Recombinant Salmonella enteritidis PT4 Cardiolipin synthase (cls)

What is the functional role of cardiolipin in Salmonella enteritidis PT4?

Cardiolipin plays multiple crucial roles in Salmonella enteritidis PT4:

• Membrane Structure: Cardiolipin is an essential component of bacterial membranes, particularly the outer membrane (OM) of Gram-negative bacteria like S. enteritidis .

• Environmental Adaptation: During infection and environmental stress, S. enteritidis regulates the cardiolipin composition of its outer membrane. This regulation allows adaptation to changing environments, including the vacuolar environment within macrophages .

• Host-Pathogen Interactions: Cardiolipin molecules can interact with host pattern recognition receptors such as Toll-like receptor 4 (Tlr4), potentially influencing inflammatory responses during infection .

• Intracellular Survival: Cardiolipin synthesis contributes to Salmonella's ability to survive within macrophage vacuoles, a critical aspect of its pathogenesis .

• Stress Response: Cardiolipin production increases during stationary phase and other stress conditions, suggesting a role in bacterial stress adaptation .

How can recombinant Salmonella enteritidis PT4 cardiolipin synthase be effectively expressed and purified?

For optimal expression and purification of recombinant S. enteritidis PT4 cardiolipin synthase:

• Expression System Selection: E. coli is the preferred heterologous host for expression, as demonstrated in multiple studies . Baculovirus expression systems can also be used for proteins requiring eukaryotic post-translational modifications.

• Vector Design: Include N-terminal or C-terminal His-tags for efficient purification. The vector should contain appropriate promoters (T7 or tac) for controlled expression.

• Expression Conditions:

  • Induce at mid-log phase (OD600 = 0.6-0.8)

  • Use lower temperatures (16-25°C) for induction to improve solubility

  • Include membrane stabilizers like glycerol (5-10%) in the growth medium

  • Consider detergents for membrane protein solubilization

• Purification Protocol:

  • Lyse cells using sonication or French press in buffer containing 50 mM Tris-HCl (pH 8.0), 300 mM NaCl, and 10% glycerol

  • Solubilize membrane fraction with appropriate detergents (DDM or CHAPS)

  • Purify using Ni-NTA affinity chromatography

  • Consider size exclusion chromatography as a polishing step

  • Store the purified protein in Tris/PBS-based buffer with 6% trehalose at -20°C or -80°C

• Quality Control:

  • Verify purity by SDS-PAGE (>90% purity is typically achievable)

  • Confirm identity by mass spectrometry or Western blotting

  • Assess activity using functional assays for cardiolipin synthesis

What strategies can be employed to create and validate cardiolipin synthase mutants in Salmonella enteritidis?

Creating and validating cardiolipin synthase mutants involves several methodological approaches:

• Mutation Generation:

  • Use λ Red recombinase-mediated homologous recombination (Datsenko and Wanner method)

  • Transform wild-type S. enteritidis PT4 with the Red helper plasmid (pKD46)

  • Generate PCR products with FRT-flanked kanamycin resistance genes and homologous regions to the target cls gene

  • Introduce PCR products into competent cells expressing the Red recombinase

  • Select for kanamycin-resistant recombinants

• Creating Non-polar Deletions:

  • Remove kanamycin resistance cassettes using FLP recombinase (pCP20 plasmid)

  • Verify gene deletion by PCR analysis

  • Confirm the absence of polar effects on downstream genes by RT-PCR

• Multiple Mutant Construction:

  • Generate single, double, and triple mutants of ClsA, ClsB, and ClsC to study functional redundancy

  • Use sequential rounds of recombination, each targeting a different cls gene

• Phenotypic Validation:

  • Analyze membrane phospholipid composition by thin-layer chromatography and mass spectrometry

  • Compare growth curves in standard and stress conditions

  • Assess changes in membrane properties (permeability, fluidity)

  • Examine bacterial morphology by light or electron microscopy

  • Test sensitivity to antimicrobial peptides and other stressors

• Complementation Studies:

  • Express wild-type cls genes from plasmids in the mutant strains

  • Verify restoration of cardiolipin synthesis and wild-type phenotypes

How do the three different cardiolipin synthases (ClsA, ClsB, ClsC) contribute to Salmonella enteritidis virulence and stress responses?

The three cardiolipin synthases in Salmonella enteritidis play distinct but overlapping roles in virulence and stress adaptation:

What role does cardiolipin play in the host-pathogen interaction during Salmonella enteritidis infection?

Cardiolipin mediates several critical aspects of host-pathogen interactions during S. enteritidis infection:

• Toll-like Receptor 4 (Tlr4) Modulation:

  • Bacterial cardiolipin molecules interact with the host Tlr4 complex

  • Purified bacterial cardiolipin with saturated fatty acids can decrease Tlr4 activation by lipid A molecules in macrophages

  • This suggests cardiolipin may help modulate the host inflammatory response during infection

• Inflammasome Activation:

  • Mitochondrial cardiolipin molecules can activate the inflammasome and its effector caspase-1, initiating pyroptosis

  • Bacterial cardiolipin synthases influence inflammasome activation, though not directly through cardiolipin production

  • The coordination of cls gene products affects the bacterial capacity to activate inflammasomes

• Intracellular Survival:

  • S. enteritidis regulates its outer membrane lipid composition during intracellular residence in macrophage vacuoles

  • This regulation contributes to bacterial survival within the hostile environment of the phagosome

  • Cardiolipin synthesis is part of the adaptive response to vacuolar stress conditions

• Experimental Approaches to Study These Interactions:

  • Infection of macrophage cell lines with wild-type and cls mutant S. enteritidis

  • Measurement of inflammatory cytokine production (TNF-α, IL-1β)

  • Assessment of inflammasome activation via caspase-1 activity assays

  • Evaluation of bacterial survival in macrophages over time

  • Analysis of cardiolipin-dependent changes in bacterial membrane properties during infection

How does cardiolipin synthase structure and function differ between Salmonella enteritidis PT4 and other bacterial pathogens?

Cardiolipin synthase exhibits important variations across bacterial species that reflect evolutionary adaptations:

• Structural Comparisons:

  • S. enteritidis PT4 possesses three distinct cardiolipin synthases (ClsA, ClsB, ClsC), similar to other Enterobacteriaceae like E. coli

  • This contrasts with Gram-positive bacteria like Bacillus subtilis and Staphylococcus aureus, which typically contain fewer (one or two) cardiolipin synthase homologs

  • The presence of multiple cls genes in Salmonella likely represents functional redundancy and specialization for different environmental conditions

• Substrate Specificity:

  • S. enteritidis ClsA uses two phosphatidylglycerol molecules

  • S. enteritidis ClsC utilizes phosphatidylglycerol and phosphatidylethanolamine

  • S. enteritidis ClsB is uniquely promiscuous, capable of synthesizing additional phospholipids including phosphatidyltrehalose

  • These substrate preferences differ from other bacterial species, reflecting metabolic adaptations

• Evolutionary Significance:

  • Phylogenetic analysis of Salmonella enterica reveals distinct lineages and evidence of recombination in the evolution of these genes

  • The relative age of S. enterica lineages correlates with varying patterns of mutation and recombination events in their genomes:

  Lineage	Age relative to TMRCA	Mutation events	Recombination events	Substitutions by recombination	Relative frequency of rec/mut	Relative effect of rec/mut
  Lineage 1	0.15	624	48	122	0.08	0.20
  Lineage 2	0.2	467	178	1013	0.38	2.17
  Lineage 3	0.66	1879	1140	5551	0.61	2.95
  Lineage 4	0.23	736	144	604	0.20	0.82
  Lineage 5	0.08	192	14	28	0.07	0.15

• Methodological Approaches for Comparative Studies:

  • Whole genome sequencing and comparative genomics

  • Protein structure prediction and modeling

  • Heterologous expression and biochemical characterization of cls enzymes from different species

  • Creation of chimeric enzymes to study domain functions

  • Evolutionary analysis through phylogenetic tree construction

How has genomic analysis contributed to our understanding of Salmonella enteritidis PT4 virulence factors, including cardiolipin synthesis?

Genomic analysis has significantly enhanced our understanding of S. enteritidis PT4 virulence mechanisms:

• Genome Characterization:

  • Complete genome sequence of S. enteritidis PT4 strain P125109 has been determined (EMBL accession no. AM933172)

  • Comparative analysis with other Salmonella strains reveals extensive core genome similarity (>90% of coding sequences form an extensive core gene-set)

  • Average nucleotide identity between shared orthologs with S. Typhimurium LT2 is 98.98%

• Virulence Factors Identified:

  • S. enteritidis PT4 contains multiple Salmonella Pathogenicity Islands (SPIs)

  • 13 fimbrial clusters have been identified, 10 of which are highly conserved with S. Typhimurium LT2

  • Unique fimbrial clusters include peg, ste, and stj, which may contribute to host specificity and virulence

  • Prophage-related elements carry genes encoding type three secretion system effector proteins

• Cardiolipin Synthesis in Genomic Context:

  • Cls genes are part of the core genome of Salmonella enterica

  • Genomic analysis reveals the evolutionary conservation of these genes, suggesting their fundamental importance

  • The presence of three functionally overlapping cls genes reflects the importance of cardiolipin synthesis for bacterial fitness and survival

• Methodological Approaches in Genomic Analysis:

  • Whole genome sequencing and annotation

  • Comparative genomics with related Salmonella strains

  • Core genome multilocus sequence typing (cgMLST)

  • Analysis of horizontally acquired elements and recombination events

  • Transcriptomic analysis to identify gene expression patterns during infection

What are the key considerations for designing experiments to investigate the immunological effects of recombinant cardiolipin synthase?

When investigating immunological effects of recombinant cardiolipin synthase from S. enteritidis PT4:

• Protein Preparation Considerations:

  • Ensure high purity (>90% as determined by SDS-PAGE)

  • Remove endotoxin contamination, which could confound immunological studies

  • Verify protein folding and activity before immunological testing

  • Consider both His-tagged and tag-free versions to control for tag effects

• Immunization Protocol Design:

  • Animal model selection (mice are commonly used)

  • Adjuvant selection (alum, Freund's, etc.)

  • Immunization schedule (typically days 0, 28, and 56)

  • Route of administration (subcutaneous immunization is effective for flagellin-based immunogens)

• Immune Response Assessment:

  • Measure antibody production (IgG, IgM, IgA) by ELISA

  • Evaluate T-cell responses via cytokine profiling

  • Test functional antibody activities (opsonophagocytosis, bacterial killing)

  • Assess protection in challenge models

• Control Groups:

  • Include adjuvant-only controls

  • Use irrelevant proteins of similar size and preparation as negative controls

  • Include known immunogens from S. enteritidis as positive controls

  • Consider using heat-inactivated enzyme to distinguish structural from enzymatic effects

• Potential Applications:

  • Development of ELISA-based detection systems for S. enteritidis

  • Investigation of cardiolipin synthase as a potential vaccine component

  • Study of host immune recognition of bacterial enzymes

What challenges might researchers encounter when studying cardiolipin synthesis in Salmonella enteritidis, and how can these be addressed?

Researchers face several technical challenges when studying cardiolipin synthesis:

• Membrane Protein Expression Challenges:

  • Problem: Cardiolipin synthases are membrane proteins that often express poorly or form inclusion bodies

  • Solution: Optimize expression conditions (lower temperature, specialized E. coli strains), use membrane-protein-specific vectors, consider cell-free expression systems

• Functional Redundancy Issues:

  • Problem: The three cls genes have overlapping functions, complicating phenotype analysis

  • Solution: Generate single, double, and triple knockouts; use conditional expression systems; perform careful phenotypic characterization under various growth conditions

• Lipid Analysis Difficulties:

  • Problem: Accurate quantification of cardiolipin and related phospholipids is technically challenging

  • Solution: Employ multiple complementary techniques (thin-layer chromatography, mass spectrometry); use isotope labeling; develop standardized extraction protocols optimized for acidic phospholipids

• In vivo Relevance Assessment:

  • Problem: Laboratory conditions may not reflect the actual environment during infection

  • Solution: Develop in vitro models that mimic infection conditions (low pH, nutrient limitation, antimicrobial peptides); use cell culture infection models; confirm findings in animal models

• Distinguishing Direct from Indirect Effects:

  • Problem: Phenotypes of cls mutants may result from indirect metabolic or regulatory effects rather than direct loss of cardiolipin

  • Solution: Perform comprehensive lipidomic analysis; use complementation with active vs. inactive enzyme variants; examine effects of exogenous cardiolipin supplementation

• Technical Approaches for Overcoming These Challenges:

  • Develop CRISPR-Cas9 based methods for more precise genetic manipulation

  • Use fluorescent lipid probes for real-time visualization of cardiolipin dynamics

  • Employ chemical biology approaches with cardiolipin synthesis inhibitors

  • Apply systems biology approaches to understand the broader metabolic context of cardiolipin synthesis

What emerging technologies could advance our understanding of cardiolipin synthase function in Salmonella enteritidis pathogenesis?

Several cutting-edge technologies offer exciting opportunities for advancing cardiolipin synthase research:

• CRISPR-Cas9 Gene Editing:

  • Precise genome editing for generating point mutations in cls genes

  • Creation of conditional knockouts to study essential functions

  • Introduction of reporter tags at endogenous loci to monitor expression and localization

• Cryo-Electron Microscopy:

  • Determination of high-resolution structures of cardiolipin synthases

  • Visualization of enzyme-substrate interactions

  • Analysis of cardiolipin distribution in bacterial membranes at nanoscale resolution

• Single-Cell Technologies:

  • Single-cell RNA sequencing to examine heterogeneity in cls gene expression during infection

  • Single-cell metabolomics to assess cell-to-cell variation in cardiolipin synthesis

  • Microfluidic approaches to study individual bacterial responses to environmental stress

• Advanced Imaging Techniques:

  • Super-resolution microscopy to visualize cardiolipin domains in bacterial membranes

  • Correlative light and electron microscopy to link enzyme localization with membrane ultrastructure

  • Live-cell imaging with cardiolipin-specific probes to monitor dynamics during infection

• Systems Biology Approaches:

  • Multi-omics integration (genomics, transcriptomics, proteomics, lipidomics) to build comprehensive models of cardiolipin metabolism

  • Network analysis to identify regulatory interactions affecting cardiolipin synthesis

  • Computational modeling of how cardiolipin affects membrane properties and protein function

How might understanding cardiolipin synthase function contribute to novel antimicrobial strategies against Salmonella enteritidis infections?

The study of cardiolipin synthases offers several promising avenues for antimicrobial development:

• Target-Based Drug Development:

  • Design of specific inhibitors targeting cardiolipin synthases

  • Focus on ClsB as a potential target due to its unique role in synthesizing immunomodulatory lipids

  • Structure-based drug design informed by protein structural studies

• Membrane Disruption Strategies:

  • Development of compounds that interfere with cardiolipin-dependent membrane organization

  • Creation of peptides that specifically bind cardiolipin-rich domains

  • Exploitation of differences between bacterial and mitochondrial cardiolipin

• Immunomodulatory Approaches:

  • Targeting of host-pathogen interactions involving cardiolipin

  • Modification of inflammasome activation pathways influenced by bacterial cardiolipin

  • Development of cardiolipin-based adjuvants for vaccines

• Diagnostic Applications:

  • Generation of antibodies against cardiolipin synthase for detection of S. enteritidis

  • Development of biosensors targeting cardiolipin or its synthases

  • Use of cardiolipin profiles as biomarkers for virulent strains

• Research Methods to Pursue These Strategies:

  • High-throughput screening of compound libraries against recombinant cardiolipin synthases

  • Whole-cell screening to identify compounds affecting cardiolipin synthesis in living bacteria

  • In vivo infection models to evaluate efficacy of targeting strategies

  • Combination therapy approaches testing cardiolipin synthesis inhibitors with conventional antibiotics