developed a high-efficiency CRISPR-Cpf1 system (from Francisella novicida) for multiplex genome editing and transcriptional repression in Streptomyces Using an all-in-one editing plasmid with homology-directed repair (HDR), our CRISPR-Cpf1 system precisely deletes single or double genes at efficiencies of 75 to 95% in Streptomyces coelicolor When no templates for HDR are present, random-sized DNA deletions are achieved by FnCpf1-induced double-strand break (DSB) repair by a reconstituted nonhomologous end joining (NHEJ) pathway. Furthermore, a DNase-deactivated Cpf1 (ddCpf1)-based integrative CRISPRi system is developed for robust, multiplex gene repression using a single customized crRNA array. Finally, we demonstrate that FnCpf1 and SpCas9 exhibit different suitability in tested industrial Streptomyces species and show that FnCpf1 can efficiently promote HDR-mediated gene deletion in the 5-oxomilbemycin-producing strain Streptomyces hygroscopicus SIPI-KF, in which SpCas9 does not work well. Collectively, FnCpf1 is a powerful and indispensable addition to the Streptomyces CRISPR toolbox.IMPORTANCE Rapid, efficient genetic engineering of Streptomyces strains is critical for genome mining of novel natural products (NPs) as well as strain improvement. Here, a novel and high-efficiency Streptomyces genome editing tool is established based on the FnCRISPR-Cpf1 system, which is an attractive and powerful alternative to the S. pyogenes CRISPR-Cas9 system due to its unique features. When combined with HDR or NHEJ, FnCpf1 enables the creation of gene(s) deletion with high efficiency. Furthermore, a ddCpf1-based integrative CRISPRi platform is established for simple, multiplex transcriptional repression. Of importance, FnCpf1-based genome editing proves to be a highly efficient tool for genetic modification of some important industrial Streptomyces strains (e.g., S. hygroscopicus SIPI-KF) that cannot utilize the SpCRISPR-Cas9 system. We expect the CRISPR-Cpf1-assisted genome editing tool to accelerate discovery and development of pharmaceutically active NPs in Streptomyces as well as other actinomycetes.

CRISPR-Cpf1-Assisted Multiplex Genome Editing and Transcriptional Repression in Streptomyces

Streptomyces has a strong capability for producing a large number of bioactive natural products and remains invaluable as a source for the discovery of novel drug leads. Although the Streptococcus pyogenes CRISPR-Cas9-assisted genome editing tool has been developed for rapid genetic engineering in Streptomyces, it has a number of limitations, including the toxicity of SpCas9 expression in some important industrial Streptomyces strains and the need for complex expression constructs when targeting multiple genomic loci. To address these problems, in this study, we developed a high-efficiency CRISPR-Cpf1 system (from Francisella novicida) for multiplex genome editing and transcriptional repression in Streptomyces Using an all-in-one editing plasmid with homology-directed repair (HDR), our CRISPR-Cpf1 system precisely deletes single or double genes at efficiencies of 75 to 95% in Streptomyces coelicolor When no templates for HDR are present, random-sized DNA deletions are achieved by FnCpf1-induced double-strand break (DSB) repair by a reconstituted nonhomologous end joining (NHEJ) pathway. Furthermore, a DNase-deactivated Cpf1 (ddCpf1)-based integrative CRISPRi system is developed for robust, multiplex gene repression using a single customized crRNA array. Finally, we demonstrate that FnCpf1 and SpCas9 exhibit different suitability in tested industrial Streptomyces species and show that FnCpf1 can efficiently promote HDR-mediated gene deletion in the 5-oxomilbemycin-producing strain Streptomyces hygroscopicus SIPI-KF, in which SpCas9 does not work well. Collectively, FnCpf1 is a powerful and indispensable addition to the Streptomyces CRISPR toolbox.IMPORTANCE Rapid, efficient genetic engineering of Streptomyces strains is critical for genome mining of novel natural products (NPs) as well as strain improvement. Here, a novel and high-efficiency Streptomyces genome editing tool is established based on the FnCRISPR-Cpf1 system, which is an attractive and powerful alternative to the S. pyogenes CRISPR-Cas9 system due to its unique features. When combined with HDR or NHEJ, FnCpf1 enables the creation of gene(s) deletion with high efficiency. Furthermore, a ddCpf1-based integrative CRISPRi platform is established for simple, multiplex transcriptional repression. Of importance, FnCpf1-based genome editing proves to be a highly efficient tool for genetic modification of some important industrial Streptomyces strains (e.g., S. hygroscopicus SIPI-KF) that cannot utilize the SpCRISPR-Cas9 system. We expect the CRISPR-Cpf1-assisted genome editing tool to accelerate discovery and development of pharmaceutically active NPs in Streptomyces as well as other actinomycetes.

CRISPR-Cas System: History and Prospects as a Genome Editing Tool in Microorganisms

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR or more precisely CRISPR-Cas) system has proven to be a highly efficient and simple tool for achieving site-specific genome modifications in comparison to Zinc Finger Nucleases (ZFNs) and Transcription Activator-Like Effector Nucleases (TALENs). The discovery of bacterial defense system that uses RNA-guided DNA cleaving enzymes for producing double-strand breaks along CRISPR has provided an exciting alternative to ZFNs and TALENs for gene editing & regulation, as the CRISPR-associated (Cas) proteins remain the same for different gene targets and only the short sequence of the guide RNA needs to be changed to redirect the site-specific cleavage. Therefore, in recent years the CRISPR-Cas system has emerged as a revolutionary engineering tool for carrying out precise and controlled genetic modifications in many microbes such as Escherichia coli, Staphylococcus aureus, Lactobacillus reuteri, Clostridium beijerinckii, Streptococcus pneumonia, and Saccharomyces cerevisiae. Though, concerns about CRISPR-Cas effectiveness in interlinked gene modifications and off-target effects need to be addressed. Nevertheless, it holds a great potential to speed up the pace of gene function discovery by interacting with previously intractable organisms and by raising the extent of genetic screens. Therefore, the potential applications of this system in microbial adaptive immune system, genome editing, gene regulations, functional genomics & biosynthesis along ethical issues, and possible harmful effects have been reviewed.

CRISPR/Cas9 Assisted Multiplex Genome Editing Technique in Escherichia coli

Genome editing for site-specific chromosome modification is one of the most significant techniques in biological research. While conventional techniques usually deal with one genomic locus at a time, multiple genomic targets are often required to be modified to develop microbial cell factories. Thus, it is necessary to develop techniques for simultaneous editing of multiple loci. In this work, the authors develop a CRISPR/Cas9 assisted multiplex genome editing (CMGE) technique in Escherichia coli. With this editing method, all functional parts are assembled into replicable plasmids, and stringent inducible expression systems are used to control Cas9 gene expression, which is to decouple transformation from editing process to increase editing efficiency. A modular assembly strategy is designed to enable construction of the complex multi-gRNA plasmid. With this technique, two and three loci are able to be modified with 100% and 88.3% efficiencies, while four loci can be edited with more than 30%, which are the best results reported. Although developed in model organism, the strategy of CMGE can be adapted to other prokaryotic cells. This is a well designed and illustrated technique with no special requirement, can be used by any biological lab easily.

High GC Content Cas9-Mediated Genome-Editing and Biosynthetic Gene Cluster Activation in Saccharopolyspora erythraea

The overexpression of bacterial secondary metabolite biosynthetic enzymes is the basis for industrial overproducing strains. Genome editing tools can be used to further improve gene expression and yield. Saccharopolyspora erythraea produces erythromycin, which has extensive clinical applications. In this study, the CRISPR-Cas9 system was used to edit genes in the S. erythraea genome. A temperature-sensitive plasmid containing the PermE promoter, to drive Cas9 expression, and the Pj23119 and PkasO promoters, to drive sgRNAs, was designed. Erythromycin esterase, encoded by S. erythraea SACE_1765, inactivates erythromycin by hydrolyzing the macrolactone ring. Sequencing and qRT-PCR confirmed that reporter genes were successfully inserted into the SACE_1765 gene. Deletion of SACE_1765 in a high-producing strain resulted in a 12.7% increase in erythromycin levels. Subsequent PermE- egfp knock-in at the SACE_0712 locus resulted in an 80.3% increase in erythromycin production compared with that of wild type. Further investigation showed that PermE promoter knock-in activated the erythromycin biosynthetic gene clusters at the SACE_0712 locus. Additionally, deletion of indA (SACE_1229) using dual sgRNA targeting without markers increased the editing efficiency to 65%. In summary, we have successfully applied Cas9-based genome editing to a bacterial strain, S. erythraea, with a high GC content. This system has potential application for both genome-editing and biosynthetic gene cluster activation in Actinobacteria.

Mathematical Modeling of RNA-Based Architectures for Closed Loop Control of Gene Expression

Feedback allows biological systems to control gene expression precisely and reliably, even in the presence of uncertainty, by sensing and processing environmental changes. Taking inspiration from natural architectures, synthetic biologists have engineered feedback loops to tune the dynamics and improve the robustness and predictability of gene expression. However, experimental implementations of biomolecular control systems are still far from satisfying performance specifications typically achieved by electrical or mechanical control systems. To address this gap, we present mathematical models of biomolecular controllers that enable reference tracking, disturbance rejection, and tuning of the temporal response of gene expression. These controllers employ RNA transcriptional regulators to achieve closed loop control where feedback is introduced via molecular sequestration. Sensitivity analysis of the models allows us to identify which parameters influence the transient and steady state response of a target gene expression process, as well as which biologically plausible parameter values enable perfect reference tracking. We quantify performance using typical control theory metrics to characterize response properties and provide clear selection guidelines for practical applications. Our results indicate that RNA regulators are well-suited for building robust and precise feedback controllers for gene expression. Additionally, our approach illustrates several quantitative methods useful for assessing the performance of biomolecular feedback control systems.

Silk Route to the Acceptance and Re-Implementation of Bacteriophage Therapy-Part II

This perspective paper follows up on earlier communications on bacteriophage therapy that we wrote as a multidisciplinary and intercontinental expert-panel when we first met at a bacteriophage conference hosted by the Eliava Institute in Tbilisi, Georgia in 2015. In the context of a society that is confronted with an ever-increasing number of antibiotic-resistant bacteria, we build on the previously made recommendations and specifically address how the Nagoya Protocol might impact the further development of bacteriophage therapy. By reviewing a number of recently conducted case studies with bacteriophages involving patients with bacterial infections that could no longer be successfully treated by regular antibiotic therapy, we again stress the urgency and significance of the development of international guidelines and frameworks that might facilitate the legal and effective application of bacteriophage therapy by physicians and the receiving patients. Additionally, we list and comment on several recently started and ongoing clinical studies, including highly desired double-blind placebo-controlled randomized clinical trials. We conclude with an outlook on how recently developed DNA editing technologies are expected to further control and enhance the efficient application of bacteriophages.

Emerging Nanomedicine Therapies to Counter the Rise of Methicillin-Resistant Staphylococcus aureus

In a recent report, the World Health Organisation (WHO) classified antibiotic resistance as one of the greatest threats to global health, food security, and development. Methicillin-resistant Staphylococcus aureus (MRSA) remains at the core of this threat, with persistent and resilient strains detectable in up to 90% of S. aureus infections. Unfortunately, there is a lack of novel antibiotics reaching the clinic to address the significant morbidity and mortality that MRSA is responsible for. Recently, nanomedicine strategies have emerged as a promising therapy to combat the rise of MRSA. However, these approaches have been wide-ranging in design, with few attempts to compare studies across scientific and clinical disciplines. This review seeks to reconcile this discrepancy in the literature, with specific focus on the mechanisms of MRSA infection and how they can be exploited by bioactive molecules that are delivered by nanomedicines, in addition to utilisation of the nanomaterials themselves as antibacterial agents. Finally, we discuss targeting MRSA biofilms using nano-patterning technologies and comment on future opportunities and challenges for MRSA treatment using nanomedicine.