CRISPR/Cas9-based efficient genome editing in Staphylococcus aureus

The CRISPR/Cas system as an antimicrobial resistance strategy in aquatic ecosystems

With the growing population, demand for food has dramatically increased, and fisheries, including aquaculture, are expected to play an essential role in sustaining demand with adequate quantities of protein and essential vitamin supplements, employment generation, and GDP growth. Unfortunately, the incidence of emerging/re-emerging AMR pathogens annually occurs because of anthropogenic activities and the frequent use of antibiotics in aquaculture. These AMR pathogens include the WHO's top 6 prioritized ESKAPE pathogens (nosocomial pathogens: Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.), extended-spectrum beta lactases (ESBLs) and carbapenemase-producing E. coli, which pose major challenges to the biomagnification of both nonnative and native antibiotic-resistant bacteria in capture and cultured fishes. Although implementing the rational use of antibiotics represents a promising mitigation measure, this approach is practically impossible due to the lack of awareness among farmers about the interplay between antimicrobial use and the emergence of antimicrobial resistance (AMR). Nevertheless, to eradicate these 'superbugs,' CRISPR/Cas (clustered regularly interspersed short palindromic repeats/CRISPR associate protein) has turned out to be a novel approach owing to its ability to perform precise site-directed targeting/knockdown/reversal of specific antimicrobial resistance genes in vitro and to distinguish AMR-resistant bacteria from a plethora of commensal aquatic bacteria. Along with highlighting the importance of virulent multidrug resistance genes in bacteria, this article aims to provide a holistic picture of CRISPR/Cas9-mediated genome editing for combating antimicrobial-resistant bacteria isolated from various aquaculture and marine systems, as well as insights into different types of CRISPR/Cas systems, delivery methods, and challenges associated with developing CRISPR/Cas9 antimicrobial agents.

Efficient Genome Editing in Most Staphylococcus aureus by Using the Restriction-Modification System Silent CRISPR-Cas9 Toolkit

Staphylococcus aureus is a clinically important pathogen that threatens human health due to its strong pathogenicity and drug resistance, leading to meningitis, endocarditis, and skin and soft tissue infections. Genetic manipulation in S. aureus is a powerful approach for characterizing the molecular mechanisms of bacterial drug resistance, pathogenicity, and virulence. However, a strong restriction barrier presents a major obstacle to the extensive utilization of genetic manipulation tools in clinical isolates of S. aureus. Here, we constructed a restriction-modification (RM) system silent CRISPR-Cas9 toolkit that synonymously eliminated the type I RM targets of S. aureus from plasmids, downsized plasmids using minicircle technology, and combined with a plasmid artificial modification (PAM) method to circumvent the type II RM system. The RM-silent CRISPR-Cas9 toolkit enables a significant improvement in transformation (105-106 transformants per microgram plasmid in strains we tested) and high-success efficiency editing for gene deletion (knockout strain obtained in one-round electroporation) in a wide range of S. aureus species including clinical isolates of unknown genetic background. The RM-silent CRISPR-Cas9 toolkits could expedite the process of mutant construction in most S. aureus strains, and this approach could be applied to the design of other genetic toolkit plasmids for utilization in a wider range of S. aureus strains.

CRISPR-Based Gene Editing in Acinetobacter baumannii to Combat Antimicrobial Resistance

Antimicrobial resistance (AMR) poses a significant threat to the health, social, environment, and economic sectors on a global scale and requires serious attention to addressing this issue. Acinetobacter baumannii was given top priority among infectious bacteria because of its extensive resistance to nearly all antibiotic classes and treatment options. Carbapenem-resistant A. baumannii is classified as one of the critical-priority pathogens on the World Health Organization (WHO) priority list of antibiotic-resistant bacteria for effective drug development. Although available genetic manipulation approaches are successful in A. baumannii laboratory strains, they are limited when employed on newly acquired clinical strains since such strains have higher levels of AMR than those used to select them for genetic manipulation. Recently, the CRISPR-Cas (Clustered regularly interspaced short palindromic repeats/CRISPR-associated protein) system has emerged as one of the most effective, efficient, and precise methods of genome editing and offers target-specific gene editing of AMR genes in a specific bacterial strain. CRISPR-based genome editing has been successfully applied in various bacterial strains to combat AMR; however, this strategy has not yet been extensively explored in A. baumannii. This review provides detailed insight into the progress, current scenario, and future potential of CRISPR-Cas usage for AMR-related gene manipulation in A. baumannii.

CRISPR-Cas9 mediated phage therapy as an alternative to antibiotics

Inappropriate use of antibiotics is globally creating public health hazards associated with antibiotic resistance. Bacteria often acquire antibiotic resistance by altering their genes through mutation or acquisition of plasmid-encoding resistance genes. To treat drug-resistant strains of bacteria, the recently developed CRISPR-Cas9 system might be an alternative molecular tool to conventional antibiotics. It disables antibiotic-resistance genes (plasmids) or deactivates bacterial virulence factors and sensitizes drug-resistant bacteria through site-specific cleavage of crucial domains of their genome. This molecular tool uses phages as vehicles for CRISPR-cas9 delivery into bacteria. Since phages are species-specific and natural predators of bacteria, they are capable of easily injecting their DNA to target bacteria. The CRISPR system is packaged into phagemid vectors, in such a way that the bacteria containing the antibiotic-resistance plasmid sequence or that containing specific DNA sequences were made to be targeted. Upon CRISPR delivery, Cas9 is programmed to recognize target sequences through the guide RNA thereby causing double-strand cleavage of targeted bacterial DNA or loss of drug resistance plasmid, which results in cell death. Remarkably, the safety and efficacy of this newly developed biotechnology tool and the biocontrol product need to be further refined for its usage in clinical translation.

Vancomycin Resistance in Enterococcus and Staphylococcus aureus

Enterococcus faecalis, Enterococcus faecium and Staphylococcus aureus are both common commensals and major opportunistic human pathogens. In recent decades, these bacteria have acquired broad resistance to several major classes of antibiotics, including commonly employed glycopeptides. Exemplified by resistance to vancomycin, glycopeptide resistance is mediated through intrinsic gene mutations, and/or transferrable van resistance gene cassette-carrying mobile genetic elements. Here, this review will discuss the epidemiology of vancomycin-resistant Enterococcus and S. aureus in healthcare, community, and agricultural settings, explore vancomycin resistance in the context of van and non-van mediated resistance development and provide insights into alternative therapeutic approaches aimed at treating drug-resistant Enterococcus and S. aureus infections.

CRISPR/Cpf1-Mediated Multiplex and Large-Fragment Gene Editing in Staphylococcus aureus

Staphylococcus aureus is a major human pathogen that causes a variety of infections, including life-threatening diseases. Research on S. aureus is constrained by complex and limited genetic manipulation methods. Here, we report a CRISPR/Cpf1-mediated system, pCpfSA, for rapid and versatile genome editing in S. aureus. In direct comparison with the existing CRISPR/Cas9-mediated genome-editing system, the pCpfSA system exhibits enhanced colony-forming units (CFUs) after editing and an expanded targetable range with comparable editing efficiency. Given the precursor crRNA (pre-crRNA) processing activity of Cpf1, the pCpfSA system also allows multiplex gene editing and large-fragment DNA knockout simply by introducing two crRNAs and the corresponding donor templates, which is difficult to achieve using the CRISPR/Cas9 system, thereby greatly expanding the genome editor toolbox for S. aureus.

Recombineering in Staphylococcus aureus

Recombineering has proven to be an extraordinarily powerful and versatile approach for the modification of bacterial genomes, but has historically not been possible in the important opportunistic pathogen Staphylococcus aureus. After evaluating the activity of various recombinases in S. aureus, we developed methods for recombineering in that organism using synthetic, single-stranded DNA oligonucleotides. This approach can be coupled to CRISPR/Cas9-mediated lethal counterselection in order to improve the efficiency with which recombinant S. aureus are recovered, which is especially useful in instances where mutants lack a selectable phenotype. These methods provide a rapid, scalable, precise, and inexpensive means to engineer point mutations, variable-length deletions, and short insertions into the S. aureus genome.

CRISPR-Cas, a Revolution in the Treatment and Study of ESKAPE Infections: Pre-Clinical Studies

One of the biggest threats we face globally is the emergence of antimicrobial-resistant (AMR) bacteria, which runs in parallel with the lack in the development of new antimicrobials. Among these AMR bacteria pathogens belonging to the ESKAPE group can be highlighted (Enterococcus spp., Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa and Enterobacter spp.) due to their profile of drug resistance and virulence. Therefore, innovative lines of treatment must be developed for these bacteria. In this review, we summarize the different strategies for the treatment and study of molecular mechanisms of AMR in the ESKAPE pathogens based on the clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated (Cas) proteins’ technologies: loss of plasmid or cellular viability, random mutation or gene deletion as well directed mutations that lead to a gene’s loss of function.

CRISPR-Cas system, antibiotic resistance and virulence in bacteria: Through a common lens

CRISPR-Cas system, antibiotic resistance and virulence are different modes of survival for the bacteria. CRISPR-Cas is an adaptive immune system that can degrade foreign DNA, antibiotic resistance helps bacteria to evade drugs that can threaten their existence and virulence determinants are offensive tools that can facilitate the establishment of infection by pathogens. This chapter focuses on these three aspects, providing insights about the CRISPR system and resistance mechanisms in brief, followed by understanding the synergistic or antagonistic relationship of resistance and virulence determinants in connection to the CRISPR system. We have addressed the discussion of this evolving topic through specific examples and studies. Different approaches for successful detection of this unique defense system in bacteria and various applications of the CRISPR-Cas systems to show how it can be harnessed to tackle the increasing problem of antibiotic resistance have been put forth. World Health Organization has declared antibiotic resistance as a serious global problem of the 21st century. As antibiotic-resistant bacteria increase their footprint across the globe, newer tools such as the CRISPR-Cas system hold immense promise to tackle this problem.

From cloning to mutant in 5 days: rapid allelic exchange in Staphylococcus aureus

In the last 10 years, the barriers preventing the uptake of foreign DNA by clinical Staphylococcus aureus isolates have been identified and powerful mutagenesis techniques such as allelic exchange are now possible in most genotypes. However, these targeted approaches can still be cumbersome, and the construction of unmarked deletions/point mutations may take many weeks or months. Here, we introduce a streamlined allelic exchange protocol using IMxxB Escherichia coli and the plasmid pIMAY-Z. With this optimized approach, a site-specific mutation can be introduced into S. aureus in 5 days, from the start of cloning to isolation of genomic DNA for confirmatory whole-genome sequencing. This streamlined protocol considerably reduces the time required to introduce a specific, unmarked mutation in S. aureus and should dramatically improve the scalability of gene-function studies.

CRISPR base editing and prime editing: DSB and template-free editing systems for bacteria and plants

CRISPR-Cas (Clustered Regularly Interspaced Short Palindromic Repeats-CRISPR associated) has been extensively exploited as a genetic tool for genome editing. The RNA guided Cas nucleases generate DNA double-strand break (DSB), triggering cellular repair systems mainly Non-homologous end-joining (NHEJ, imprecise repair) or Homology-directed repair (HDR, precise repair). However, DSB typically leads to unexpected DNA changes and lethality in some organisms. The establishment of bacteria and plants into major bio-production platforms require efficient and precise editing tools. Hence, in this review, we focus on the non-DSB and template-free genome editing, i.e., base editing (BE) and prime editing (PE) in bacteria and plants. We first highlight the development of base and prime editors and summarize their studies in bacteria and plants. We then discuss current and future applications of BE/PE in synthetic biology, crop improvement, evolutionary engineering, and metabolic engineering. Lastly, we critically consider the challenges and prospects of BE/PE in PAM specificity, editing efficiency, off-targeting, sequence specification, and editing window.

Emerging Strategies to Combat β-Lactamase Producing ESKAPE Pathogens

Since the discovery of penicillin by Alexander Fleming in 1929 as a therapeutic agent against staphylococci, β-lactam antibiotics (BLAs) remained the most successful antibiotic classes against the majority of bacterial strains, reaching a percentage of 65% of all medical prescriptions. Unfortunately, the emergence and diversification of β-lactamases pose indefinite health issues, limiting the clinical effectiveness of all current BLAs. One solution is to develop β-lactamase inhibitors (BLIs) capable of restoring the activity of β-lactam drugs. In this review, we will briefly present the older and new BLAs classes, their mechanisms of action, and an update of the BLIs capable of restoring the activity of β-lactam drugs against ESKAPE (Enterococcus spp., Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp.) pathogens. Subsequently, we will discuss several promising alternative approaches such as bacteriophages, antimicrobial peptides, nanoparticles, CRISPR (clustered regularly interspaced short palindromic repeats) cas technology, or vaccination developed to limit antimicrobial resistance in this endless fight against Gram-negative pathogens.

Genome editing of Francisella tularensis using (CRISPR-Cas9)

The clustered regularly interspaced short palindromic repeats (CRISPR)-CRISPR-associated protein 9 (Cas9) system is a powerful tool for gene editing in eukaryotic genomes but is still being developed for editing bacterial genomes. Here we describe the construction of an all-in-one vector for generating potentially scarless deletion mutants in Francisella tularensis LVS using a CRISPR-Cas9-based system.

Antibiotic Resistance Profiles, Molecular Mechanisms and Innovative Treatment Strategies of Acinetobacter baumannii

Antibiotic resistance is one of the biggest challenges for the clinical sector and industry, environment and societal development. One of the most important pathogens responsible for severe nosocomial infections is Acinetobacter baumannii, a Gram-negative bacterium from the Moraxellaceae family, due to its various resistance mechanisms, such as the β-lactamases production, efflux pumps, decreased membrane permeability and altered target site of the antibiotic. The enormous adaptive capacity of A. baumannii and the acquisition and transfer of antibiotic resistance determinants contribute to the ineffectiveness of most current therapeutic strategies, including last-line or combined antibiotic therapy. In this review, we will present an update of the antibiotic resistance profiles and underlying mechanisms in A. baumannii and the current progress in developing innovative strategies for combating multidrug-resistant A. baumannii (MDRAB) infections.

Targeting Plasmids to Limit Acquisition and Transmission of Antimicrobial Resistance

Antimicrobial resistance (AMR) is a significant global threat to both public health and the environment. The emergence and expansion of AMR is sustained by the enormous diversity and mobility of antimicrobial resistance genes (ARGs). Different mechanisms of horizontal gene transfer (HGT), including conjugation, transduction, and transformation, have facilitated the accumulation and dissemination of ARGs in Gram-negative and Gram-positive bacteria. This has resulted in the development of multidrug resistance in some bacteria. The most clinically significant ARGs are usually located on different mobile genetic elements (MGEs) that can move intracellularly (between the bacterial chromosome and plasmids) or intercellularly (within the same species or between different species or genera). Resistance plasmids play a central role both in HGT and as support elements for other MGEs, in which ARGs are assembled by transposition and recombination mechanisms. Considering the crucial role of MGEs in the acquisition and transmission of ARGs, a potential strategy to control AMR is to eliminate MGEs. This review discusses current progress on the development of chemical and biological approaches for the elimination of ARG carriers.

Genome Editing in Staphylococcus aureus by Conditional Recombineering and CRISPR/Cas9-Mediated Counterselection

Methods for the genetic manipulation of S. aureus have historically proven challenging, which has hindered experimental studies of this organism. We recently developed a system for recombineering and CRISPR/Cas9-mediated counterselection in S. aureus which utilizes commercially synthesized synthetic DNA oligonucleotides as substrates for introducing precise genomic modifications into the organism and for performing lethal counterselection of unedited cells. These techniques make it possible to scalably and inexpensively engineer desired genomic changes into laboratory or clinical S. aureus strains, using electroporation to introduce the effector plasmid vectors and oligonucleotides. Here we describe detailed protocols for performing genome editing of S. aureus in order to produce isogenic strains using this system and detail general principles which are broadly applicable across a range of organisms for which equivalent systems have been established.

Editing the microbiome the CRISPR way

Our bodies are colonized by a complex ecosystem of bacteria, unicellular eukaryotes and their viruses that together play a major role in our health. Over the past few years tools derived from the prokaryotic immune system known as CRISPR-Cas have empowered researchers to modify and study organisms with unprecedented ease and efficiency. Here we discuss how various types of CRISPR-Cas systems can be used to modify the genome of gut microorganisms and bacteriophages. CRISPR-Cas systems can also be delivered to bacterial population and programmed to specifically eliminate members of the microbiome. Finally, engineered CRISPR-Cas systems can be used to control gene expression and modulate the production of metabolites and proteins. Together these tools provide exciting opportunities to investigate the complex interplay between members of the microbiome and our bodies, and present new avenues for the development of drugs that target the microbiome. This article is part of a discussion meeting issue 'The ecology and evolution of prokaryotic CRISPR-Cas adaptive immune systems'.

Phage Therapy: What Have We Learned?

In this article we explain how current events in the field of phage therapy may positively influence its future development. We discuss the shift in position of the authorities, academia, media, non-governmental organizations, regulatory agencies, patients, and doctors which could enable further advances in the research and application of the therapy. In addition, we discuss methods to obtain optimal phage preparations and suggest the potential of novel applications of phage therapy extending beyond its anti-bacterial action.

Highly efficient base editing in Staphylococcus aureus using an engineered CRISPR RNA-guided cytidine deaminase

The base editor pnCasSA-BEC enables highly efficient base editing in Staphylococcus aureus.

Novel therapeutic means against Staphylococcus aureus infections are urgently needed due to the emergence of drug-resistant S. aureus. We report the development of a CRISPR RNA-guided cytidine deaminase (pnCasSA–BEC), enabling highly efficient gene inactivation and point mutations in S. aureus. We engineered a fusion of a Cas9 nickase (Cas9D10A) and a cytidine deaminase (APOBEC1) that can be guided to a target genomic locus for gene inactivation via generating a premature stop codon. The pnCasSA–BEC system nicks the non-edited strand of the genomic DNA, directly catalyzes the conversion of cytidine (C) to uridine (U), and relies on DNA replication to achieve C → T (G → A) conversion without using donor repair templates. The development of the base-editing system will dramatically accelerate drug-target exploration in S. aureus and provides critical insights into the development of base-editing tools in other microbes.

Efficient and Scalable Precision Genome Editing in Staphylococcus aureus through Conditional Recombineering and CRISPR/Cas9-Mediated Counterselection

Staphylococcus aureus is an important human pathogen, but studies of the organism have suffered from the lack of a robust tool set for its genetic and genomic manipulation. Here we report the development of a system for the facile and high-throughput genomic engineering of S. aureus using single-stranded DNA (ssDNA) oligonucleotide recombineering coupled with clustered regularly interspaced short palindromic repeat (CRISPR)/Cas9-mediated counterselection. We identify recombinase EF2132, derived from Enterococcus faecalis, as being capable of integrating single-stranded DNA oligonucleotides into the S. aureus genome. We found that EF2132 can readily mediate recombineering across multiple characterized strains (3 of 3 tested) and primary clinical isolates (6 of 6 tested), typically yielding thousands of recombinants per transformation. Surprisingly, we also found that some S. aureus strains are naturally recombinogenic at measurable frequencies when oligonucleotides are introduced by electroporation, even without exogenous recombinase expression. We construct a temperature-sensitive, two-vector system which enables conditional recombineering and CRISPR/Cas9-mediated counterselection in S. aureus without permanently introducing exogenous genetic material or unintended genetic lesions. We demonstrate the ability of this system to efficiently and precisely engineer point mutations and large single-gene deletions in the S. aureus genome and to yield highly enriched populations of engineered recombinants even in the absence of an externally selectable phenotype. By virtue of utilizing inexpensive, commercially synthesized synthetic DNA oligonucleotides as substrates for recombineering and counterselection, this system provides a scalable, versatile, precise, inexpensive, and generally useful tool for producing isogenic strains in S. aureus which will enable the high-throughput functional assessment of genome variation and gene function across multiple strain backgrounds.

Engineering genetic changes in bacteria is critical to understanding the function of particular genes or mutations but is currently a laborious and technically challenging process to perform for the important human pathogen Staphylococcus aureus. In an effort to develop methods which are rapid, easy, scalable, versatile, and inexpensive, here we describe a system for incorporating synthetic, mutagenic DNA molecules into the S. aureus genome and for eliminating cells that lack the engineered mutation. This method allows efficient, precise, and high-throughput genetic engineering of S. aureus strains and will facilitate studies seeking to address a variety of issues about the function of particular genes and specific mutations.