Recombinant Salmonella gallinarum Cardiolipin synthase (cls)

What is Cardiolipin synthase (cls) in Salmonella gallinarum and what is its functional significance?

Cardiolipin synthase (cls) in Salmonella gallinarum is an enzyme responsible for synthesizing cardiolipins, which are acidic glycerophospholipids found in bacterial membranes. The enzyme contains phospholipase-D motifs and is encoded by the cls gene (locus tag SG1376 in strain 287/91) .

The functional significance of cls lies in membrane integrity maintenance and adaptation to environmental stresses. Cardiolipins are particularly important for the outer membrane (OM) composition, which serves as a barrier to the environment. During infection, Gram-negative bacteria like Salmonella remodel their OM to promote survival within host tissues that activate the PhoPQ two-component regulatory system . This remodeling process involves regulated trafficking of cardiolipins to the OM, which is necessary for Salmonella to survive within hostile host environments.

How many types of cls genes exist in Salmonella species and what are their biochemical differences?

Salmonella species, including S. gallinarum, contain three distinct cardiolipin synthase genes: clsA, clsB, and clsC. These genes encode enzymes with different biochemical characteristics :

When analyzing the contribution of each enzyme to the total cardiolipin pool, research has shown that deletion of clsA results in "an undetectable amount of CL in the log phase of growth" while "the ΔclsB and ΔclsC mutants produced amounts of CL equivalent to those of the wild type" . This indicates that ClsA is the primary synthase during normal growth conditions, while ClsB and ClsC become more important during stress conditions.

What methods are used to express and purify recombinant Salmonella gallinarum cls protein?

To express and purify recombinant S. gallinarum cls protein, researchers employ several methodological approaches:

• Expression Systems:

  • E. coli is the most common expression host, though yeast, baculovirus, and mammalian cells can also be used depending on research requirements

  • For membrane proteins like cls, specialized E. coli strains (e.g., C41(DE3), C43(DE3)) may improve expression yields

• Expression Vectors:

  • pET vector systems (e.g., pETT22b) are commonly used for high-level expression in E. coli

  • For S. gallinarum-specific expression, specialized vectors like pYA3342 have been successfully employed

• Culture and Induction Conditions:

  • S. gallinarum strains are typically "grown in Luria–Bertani (LB) broth or on agar plates at 37°C"

  • For temperature-sensitive constructs, cultivation at 30°C is recommended

  • Induction conditions must be optimized for membrane proteins (typically lower temperatures and inducer concentrations)

• Purification Strategy:

  • Membrane extraction using appropriate detergents

  • Affinity chromatography using fusion tags

  • Size exclusion chromatography for final polishing

  • Storage in stabilizing buffers containing glycerol (50%) and appropriate detergents

• Quality Control:

  • SDS-PAGE and Western blotting to confirm expression and purity

  • Mass spectrometry to verify protein identity

  • Activity assays to confirm functionality

To verify the successful expression and purification, researchers typically perform thin-layer chromatography (TLC) or liquid chromatography-mass spectrometry (LC-MS/MS) to assess the enzyme's ability to synthesize cardiolipin when provided with appropriate substrates.

How can researchers generate cls gene modifications in Salmonella gallinarum?

Based on the research literature, several approaches are effective for generating cls gene modifications in S. gallinarum:

• CRISPR/Cas9-Based Gene Editing:
  The CRISPR/Cas9 system has been successfully applied for gene deletion in Salmonella as demonstrated in the SpvB deletion study . This approach includes:

  • Designing gRNA targeting the cls gene sequence

  • Constructing a plasmid containing both Cas9 and the gRNA (e.g., pCas9 or pCasSA vectors)

  • Providing a DNA template for homology-directed repair

  • Co-transformation and selection of transformants

  • Confirmation by PCR and sequencing

• λ-Red Recombination System:
  This homologous recombination approach was used for constructing SG102 strain :

  • PCR amplification of the cls gene with designed restriction sites

  • Transformation of the pKD46 plasmid into S. gallinarum

  • Expression of recombinases (Exo, Beta, and Gam) induced by L-arabinose

  • Transformation with the homologous recombination fragment

  • Selection of recombinants and confirmation by PCR

• Site-Specific Integration:
  As demonstrated for other genes, site-specific integration using temperature-sensitive plasmids can be adapted for cls genes :

  • Amplification of internal fragments of the cls gene

  • Cloning into a non-replicating plasmid (e.g., pORI28)

  • Use of a helper plasmid (e.g., pVE6007) for initial transformation

  • Temperature shift to induce plasmid integration

  • Selection and verification by PCR

• Balanced-Lethal Systems:
  For stable expression of recombinant cls, a chromosome-plasmid-balanced lethal system can be employed :

  • Creation of an asd gene deletion in S. gallinarum

  • Construction of a plasmid containing both the asd gene and the modified cls gene

  • Transformation and selection in media without diaminopimelic acid (DAP)

These genetic modification approaches allow researchers to create precise cls gene deletions, insertions, or replacements to study the function of cardiolipin synthase or develop attenuated strains for vaccine applications.

How are the effects of cls mutations on bacterial membrane composition analyzed?

Several analytical approaches are employed to assess the impact of cls mutations on membrane composition:

• Lipid Extraction and Analysis:

  • Thin-layer chromatography (TLC): Provides qualitative assessment of major lipid species, as shown in study where "the ΔclsA mutant produced an undetectable amount of CL in the log phase of growth"

  • Liquid chromatography-mass spectrometry (LC-MS/MS): Enables more precise quantification, as demonstrated where "the ΔclsA mutant routinely measured a reduction in CL molecules in stationary phase by LC-MS/MS"

• Membrane Fractionation:

  • Differential centrifugation to separate inner and outer membranes

  • Analysis of cardiolipin distribution between membrane fractions

  • Examination of membrane trafficking, as seen where "PbgA binds cardiolipin glycerophospholipids near the inner membrane and promotes their PhoPQ-regulated trafficking to the OM"

• Growth and Stress Response Analysis:

  • Growth curves under various conditions (normal growth, high osmolarity, acid stress)

  • Survival assays under stressful conditions

  • Assessment of membrane integrity using fluorescent dyes

• Genetic Complementation:

  • Expression of wild-type cls genes in mutant strains to confirm phenotype restoration, as shown where "supplying the clsA operon on a plasmid restored CL production to the ΔclsA mutant in the log phase of growth"

In interpreting these analyses, researchers must consider the complex interplay between different cls enzymes and their activation under various growth conditions. For example, while ClsA is the primary synthase during logarithmic growth, ClsB and ClsC contribute significantly during stationary phase and stress conditions .

What is the role of cardiolipin in Salmonella pathogenesis and host-pathogen interactions?

Cardiolipins play several important roles in Salmonella pathogenesis and host-pathogen interactions:

• Membrane Remodeling during Infection:
  Research demonstrates that "increased cardiolipin trafficking to the OM is necessary for Salmonella to survive within host tissues that activate PhoPQ" . The PbgA protein forms complexes that bridge the bacterial envelope to enable regulated cardiolipin delivery to the outer membrane.

• Immune System Interactions:
  Cardiolipins can interact with host pattern recognition receptors like Toll-like receptor 4 (Tlr4). Purified bacterial cardiolipin molecules with saturated fatty acids can modulate Tlr4 activation by lipid A molecules in macrophages , potentially influencing inflammatory responses.

• Inflammasome Activation:
  While mitochondrial cardiolipins can prime and activate host inflammasomes, bacterial cardiolipins may also influence this process. Study reports that "the genes encoding the CL synthases work coordinately to promote intracellular survival in macrophages and to activate the inflammasome."

• Adaptation to Host Environments:
  The multiple cls genes allow Salmonella to modulate membrane composition in response to environmental challenges encountered during infection, which may contribute to its survival within host cells.

• Virulence Regulation:
  Cardiolipin synthesis may interact with other virulence mechanisms. For instance, the SpvB protein, which is involved in Salmonella virulence and "promotes macrophage apoptosis and P-body disassembly" , may function in concert with membrane remodeling processes during infection.

Understanding these interactions is crucial for developing targeted interventions, such as vaccines or antimicrobial treatments, that could disrupt cardiolipin-dependent aspects of Salmonella pathogenesis.

How do cls gene mutations affect Salmonella gallinarum virulence in animal models?

The impact of cls gene mutations on S. gallinarum virulence must be assessed through appropriate animal models, primarily chicken infection studies. While the search results don't directly address cls mutations in S. gallinarum, insights can be drawn from related studies:

For S. gallinarum virulence assessment, chicken models are appropriate as demonstrated in gene deletion studies, where researchers used:

• Experimental Design:

  • "Disease-free male Cobb 500 broiler chickens (n = 120)" divided into experimental groups

  • Housing in "pre-sterile cages in an environmentally controlled room"

  • Oral inoculation with "0.5 ml of normal saline containing approximately 1 × 10^8 CFU"

  • Monitoring for 3 weeks post-infection

• Assessment Parameters:

  • Mortality rates and clinical signs

  • Weight gain (measured "three times per week")

  • Bacterial colonization in organs (liver, spleen, intestine)

  • Histopathological examination

Gene modification effects on virulence can be dramatic, as seen with the SpvB deletion where "the SpvB-deleted S. gallinarum strain (ΔSpvB_SG18), when tested for its virulence in broiler chicken showed no diseases signs and mortality" .

To specifically assess cls mutation effects, researchers would need to:

• Create single and multiple cls gene deletions in S. gallinarum

• Compare colonization, persistence, and pathology with wild-type strains

• Examine organ-specific bacterial loads at different time points post-infection

• Assess immune responses in infected birds

How can recombinant cls be utilized in vaccine development against fowl typhoid?

Recombinant cls proteins or modified S. gallinarum strains with altered cls expression could be valuable in developing vaccines against fowl typhoid. Based on the research literature:

• Attenuated Live Vaccine Approach:
  Modifications to cls genes could potentially create attenuated strains similar to the successful approach demonstrated with SpvB deletion, where "the avirulent strain does not affect the bird's weight and was rapidly cleared from the liver after infection" . Such strains might provide protective immunity while avoiding disease symptoms.

• Balanced-Lethal Systems:
  The chromosome-plasmid-balanced lethal system used to develop the recombinant strain SG102 could be adapted to create cls-modified vaccine strains. This system involved "expressing the APEC type I fimbriae gene cluster (fim) on the cell surface of an avirulent Salmonella gallinarum (S. gallinarum) vector strain" .

• Multicomponent Vaccines:
  Recombinant cls could be combined with other immunogenic components to create multivalent vaccines. For example, the SG102 strain provided protection against both S. gallinarum and avian pathogenic E. coli challenges .

• Immune Response Considerations:
  Effective vaccines must induce appropriate immune responses. The SG102 strain demonstrated that "highly antigen-specific humoral and mucosal immune responses... were detected in SG102-immunized chickens, with IgG and secretory IgA (sIgA) concentrations of 221.50 μg/mL and 1.68 μg/mL, respectively" .

• Safety Assessment:
  Any cls-based vaccine would require thorough safety evaluation. Current S. gallinarum vaccines like strain 9R have safety concerns, as "the genome of SG9R isolates from a fowl typhoid outbreak in Belgium has been elucidated through whole-genome sequencing, highlighting significant safety considerations" .

The development of cls-based vaccines would require investigation of whether modified cls expression affects immunogenicity while maintaining an appropriate safety profile. The ability of cls-modified strains to persist long enough to induce immunity without causing disease would be a critical consideration.

What techniques are used to evaluate the enzymatic activity of recombinant cls proteins?

Several complementary techniques are employed to evaluate the enzymatic activity of recombinant cls proteins:

• Thin-Layer Chromatography (TLC):

  • Provides qualitative assessment of cardiolipin production

  • Allows visualization of reaction products separated by polarity

  • Used to compare wild-type and mutant strains, as demonstrated where "the ΔclsA, ΔclsB, and ΔclsC mutants were qualitatively similar by TLC analysis"

• Liquid Chromatography-Mass Spectrometry (LC-MS/MS):

  • Enables precise quantification of cardiolipin species

  • Can distinguish between cardiolipins with different fatty acid compositions

  • Provides higher sensitivity than TLC, as shown where subtle differences in cardiolipin levels "were not statistically significant from the wild-type strain" by LC-MS/MS

• Radioactive Substrate Incorporation:

  • Uses radiolabeled precursors (e.g., ^32P-phosphatidylglycerol) to track product formation

  • Allows kinetic analysis of enzyme activity

  • Provides quantitative data on reaction rates

• Fluorescent Substrate Analogs:

  • Enables real-time monitoring of enzyme kinetics

  • May allow high-throughput screening of conditions or inhibitors

  • Can be used for cellular localization studies

• Complementation Studies:

  • Functional validation through restoration of cardiolipin production in cls-deficient strains

  • Analysis by TLC or LC-MS/MS following genetic complementation

  • As demonstrated where "supplying the clsA operon on a plasmid restored CL production to the ΔclsA mutant"

For recombinant cls proteins specifically, researchers must consider several factors:

• Appropriate detergent selection for solubilization and activity maintenance

• Buffer optimization to preserve enzyme structure and function

• Substrate availability and presentation in in vitro assays

• Potential requirement for specific membrane environments for optimal activity

These enzymatic activity assays are crucial for confirming that recombinant cls proteins retain their functional properties, which is essential for structure-function studies and applications in vaccine development.

How do cardiolipin synthesis genes interact with other virulence factors in Salmonella gallinarum?

The interaction between cardiolipin synthesis genes and other virulence factors in Salmonella gallinarum involves complex regulatory networks:

• Interaction with Pathogenicity Islands:
  Salmonella Pathogenicity Islands (SPIs) encode numerous virulence factors. While direct interactions with cls genes aren't fully characterized, the membrane remodeling facilitated by cardiolipin synthesis likely affects the function of membrane-associated virulence systems like Type III Secretion Systems (T3SS) encoded by SPI-1 and SPI-2.

• SpvB and Virulence Plasmid:
  The SpvB gene, located on the virulence plasmid, is a key virulence factor that "exhibits delayed cell death by preventing actin polymerization followed by apoptosis during intracellular infection" . Cardiolipin synthesis may influence the effectiveness of SpvB and other plasmid-encoded virulence factors by affecting membrane properties and bacterial survival within host cells.

• PhoPQ Two-Component System:
  Research shows that "Salmonella rely on the PhoPQ two-component regulators to coordinate OM remodeling in response to environmental cues" . The PhoPQ system regulates cardiolipin trafficking to the outer membrane via PbgA, and this process is "necessary for Salmonella to survive within host tissues that activate PhoPQ" .

• Intracellular Survival Mechanisms:
  Cardiolipin synthesis genes work "coordinately to promote intracellular survival in macrophages" , which is crucial for Salmonella pathogenesis. This likely involves interactions with other virulence factors involved in resisting host defense mechanisms, such as SpiC which "interferes with vesicular trafficking in host cells to prevent SCV-lysosome fusion" .

• Inflammasome Activation:
  Cls genes contribute to "activate the inflammasome" , which interacts with virulence factors like SifA that "subverts human NLRP3 and NLRC4 inflammasome" .

The table below summarizes potential interactions between cardiolipin synthesis and key virulence factors:

These interactions highlight the integrated nature of bacterial virulence mechanisms and suggest that targeting cardiolipin synthesis could have pleiotropic effects on multiple virulence systems.

What are the structural and functional differences between ClsA, ClsB, and ClsC enzymes in Salmonella gallinarum?

The three cardiolipin synthases in Salmonella gallinarum exhibit distinct structural features and functional characteristics:

These differences enable Salmonella to fine-tune membrane composition across various growth conditions and stress environments, likely contributing to its adaptability during host infection.

How can CRISPR/Cas9 technology be applied to study cls gene function in Salmonella gallinarum?

CRISPR/Cas9 technology offers powerful approaches for studying cls gene function in Salmonella gallinarum:

• Gene Deletion Strategy:
  Similar to the SpvB deletion approach , researchers can use CRISPR/Cas9 to create precise cls gene knockouts:

  • Design gRNA targeting specific cls gene sequences

  • Construct expression vectors for Cas9 and gRNA (e.g., "pCas9 (Plasmid #42876)" or "pCasSA (Plasmid #98211)" )

  • Provide DNA templates with homologous arms for recombination

  • Transform into S. gallinarum using electroporation

  • Select transformants on appropriate antibiotics

  • Verify deletions by PCR and sequencing

• Multiplex Gene Editing:
  CRISPR/Cas9 can target multiple cls genes simultaneously:

  • Design gRNAs for clsA, clsB, and clsC

  • Create single, double, and triple mutants to assess functional redundancy

  • Compare phenotypes across different combinations of mutations

• Base Editing Applications:
  Rather than complete gene deletion, CRISPR base editors can introduce specific mutations:

  • Target catalytic residues in phospholipase-D motifs

  • Create point mutations in regulatory regions

  • Modify substrate binding sites to alter enzyme specificity

• CRISPRi for Gene Silencing:
  CRISPR interference using catalytically inactive Cas9 (dCas9):

  • Allows tunable repression of cls gene expression

  • Enables temporal control of gene silencing

  • Avoids complete gene deletion that might have polar effects

• Gene Tagging:
  CRISPR/Cas9 can facilitate precise insertion of epitope or fluorescent tags:

  • Create C-terminal fusions to track protein localization

  • Add purification tags for interaction studies

  • Introduce reporter constructs to monitor expression

• Implementation Considerations:
  Based on the SpvB deletion study :

  • "The homology-directed repair method was used for complete deletion using the modified pCas9 plasmid"

  • Random colonies should be screened by PCR for successful editing

  • Positive clones require sequence verification

  • Multiple independent mutants should be characterized to rule out off-target effects

• Phenotypic Analysis:
  Following successful gene editing, comprehensive phenotypic analysis would include:

  • Cardiolipin profiling using TLC and LC-MS/MS

  • Growth characteristics under various conditions

  • Stress tolerance assessment

  • Virulence testing in appropriate chicken models

CRISPR/Cas9 technology offers significant advantages over traditional mutagenesis approaches, including greater precision, efficiency, and versatility for studying cls gene function in Salmonella gallinarum.

What is the relationship between cardiolipin synthesis and antimicrobial resistance in Salmonella gallinarum?

While the direct relationship between cardiolipin synthesis and antimicrobial resistance in Salmonella gallinarum isn't extensively covered in the search results, several key connections can be identified based on membrane biology principles and related research:

• Membrane Permeability Barrier:
  Cardiolipins contribute to the outer membrane permeability barrier, which affects the entry of antibiotics. Modifications in cardiolipin synthesis could alter membrane properties, potentially affecting susceptibility to antimicrobials that must cross the membrane to reach their targets.

• PhoPQ Regulation:
  The PhoPQ two-component system, which regulates cardiolipin trafficking via PbgA , also controls multiple antimicrobial resistance mechanisms. PhoPQ activation "coordinates OM remodeling in response to environmental cues" , which includes modifications that can reduce susceptibility to cationic antimicrobial peptides and some antibiotics.

• Stress Response Connection:
  ClsB and ClsC are activated during stress conditions , which often coincide with antibiotic exposure. The stress response adaptations mediated by these enzymes may contribute to survival during antimicrobial therapy.

• Biofilm Formation:
  Alterations in membrane composition can affect bacterial adhesion and biofilm formation, which are known to enhance antimicrobial resistance. Cardiolipin synthesis may influence these processes, though specific evidence for S. gallinarum is limited.

• Drug Target Potential:
  The cls enzymes themselves could be potential targets for novel antimicrobials. Their essential role in membrane homeostasis makes them candidates for therapeutic intervention, particularly if selective inhibition could be achieved.

• Multi-Drug Resistance Context:
  In the broader context of Salmonella species, reintroduction of older drugs raises concerns about "the re-emergence of MDR upon increased consumption of first-line antimicrobials" . The role of membrane adaptations, including cardiolipin modifications, in this process remains to be fully elucidated.

• Experimental Approaches:
  To investigate this relationship, researchers could:

  • Compare antimicrobial susceptibility profiles between wild-type and cls mutant strains

  • Examine changes in cls gene expression following antibiotic exposure

  • Assess the impact of cls overexpression on antimicrobial resistance

  • Analyze membrane properties and antibiotic penetration in strains with altered cardiolipin levels

Understanding the relationship between cardiolipin synthesis and antimicrobial resistance could potentially reveal new strategies for combating resistant Salmonella gallinarum strains and improving treatment outcomes for fowl typhoid.