Colonization efficiency of multidrug-resistant Neisseria gonorrhoeae in a female mouse model

The rapid occurrence of gonococcal resistance to all classes of antibiotics could lead to untreatable gonorrhea. Thus, development of novel anti-Neisseria gonorrhoeae drugs is urgently needed. Neisseria gonorrhoeae FA1090 is the most used in gonococcal infection mouse models because of its natural resistance to streptomycin. Streptomycin inhibits the urogenital commensal flora that permits gonococcal colonization. However, this strain is drug-susceptible and cannot be used to investigate the efficacy of novel agents against multidrug-resistant N. gonorrhoeae. Hence, to test the in vivo efficacy of new therapeutics against N. gonorrhoeae resistant to the frontline antibiotics, azithromycin, or ceftriaxone, we constructed streptomycin-resistant mutants of N. gonorrhoeae CDC-181 (azithromycin-resistant) and WHO-X (ceftriaxone-resistant). We identified the inoculum size needed to successfully colonize mice. Both mutants, CDC-181-rpsLA128G and WHO-X-rpsLA128G, colonized the genital tract of mice for 14 days with 100% colonization observed for at least 7 days. CDC-181-rpsLA128G demonstrated better colonization of the murine genital tract compared to WHO-X-rpsLA128G. Lower inoculum of WHO-X-rpsLA128G (105 and 106 CFU) colonized mice better than higher inoculum. Overall, our results indicate that CDC-181-rpsLA128G and WHO-X-rpsLA128G can colonize the lower genital tract of mice and are suitable to be used in mouse models to investigate the efficacy of antigonococcal agents.

Development and Properties of Francisella tularensis Subsp. holarctica 15 NIIEG Vaccine Strain without the recD Gene

The genomic analysis of all subspecies F. tularensis, as found in Gen Bank NCBI, reveals the presence of genes encoding proteins like to the multifunctional RecBCD enzyme complex in E. coli and other bacteria. To date, the role of the recD gene in F. tularensis, which encodes the alpha chain of exonuclease V, in DNA metabolism processes, has not been studied either in vitro or in vivo. F. tularensis subsp. holarctica 15 NIIEG, a vaccine strain, served as the basis to create the F. tularensis 15D strain with recD deletion. The lack of the recD gene suppresses the integration of suicide plasmids with F. tularensis genome fragments into the chromosome. The modified strain showed reduced growth in vitro and in vivo. This study shows that such deletion significantly reduces the virulence of the strain in BALB/c mice.

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.

Counter-Selection Method for Markerless Allelic Exchange in Bordetella bronchiseptica Based on sacB Gene From Bacillus subtilis

Bordetella bronchiseptica is a gram-negative bacterium that causes respiratory tract infections. It is a natural pathogen of a wide variety of mammals, including some used as laboratory models. This makes B. bronchiseptica an ideal organism to study pathogen-host interactions in order to unveil molecular mechanisms behind pathogenic processes. Even though genetic engineering is an essential tool in this area, there are just a few reports about genome manipulation techniques in this organism. In this article we describe an allelic exchange protocol based on double crossover recombination facilitated by the Bacillus subtilis sacB gene that can be applied for partial or complete gene knockouts, single-nucleotide mutations, or even introduction of coding sequences for transcriptional fusions. In contrast to previously employed techniques, this protocol renders genetically manipulated chromosomes without foreign DNA and enables the construction of successive genome manipulation using the same vector backbone. The entire procedure has been developed for fast and reliable manipulations with a total duration of 2 weeks. © 2020 Wiley Periodicals LLC. Basic Protocol 1: Setting up strains Basic Protocol 2: Homologous recombination (first crossing-over) Alternate Protocol: B. bronchiseptica electroporation Basic Protocol 3: Screening for sucrose-sensitive clones Basic Protocol 4: Homologous recombination (second crossing-over) Basic Protocol 5: PCR screening of putative marker-exchange mutants Support Protocol: Electrocompetent cell preparation.

A New Suite of Allelic-Exchange Vectors for the Scarless Modification of Proteobacterial Genomes

Despite the advent of new techniques for genetic engineering of bacteria, allelic exchange through homologous recombination remains an important tool for genetic analysis. Currently, sacB-based vector systems are often used for allelic exchange, but counterselection escape, which prevents isolation of cells with the desired mutation, occasionally limits their utility. To circumvent this, we engineered a series of "pTOX" allelic-exchange vectors. Each plasmid encodes one of a set of inducible toxins, chosen for their potential utility in a wide range of medically important proteobacteria. A codon-optimized rhaS transcriptional activator with a strong synthetic ribosome-binding site enables tight toxin induction even in organisms lacking an endogenous rhamnose regulon. Expression of the gene encoding blue AmilCP or magenta TsPurple nonfluorescent chromoprotein facilitates monitoring of successful single- and double-crossover events using these vectors. The versatility of these vectors was demonstrated by deleting genes in Serratia marcescens, Escherichia coli O157:H7, Enterobacter cloacae, and Shigella flexneri Finally, pTOX was used to characterize the impact of disruption of all combinations of the 3 paralogous S. marcescens peptidoglycan amidohydrolases on chromosomal ampC β-lactamase activity and the corresponding β-lactam antibiotic resistance. Mutation of multiple amidohydrolases was necessary for high-level ampC derepression and β-lactam resistance. These data suggest why β-lactam resistance may emerge during treatment less frequently in S. marcescens than in other AmpC-producing pathogens, like E. cloacae Collectively, our findings suggest that the pTOX vectors should be broadly useful for genetic engineering of Gram-negative bacteria.IMPORTANCE Targeted modification of bacterial genomes is critical for genetic analysis of microorganisms. Allelic exchange is a technique that relies on homologous recombination to replace native loci with engineered sequences. However, current allelic-exchange vectors often enable only weak selection for successful homologous recombination. We developed a suite of new allelic-exchange vectors, pTOX, which were validated in several medically important proteobacteria. They encode visible nonfluorescent chromoproteins that enable easy identification of colonies bearing integrated vectors and permit stringent selection for the second step of homologous recombination. We demonstrate the utility of these vectors by using them to investigate the effect of inactivation of Serratia marcescens peptidoglycan amidohydrolases on β-lactam antibiotic resistance.

Generation of Markerless Deletions in the Nosocomial Pathogen Clostridium difficile by Induction of DNA Double-Strand Breaks

Most sequenced bacterial genomes contain genes encoding proteins of unknown or hypothetical function. To identify a phenotype for mutations in such genes, deletion is the preferred method for mutagenesis because it reduces the likelihood of polar effects, although it does not eliminate the possibility. Allelic exchange to produce deletions is dependent on the length of homologous regions used to generate merodiploids. Shorter regions of homology resolve at lower frequencies. The work presented here demonstrates the utility of inducing DNA double-strand breaks to increase the frequency of merodiploid resolution in Clostridium difficile. Using this approach, we reveal the roles of two genes, encoding homologues of AddAB, in survival following DNA damage. The method is readily applicable to the production of deletions in C. difficile and expands the toolbox available for genetic analysis of this important anaerobic pathogen.

Clostridium difficile is an important nosocomial pathogen associated with potentially fatal disease induced by the use of antibiotics. Genetic characterization of such clinically important bacteria is often hampered by lack of availability of suitable tools. Here, we describe the use of I-SceI to induce DNA double-strand breaks, which increase the frequency of allelic exchange and enable the generation of markerless deletions in C. difficile. The usefulness of the system is illustrated by the deletion of genes encoding putative AddAB homologues. The ΔaddAB mutants are sensitive to ultraviolet light and the antibiotic metronidazole, indicating a role in homologous recombination and the repair of DNA breaks. Despite the impairment in recombination, the mutants are still proficient for induction of the SOS response. In addition, deletion of the fliC gene, and subsequent complementation, reveals the importance of potential regulatory elements required for expression of a downstream gene encoding the flagellin glycosyltransferase.

IMPORTANCE Most sequenced bacterial genomes contain genes encoding proteins of unknown or hypothetical function. To identify a phenotype for mutations in such genes, deletion is the preferred method for mutagenesis because it reduces the likelihood of polar effects, although it does not eliminate the possibility. Allelic exchange to produce deletions is dependent on the length of homologous regions used to generate merodiploids. Shorter regions of homology resolve at lower frequencies. The work presented here demonstrates the utility of inducing DNA double-strand breaks to increase the frequency of merodiploid resolution in Clostridium difficile. Using this approach, we reveal the roles of two genes, encoding homologues of AddAB, in survival following DNA damage. The method is readily applicable to the production of deletions in C. difficile and expands the toolbox available for genetic analysis of this important anaerobic pathogen.

Modernized Tools for Streamlined Genetic Manipulation and Comparative Study of Wild and Diverse Proteobacterial Lineages

A great challenge in microbiota research is the immense diversity of symbiotic bacteria with the capacity to impact the lives of plants and animals. Moving beyond correlative DNA sequencing-based studies to define the cellular and molecular mechanisms by which symbiotic bacteria influence the biology of their hosts is stalling because genetic manipulation of new and uncharacterized bacterial isolates remains slow and difficult with current genetic tools. Moreover, developing tools de novo is an arduous and time-consuming task and thus represents a significant barrier to progress. To address this problem, we developed a suite of engineering vectors that streamline conventional genetic techniques by improving postconjugation counterselection, modularity, and allelic exchange. Our modernized tools and step-by-step protocols will empower researchers to investigate the inner workings of both established and newly emerging models of bacterial symbiosis.

Correlating the presence of bacteria and the genes they carry with aspects of plant and animal biology is rapidly outpacing the functional characterization of naturally occurring symbioses. A major barrier to mechanistic studies is the lack of tools for the efficient genetic manipulation of wild and diverse bacterial isolates. To address the need for improved molecular tools, we used a collection of proteobacterial isolates native to the zebrafish intestinal microbiota as a testbed to construct a series of modernized vectors that expedite genetic knock-in and knockout procedures across lineages. The innovations that we introduce enhance the flexibility of conventional genetic techniques, making it easier to manipulate many different bacterial isolates with a single set of tools. We developed alternative strategies for domestication-free conjugation, designed plasmids with customizable features, and streamlined allelic exchange using visual markers of homologous recombination. We demonstrate the potential of these tools through a comparative study of bacterial behavior within the zebrafish intestine. Live imaging of fluorescently tagged isolates revealed a spectrum of distinct population structures that differ in their biogeography and dominant growth mode (i.e., planktonic versus aggregated). Most striking, we observed divergent genotype-phenotype relationships: several isolates that are predicted by genomic analysis and in vitro assays to be capable of flagellar motility do not display this trait within living hosts. Together, the tools generated in this work provide a new resource for the functional characterization of wild and diverse bacterial lineages that will help speed the research pipeline from sequencing-based correlations to mechanistic underpinnings.

Forward Genetic Dissection of Biofilm Development by Fusobacterium nucleatum: Novel Functions of Cell Division Proteins FtsX and EnvC

Fusobacterium nucleatum is a key member of the human oral biofilm. It is also implicated in preterm birth and colorectal cancer. To facilitate basic studies of fusobacterial virulence, we describe here a versatile transposon mutagenesis procedure and a pilot screen for mutants defective in biofilm formation. Out of 10 independent biofilm-defective mutants isolated, the affected genes included the homologs of the Escherichia coli cell division proteins FtsX and EnvC, the electron transport protein RnfA, and four proteins with unknown functions. Next, a facile new gene deletion method demonstrated that nonpolar, in-frame deletion of ftsX or envC produces viable bacteria that are highly filamentous due to defective cell division. Transmission electron and cryo-electron microscopy revealed that the ΔftsX and ΔenvC mutant cells remain joined with apparent constriction, and scanning electron microscopy (EM) uncovered a smooth cell surface without the microfolds present in wild-type cells. FtsX and EnvC proteins interact with each other as well as a common set of interacting partners, many with unknown function. Last, biofilm development is altered when cell division is blocked by MinC overproduction; however, unlike the phenotypes of ΔftsX and ΔenvC mutants, a weakly adherent biofilm is formed, and the wild-type rugged cell surface is maintained. Therefore, FtsX and EnvC may perform novel functions in Fusobacterium cell biology. This is the first report of an unbiased approach to uncover genetic determinants of fusobacterial biofilm development. It points to an intriguing link among cytokinesis, cell surface dynamics, and biofilm formation, whose molecular underpinnings remain to be elucidated.

Little is known about the virulence mechanisms and associated factors in F. nucleatum, due mainly to the lack of convenient genetic tools for this organism. We employed two efficient genetic strategies to identify F. nucleatum biofilm-defective mutants, revealing FtsX and EnvC among seven biofilm-associated factors. Electron microscopy established cell division defects of the ΔftsX and ΔenvC mutants, accompanied with a smooth cell surface, unlike the microfold, rugged appearance of wild-type bacteria. Proteomic studies demonstrated that FtsX and EnvC interact with each other as well as a set of common and unique interacting proteins, many with unknown functions. Importantly, blocking cell division by MinC overproduction led to formation of a weakly adherent biofilm, without alteration of the wild-type cell surface. Thus, this work links cell division and surface dynamics to biofilm development and lays a foundation for future genetic and biochemical investigations of basic cellular processes in this clinically significant pathogen.

Avirulent Bacillus anthracis Strain with Molecular Assay Targets as Surrogate for Irradiation-Inactivated Virulent Spores

The revelation in May 2015 of the shipment of γ irradiation–inactivated wild-type Bacillus anthracis spore preparations containing a small number of live spores raised concern about the safety and security of these materials. The finding also raised doubts about the validity of the protocols and procedures used to prepare them. Such inactivated reference materials were used as positive controls in assays to detect suspected B. anthracis in samples because live agent cannot be shipped for use in field settings, in improvement of currently deployed detection methods or development of new methods, or for quality assurance and training activities. Hence, risk-mitigated B. anthracis strains are needed to fulfill these requirements. We constructed a genetically inactivated or attenuated strain containing relevant molecular assay targets and tested to compare assay performance using this strain to the historical data obtained using irradiation-inactivated virulent spores.

galK-based suicide vector mediated allelic exchange in Mycobacterium abscessus

Mycobacterium abscessus is a fast-growing environmental organism and an important emerging pathogen. It is highly resistant to many antibiotics and undergoes a smooth to rough colony morphology change that appears to be important for pathogenesis. Smooth environmental strains have a glycopeptidolipid (GPL) on the surface, while certain types of clinical strains are often rough and lack this GPL, due to mutations in biosynthetic genes or the mmpL4b transporter gene. We report here the development and evaluation of an allelic exchange system for unmarked alleles in M. abscessus ATCC19977, using a suicide vector bearing the E. coli galK gene and 2-deoxygalactose counterselection. We describe here two variant galK suicide vectors, and demonstrate their utility in constructing a variety of mutants with deletion alleles of the mmpL4b GPL transporter gene, the mbtH GPL biosynthesis gene, the known β-lactamase gene MAB_2875 and a putative β-lactamase gene, MAB_2833. We also show that a novel allele of the E. coli aacC4 gene, conferring apramycin resistance (aacC41), can be used as a selectable marker in M. abscessus ATCC19977 at single copy.

Burkholderia thailandensis: Genetic Manipulation

Burkholderia thailandensis is a Gram-negative bacterium endemic to Southeast Asian and northern Australian soils. It is non-pathogenic; therefore, it is commonly used as a model organism for the related human pathogens Burkholderia mallei and Burkholderia pseudomallei. B. thailandensis is relatively easily genetically manipulated and a variety of robust genetic tools can be used in this organism. This unit describes protocols for conjugation, natural transformation, mini-Tn7 insertion, and allelic exchange in B. thailandensis. © 2017 by John Wiley & Sons, Inc.

An Effective Counterselection System for Listeria monocytogenes and Its Use To Characterize the Monocin Genomic Region of Strain 10403S

Construction of Listeria monocytogenes mutants by allelic exchange has been laborious and time-consuming due to lack of proficient selection markers for the final recombination event, that is, a marker conveying substance sensitivity to the bacteria bearing it, enabling the exclusion of merodiploids and selection for plasmid loss. In order to address this issue, we engineered a counterselection marker based on a mutated phenylalanyl-tRNA synthetase gene (pheS*). This mutation renders the phenylalanine-binding site of the enzyme more promiscuous and allows the binding of the toxic p-chloro-phenylalanine analog (p-Cl-phe) as a substrate. When pheS* is introduced into L. monocytogenes and highly expressed under control of a constitutively active promoter, the bacteria become sensitive to p-Cl-phe supplemented in the medium. This enabled us to utilize pheS* as a negative selection marker and generate a novel, efficient suicide vector for allelic exchange in L. monocytogenes We used this vector to investigate the monocin genomic region in L. monocytogenes strain 10403S by constructing deletion mutants of the region. We have found this region to be active and to cause bacterial lysis upon mitomycin C treatment. The future applications of such an effective counterselection system, which does not require any background genomic alterations, are vast, as it can be modularly used in various selection systems (e.g., genetic screens). We expect this counterselection marker to be a valuable genetic tool in research on L. monocytogenesIMPORTANCEL. monocytogenes is an opportunistic intracellular pathogen and a widely studied model organism. An efficient counterselection marker is a long-standing need in Listeria research for improving the ability to design and perform various genetic manipulations and screening systems for different purposes. We report the construction and utilization of an efficient suicide vector for allelic exchange which can be conjugated, leaves no marker in the bacterial chromosome, and does not require the use of sometimes leaky inducible promoters. This highly efficient genome editing tool for L. monocytogenes will allow for rapid sequential mutagenesis, introduction of point mutations, and design of screening systems. We anticipate that it will be extensively used by the research community and yield novel insights into the diverse fields studied using this model organism.

Genetic manipulation of Staphylococci-breaking through the barrier

Most strains of Staphylococcus aureus and Staphylococcus epidermidis possess a strong restriction barrier that hinders exchange of DNA. Recently, major advances have been made in identifying and characterizing the restriction-modification (RM) systems involved. In particular a novel type IV restriction enzyme that recognizes cytosine methylated DNA has been shown to be the major barrier to transfer of plasmid DNA from Escherichia coli into S. aureus and S. epidermidis. While the conserved type I RM system provides a further barrier. Here we review the recent advances in understanding of restriction systems in staphylococci and highlight how this has been exploited to improve our ability to manipulate genetically previously untransformable strains.

Advances in genetic manipulation of obligate intracellular bacterial pathogens

Infections by obligate intracellular bacterial pathogens result in significant morbidity and mortality worldwide. These bacteria include Chlamydia spp., which causes millions of cases of sexually transmitted disease and blinding trachoma annually, and members of the α-proteobacterial genera Anaplasma, Ehrlichia, Orientia, and Rickettsia, agents of serious human illnesses including epidemic typhus. Coxiella burnetii, the agent of human Q fever, has also been considered a prototypical obligate intracellular bacterium, but recent host cell-free (axenic) growth has rescued it from obligatism. The historic genetic intractability of obligate intracellular bacteria has severely limited molecular dissection of their unique lifestyles and virulence factors involved in pathogenesis. Host cell restricted growth is a significant barrier to genetic transformation that can make simple procedures for free-living bacteria, such as cloning, exceedingly difficult. Low transformation efficiency requiring long-term culture in host cells to expand small transformant populations is another obstacle. Despite numerous technical limitations, the last decade has witnessed significant gains in genetic manipulation of obligate intracellular bacteria including allelic exchange. Continued development of genetic tools should soon enable routine mutation and complementation strategies for virulence factor discovery and stimulate renewed interest in these refractory pathogens. In this review, we discuss the technical challenges associated with genetic transformation of obligate intracellular bacteria and highlight advances made with individual genera.

Genetic manipulation of francisella tularensis

Francisella tularensis is a facultative intracellular pathogen that causes the disease tularemia. F. tularensis subsp. tularensis causes the most severe disease in humans and has been classified as a Category A select agent and potential bioweapon. There is currently no vaccine approved for human use, making genetic manipulation of this organism critical to unraveling the genetic basis of pathogenesis and developing countermeasures against tularemia. The development of genetic techniques applicable to F. tularensis have lagged behind those routinely used for other bacteria, primarily due to lack of research and the restricted nature of the biocontainment required for studying this pathogen. However, in recent years, genetic techniques, such as transposon mutagenesis and targeted gene disruption, have been developed, that have had a dramatic impact on our understanding of the genetic basis of F. tularensis virulence. In this review, we describe some of the methods developed for genetic manipulation of F. tularensis.

Oligonucleotide recombination: a hidden treasure

In Swingle et al. we demonstrate that it is possible to use recombineering to direct a variety of changes in wild-type bacterial cells without the addition of phage-encoded proteins. This discovery is potentially applicable to biological engineering in a wide variety of bacterial species. Here we describe key features of oligo recombination as it is currently understood, and propose strategies for expanding the utility of oligo recombination for bioengineering.

Borrelia burgdorferi periplasmic flagella have both skeletal and motility functions

Bacterial shape usually is dictated by the peptidoglycan layer of the cell wall. In this paper, we show that the morphology of the Lyme disease spirochete Borrelia burgdorferi is the result of a complex interaction between the cell cylinder and the internal periplasmic flagella. B. burgdorferi has a bundle of 7-11 helically shaped periplasmic flagella attached at each end of the cell cylinder and has a flat-wave cell morphology. Backward moving, propagating waves enable these bacteria to swim in both low viscosity media and highly viscous gel-like media. Using targeted mutagenesis, we inactivated the gene encoding the major periplasmic flagellar filament protein FlaB. The resulting flaB mutants not only were nonmotile, but were rod-shaped. Western blot analysis indicated that FlaB was no longer synthesized, and electron microscopy revealed that the mutants were completely deficient in periplasmic flagella. Wild-type cells poisoned with the protonophore carbonyl cyanide-m-chlorophenylhydrazone retained their flat-wave morphology, indicating that the periplasmic flagella do not need to be energized for the cell to maintain this shape. Our results indicate that the periplasmic flagella of B. burgdorferi have a skeletal function. These organelles dynamically interact with the rod-shaped cell cylinder to enable the cell to swim, and to confer in part its flat-wave morphology.