Recombinant Salmonella paratyphi B Phosphoglycerol transferase I (mdoB)

What is Phosphoglycerol transferase I (mdoB) and what is its function in bacterial cells?

Phosphoglycerol transferase I, encoded by the mdoB gene, is an enzyme located in the inner cytoplasmic membrane of gram-negative bacteria. In Enterobacteriaceae such as Escherichia coli and Salmonella species, this enzyme catalyzes the transfer of phosphoglycerol residues from phosphatidylglycerol to membrane-derived oligosaccharides (MDOs) or to artificial substrates like arbutin (p-hydroxyphenyl-beta-D-glucoside) in experimental settings . The reaction produces phosphoglycerol diester derivatives of the oligosaccharides and sn-1,2-diglyceride as a byproduct . Importantly, the enzyme has its active site on the outer aspect of the inner membrane, enabling it to interact with substrates in the periplasmic space . The phosphoglycerol modifications of membrane-derived oligosaccharides are considered important for membrane integrity and potentially for bacterial adaptation to environmental stresses.

How does Phosphoglycerol transferase I in Salmonella paratyphi B compare structurally and functionally to its homologs in other Enterobacteriaceae?

While direct structural comparisons between Salmonella paratyphi B Phosphoglycerol transferase I and its homologs in other Enterobacteriaceae require specific crystallographic data not present in the provided resources, functional studies suggest considerable conservation of enzymatic activity. The enzyme in Enterobacteriaceae shares the core function of transferring phosphoglycerol residues to periplasmic oligosaccharides . In E. coli, where it has been more extensively characterized, mutations in the mdoB gene result in membrane-derived oligosaccharides devoid of phosphoglycerol residues, indicating the enzyme's essential role in this modification process . Based on genomic analyses of different Salmonella strains, including S. paratyphi B strains of various sequence types (ST42, ST86, ST2814, and ST135), we can infer that while the core enzyme function is likely conserved, potential variations might contribute to differences in membrane composition and potentially influence aspects of bacterial virulence or stress response .

What role might mdoB play in Salmonella paratyphi B pathogenesis?

The role of mdoB in S. paratyphi B pathogenesis represents an area requiring further investigation, but several hypotheses can be formulated based on current understanding:

• Membrane integrity maintenance: The phosphoglycerol modifications introduced by mdoB may contribute to membrane stability under the stressful conditions encountered during infection.

• Environmental adaptation: S. paratyphi B causes enteric fever and can establish both acute infection and chronic carriage states . The mdoB-mediated modifications might help the bacterium adapt to different host environments.

• Host immune evasion: Modified membrane-derived oligosaccharides could potentially affect the bacterium's surface properties, potentially influencing recognition by host immune factors.

The clinical significance of S. paratyphi B infections is well documented, with cases ranging from mild gastrointestinal symptoms to invasive disease requiring hospitalization . The bacterium has demonstrated person-to-person transmission capabilities, with genetic evidence (whole genome sequencing) confirming transmission links between cases separated by extended time periods, suggesting the importance of its adaptive mechanisms .

What are the optimal expression systems for producing recombinant Salmonella paratyphi B Phosphoglycerol transferase I?

When designing expression systems for recombinant S. paratyphi B Phosphoglycerol transferase I, researchers should consider:

• Host selection: E. coli BL21(DE3) remains a primary choice for membrane protein expression due to its reduced protease activity and compatibility with T7 promoter-based systems. Alternative hosts such as C41(DE3) or C43(DE3), specifically designed for membrane protein expression, may yield better results.

• Vector design: Vectors containing inducible promoters (like T7 or araBAD) allow controlled expression. For membrane proteins like Phosphoglycerol transferase I, fusion tags that aid in protein folding and purification are beneficial:

  • N-terminal His₆-tag for purification

  • Fusion partners such as MBP (maltose-binding protein) to improve solubility

  • Cleavage sites for tag removal post-purification

• Expression conditions: Based on principles of Design of Experiments (DoE), researchers should optimize:

  • Induction temperature (typically lower temperatures like 16-20°C for membrane proteins)

  • Inducer concentration

  • Expression duration

  • Media composition

A factorial experimental design allows for systematic evaluation of these parameters to maximize functional protein yield . The experiment should include validation of enzyme activity using the arbutin transfer assay described in related phosphoglycerol transferase I studies .

What are the best methods for assessing Phosphoglycerol transferase I activity in vitro?

Several complementary approaches can be employed to assess the enzymatic activity of recombinant Phosphoglycerol transferase I:

• Arbutin transfer assay: This established method measures the transfer of phosphoglycerol residues from phosphatidylglycerol to arbutin (p-hydroxyphenyl-beta-D-glucoside), which serves as a model substrate . The reaction generates sn-1,2-diglyceride as a byproduct, which can be quantified.

• Radioisotope labeling: Utilizing ³²P-labeled phosphatidylglycerol as a substrate allows sensitive detection of phosphoglycerol transfer to acceptor molecules.

• Mass spectrometry-based approaches: LC-MS/MS can characterize both the substrates and products of the enzymatic reaction, providing structural insights.

• Fluorescence-based assays: Development of fluorescently labeled substrates may enable real-time monitoring of enzyme kinetics.

For rigorous characterization, a combination of these methods should be employed with appropriate controls:

• Positive control: E. coli Phosphoglycerol transferase I with established activity

• Negative control: Heat-inactivated enzyme

• Substrate controls: Reactions without acceptor molecule

How can researchers apply Design of Experiments (DoE) approaches to optimize expression and purification of recombinant mdoB protein?

Design of Experiments (DoE) offers a systematic framework for optimizing the expression and purification of recombinant mdoB protein with minimal experimental runs. Following the key principles of DoE - randomization, replication, blocking, orthogonality, and factorial experimentation - researchers can:

• Define critical factors affecting expression:

  • Temperature (16°C, 25°C, 37°C)

  • Inducer concentration (e.g., IPTG: 0.1mM, 0.5mM, 1.0mM)

  • Expression time (4h, 8h, overnight)

  • Media composition (LB, TB, M9 minimal)

  • Host strain (BL21(DE3), C41(DE3), Rosetta)

• Select appropriate DoE model: For initial screening, a fractional factorial design allows assessment of multiple factors with fewer experiments. For optimization, response surface methodology (RSM) provides more detailed information on optimal conditions .

• Define response variables:

  • Protein yield (mg/L culture)

  • Enzyme activity (nmol product/min/mg enzyme)

  • Protein purity (%)

  • Membrane incorporation efficiency

• Execute and analyze:

  • Conduct experiments in randomized order

  • Include center points and replicates

  • Use statistical software to analyze results and identify significant factors and interactions

A sample factorial design for optimizing mdoB expression might look like:

This approach allows researchers to identify optimal conditions while understanding interactions between factors, saving significant time and resources while improving reproducibility .

How does mdoB expression differ between antibiotic-resistant and susceptible strains of Salmonella paratyphi B?

Recent investigations into extensively drug-resistant (XDR) Salmonella paratyphi B strains have revealed important insights that may relate to mdoB expression patterns. Four XDR S. paratyphi B ST2814 strains were identified in Jiangsu Province, showing resistance to three major antibiotic classes used against Salmonella . While direct mdoB expression data across resistant and susceptible strains is not explicitly provided in the search results, several relevant observations can guide research:

• Strain variation: Different sequence types of S. paratyphi B (ST42, ST86, ST2814, and ST135) show associations with specific biotypes (Java and sensu stricto), suggesting potential variations in membrane composition genes including mdoB .

• Hypothesized mechanisms: Alterations in membrane composition, potentially involving mdoB-mediated modifications, could affect:

  • Membrane permeability to antibiotics

  • Efflux pump efficiency

  • Cell envelope stress responses

• Research approach: To investigate this relationship, researchers should:

  • Perform comparative transcriptomics/proteomics of mdoB expression in resistant vs. susceptible isolates

  • Generate mdoB knockout mutants in resistant strains to assess impact on minimum inhibitory concentrations (MICs)

  • Examine membrane-derived oligosaccharide profiles in different strain backgrounds

The emergence of XDR strains represents a significant clinical concern, particularly as S. paratyphi B can establish chronic carriage states . Understanding the potential contribution of mdoB to this phenotype could inform new therapeutic strategies.

What techniques are most effective for studying the in vivo role of mdoB in Salmonella paratyphi B infections?

Investigating the in vivo role of mdoB in S. paratyphi B infections requires specialized approaches that account for the pathogen's unique biology. Researchers should consider:

• Genetic manipulation strategies:

  • CRISPR-Cas9 genome editing for precise mdoB deletion/modification

  • Complementation studies with wild-type and mutant alleles

  • Conditional expression systems for temporal control

• Infection models:

  • Murine typhoid models, while imperfect for human-adapted pathogens, can provide insights

  • Humanized mouse models with human immune components

  • Ex vivo organ culture systems

  • Cell culture infection models using relevant human cell types

• Analysis techniques:

  • Whole genome sequencing for tracking bacterial evolution during infection

  • Metabolomic profiling to detect infection-specific markers

  • Imaging techniques (electron microscopy, confocal microscopy) to visualize host-pathogen interactions

• Clinical isolate studies:

  • Comparative genomics of outbreak isolates like those described in the English Paratyphoid B cluster

  • Phenotypic characterization of chronic carrier isolates

  • Analysis of bacterial adaptation during person-to-person transmission

The challenge of studying human-restricted pathogens necessitates creative experimental approaches. The documented person-to-person transmission of S. paratyphi B in England, confirmed through whole genome sequencing (showing only 0-5 single-nucleotide polymorphisms between isolates), demonstrates the importance of combining molecular and epidemiological approaches .

How can metabolomic approaches be integrated with mdoB functional studies to better understand Salmonella paratyphi pathogenesis?

Metabolomic approaches offer powerful tools for understanding S. paratyphi pathogenesis when integrated with mdoB functional studies:

• Metabolic signature identification: Metabolomic studies using two-dimensional gas chromatography with time-of-flight mass spectrometry (GCxGC/TOFMS) have successfully distinguished between S. Typhi and S. Paratyphi A infections through unique metabolite profiles . Similar approaches could:

  • Identify metabolic signatures specific to S. paratyphi B infection

  • Compare wild-type and mdoB-mutant strains to detect metabolic consequences

  • Track metabolic changes during infection progression

• Integration of membrane lipid analysis: Since mdoB affects membrane phospholipid composition, specialized lipidomic analyses can:

  • Characterize changes in phosphatidylglycerol and other membrane lipids

  • Correlate lipid alterations with metabolic adaptations

  • Link membrane composition to virulence phenotypes

• Experimental design approach:

  • Apply orthogonal partial least squares-discriminant analysis (OPLS-DA) to differentiate metabolite profiles

  • Use supervised pattern recognition to identify biomarkers associated with mdoB function

  • Develop predictive models based on key metabolite combinations

A prior study examining S. Typhi and S. Paratyphi A infections determined that a combination of just six metabolites could accurately define the etiological agent . This suggests that targeted metabolomic approaches focusing on mdoB-influenced pathways could yield substantial insights with relatively focused analysis.

What are common challenges in purifying active recombinant Phosphoglycerol transferase I and how can they be addressed?

Purifying active recombinant Phosphoglycerol transferase I presents several challenges due to its membrane-associated nature. Common issues and solutions include:

• Protein aggregation/inclusion body formation:

  • Solution: Lower expression temperature (16-20°C), use solubility-enhancing fusion tags (MBP, SUMO), or optimize inducer concentration through DoE approaches

  • Alternative: Develop inclusion body refolding protocols with gradual dialysis against decreasing concentrations of chaotropic agents

• Low expression levels:

  • Solution: Codon optimization for expression host, use of strong promoters with tight regulation, or screening multiple host strains

  • Validation: Western blot analysis to confirm expression before proceeding to purification

• Loss of activity during purification:

  • Solution: Include appropriate phospholipids in purification buffers to stabilize the protein

  • Strategy: Rapid purification protocols minimizing time at room temperature

  • Additives: Glycerol (10-20%), reducing agents, and protease inhibitors

• Membrane extraction challenges:

  • Solution: Screen detergents (DDM, LDAO, digitonin) for optimal extraction efficiency while maintaining activity

  • Approach: Use detergent screening kits to identify conditions that maintain enzyme stability

• Activity verification:

  • Challenge: Standard activity assays may be difficult to adapt to purified recombinant enzyme

  • Solution: Develop miniaturized arbutin transfer assays compatible with purified enzyme preparations

  • Control: Always compare to crude membrane preparations with known activity

The approaches should be systematically tested using DoE principles to efficiently identify optimal conditions across multiple variables simultaneously .

How should researchers address contradictory results between in vitro studies of mdoB and observations in clinical S. paratyphi B isolates?

Addressing contradictions between in vitro mdoB studies and clinical observations requires a systematic approach:

• Identify the specific contradictions:

  • Enzyme activity vs. phenotypic outcomes

  • Gene expression levels vs. functional impact

  • In vitro growth characteristics vs. in vivo behavior

• Consider strain differences:

  • Different S. paratyphi B sequence types (ST42, ST86, ST2814, ST135) may exhibit variable mdoB expression or function

  • Java biotype vs. sensu stricto biotype differences may influence experimental outcomes

  • Clinical isolates may have acquired compensatory mutations

• Methodological approaches to resolve contradictions:

  • Whole genome sequencing of clinical isolates to identify potential modifying factors

  • Complementation studies in defined genetic backgrounds

  • Creation of isogenic strains differing only in mdoB alleles

  • Transcriptomic analysis under various environmental conditions

• Contextual factors:

  • Host environment effects on gene expression

  • Interaction with other bacterial factors in clinical settings

  • Strain adaptation during chronic carriage states

The documented transmission of S. paratyphi B between cases in England with minimal genetic changes (0-5 SNPs) demonstrates how bacterial factors, potentially including mdoB-related functions, can remain stable during person-to-person transmission while still allowing adaptation to new hosts .

What bioinformatic tools are most valuable for analyzing mdoB sequence variations across Salmonella strains?

For comprehensive analysis of mdoB sequence variations across Salmonella strains, researchers should utilize:

• Comparative sequence analysis tools:

  • BLAST and DIAMOND for basic sequence similarity searches

  • MEGA X for phylogenetic analysis and evolutionary rate calculations

  • Clustal Omega or MUSCLE for multiple sequence alignments

  • JalView for visualization and analysis of sequence conservation

• Structural prediction and analysis:

  • AlphaFold2 or RoseTTAFold for protein structure prediction

  • PyMOL or UCSF Chimera for structural visualization and comparison

  • ConSurf for mapping sequence conservation onto predicted structures

  • CAVER for prediction of substrate tunnels and channels

• Whole genome analysis approaches:

  • SnapperDB for single-nucleotide polymorphism (SNP) analysis

  • Salmonella eBURST groups and sequence typing databases for strain classification

  • Pairwise SNP analysis with single-linkage hierarchical clustering for population structure assessment

• Functional impact prediction:

  • PROVEAN or SIFT for assessing functional impact of amino acid substitutions

  • I-TASSER for enzyme function prediction based on structure

  • MetaPocket for ligand binding site prediction

When analyzing S. paratyphi B isolates, these tools have proven valuable for tracking transmission and population structure. For example, pairwise SNP analysis of 93 S. paratyphi B isolates enabled researchers to identify related strains with as few as 0-5 SNPs difference, confirming person-to-person transmission in a cluster of cases . Similarly, sequence type determination helped categorize S. paratyphi B strains into distinct lineages (ST42, ST86, ST2814, and ST135) with specific biotype associations .

What emerging technologies could advance our understanding of mdoB function in S. paratyphi B?

Several cutting-edge technologies have the potential to significantly advance our understanding of mdoB function in S. paratyphi B:

• CRISPR interference (CRISPRi) and CRISPR activation (CRISPRa):

  • Enables precise temporal control of mdoB expression without permanent genetic modification

  • Allows titration of expression levels to determine threshold effects

  • Can be applied in infection models to study dynamic regulation

• Advanced imaging techniques:

  • Cryo-electron microscopy (cryo-EM) for high-resolution structural analysis of the enzyme

  • Super-resolution microscopy to visualize membrane localization and dynamics

  • Live-cell imaging with fluorescent reporters to track enzyme activity in real-time

• Single-cell technologies:

  • Single-cell RNA-seq to examine heterogeneity in mdoB expression within bacterial populations

  • Mass cytometry (CyTOF) to correlate mdoB expression with other cellular markers

  • Microfluidics platforms for studying single-cell behavior under controlled conditions

• Metabolomic integration:

  • Building on established metabolomic approaches that have successfully distinguished between Salmonella serovars

  • Development of targeted metabolomic assays specific to phosphoglycerol transferase activity

  • Spatial metabolomics to map metabolite distributions within infected tissues

• Synthetic biology approaches:

  • Engineering orthogonal phosphoglycerol transferase systems with novel specificities

  • Creation of biosensors for real-time monitoring of enzyme activity

  • Development of tunable expression systems for precise control of mdoB levels

These technologies, particularly when applied in combination, could resolve outstanding questions about how mdoB contributes to S. paratyphi B pathogenesis and potentially identify new targets for therapeutic intervention.

How might mdoB function contribute to the documented person-to-person transmission of S. paratyphi B?

The potential contribution of mdoB function to person-to-person transmission of S. paratyphi B represents an intriguing research direction:

• Environmental persistence mechanisms:

  • Membrane-derived oligosaccharide modifications by mdoB may enhance bacterial survival outside the host

  • Phosphoglycerol modifications could stabilize bacterial membranes under desiccation or osmotic stress

  • These adaptations may facilitate indirect transmission through contaminated environments

• Host adaptation during chronic carriage:

  • Documented cases of person-to-person transmission in England involved a primary case with chronic carriage

  • mdoB-mediated membrane modifications may contribute to establishing and maintaining chronic infection states

  • Alterations in membrane composition could reduce recognition by host immune system

• Virulence modulation:

  • Membrane modifications may regulate expression or function of virulence factors

  • Adaptation of membrane properties during host transition could optimize infectivity

• Research approach:

  • Compare mdoB sequence and expression between initial and secondary cases in transmission chains

  • Assess membrane-derived oligosaccharide profiles in isolates from chronic carriers

  • Develop animal models to test transmission efficiency of wild-type vs. mdoB mutants

The documented transmission cluster in England, where whole genome sequencing confirmed close genetic relationships (0-5 SNPs) between cases separated by a year, provides a valuable model for studying how bacterial factors, potentially including mdoB function, contribute to transmission dynamics .

What are the implications of mdoB function for developing new strategies against extensively drug-resistant S. paratyphi B strains?

The emergence of extensively drug-resistant (XDR) S. paratyphi B strains creates an urgent need for novel therapeutic approaches. The function of mdoB offers several potential avenues for intervention:

• Direct enzyme inhibition strategies:

  • Development of small molecule inhibitors targeting the phosphoglycerol transferase active site

  • Structure-based drug design utilizing predicted or experimentally determined enzyme structures

  • Screening of natural product libraries for inhibitory compounds

• Membrane vulnerability exploitation:

  • mdoB mutants may exhibit altered membrane properties that could be targeted by novel antimicrobials

  • Combination therapies pairing mdoB inhibitors with existing antibiotics may overcome resistance

  • Development of antimicrobial peptides specifically designed to interact with membranes lacking phosphoglycerol modifications

• Diagnostic applications:

  • Metabolomic profiles related to mdoB activity could serve as biomarkers for infection

  • Rapid identification of XDR strains through detection of specific membrane characteristics

  • Development of targeted diagnostic tools for epidemiological surveillance

• Research priorities:

  • Comparative analysis of membrane composition in XDR vs. susceptible strains

  • Screening for synergistic effects between mdoB inhibition and conventional antibiotics

  • Investigation of cross-resistance patterns in relation to membrane modifications

The alarming spread of XDR S. paratyphi B ST2814 strains, which show resistance to all three major antibiotic classes used against Salmonella , underscores the urgency of developing alternative therapeutic approaches. Understanding and targeting mdoB function represents one promising direction that merits further investigation.

What are the key knowledge gaps in our understanding of S. paratyphi B Phosphoglycerol transferase I that should be prioritized?

Despite the available information on phosphoglycerol transferase I and S. paratyphi B biology, several critical knowledge gaps remain that warrant prioritized research attention:

• Structural characterization: The three-dimensional structure of S. paratyphi B Phosphoglycerol transferase I remains undetermined, limiting structure-based drug design efforts and mechanistic understanding.

• Regulatory networks: The environmental and genetic factors controlling mdoB expression in S. paratyphi B during infection and transmission are poorly understood.

• Biotype-specific variations: The functional differences in mdoB between Java and sensu stricto biotypes and their implications for pathogenesis require further investigation.

• Host-pathogen interactions: How mdoB-mediated membrane modifications influence host immune recognition and bacterial persistence during chronic carriage needs clarification.

• Antimicrobial resistance links: The potential relationship between mdoB function and the emergence of extensively drug-resistant strains represents a critical area for investigation.

• Metabolomic signatures: While metabolomic approaches have successfully differentiated S. Typhi and S. Paratyphi A infections , equivalent profiles for S. paratyphi B remain to be established.

Addressing these knowledge gaps through integrated research approaches will advance both fundamental understanding of bacterial membrane biology and potential applications in diagnosis and treatment of S. paratyphi B infections.

What methodological approaches should researchers prioritize when studying recombinant S. paratyphi B Phosphoglycerol transferase I?

Researchers investigating recombinant S. paratyphi B Phosphoglycerol transferase I should prioritize the following methodological approaches:

• Integrated structural biology:

  • Combine X-ray crystallography, cryo-EM, and computational modeling for comprehensive structural characterization

  • Implement molecular dynamics simulations to understand enzyme dynamics in membrane environments

  • Apply hydrogen-deuterium exchange mass spectrometry to identify substrate binding sites

• Systematic mutagenesis:

  • Use alanine scanning and site-directed mutagenesis to identify catalytic residues

  • Generate chimeric enzymes between different species to identify determinants of specificity

  • Apply deep mutational scanning to comprehensively map sequence-function relationships

• Design of Experiments optimization:

  • Implement DoE principles for efficient optimization of expression and purification conditions

  • Develop fractional factorial designs to screen multiple variables simultaneously

  • Apply response surface methodology for fine-tuning optimal conditions

• Advanced biochemical characterization:

  • Develop real-time activity assays to determine enzyme kinetics

  • Implement isothermal titration calorimetry for thermodynamic analysis of substrate binding

  • Use native mass spectrometry to analyze protein-ligand interactions

• Translational approaches:

  • Screen chemical libraries for potential inhibitors

  • Assess enzyme activity in clinically relevant environments

  • Develop high-throughput screening platforms for inhibitor discovery

These approaches should be implemented within a framework that connects biochemical findings to physiological relevance, particularly in the context of S. paratyphi B pathogenesis and the emerging threat of extensively drug-resistant strains .

How should the scientific community coordinate efforts to address the emerging threat of extensively drug-resistant S. paratyphi B strains?

Addressing the emerging threat of extensively drug-resistant (XDR) S. paratyphi B strains requires coordinated scientific efforts across multiple disciplines:

• Global surveillance networks:

  • Implement standardized whole genome sequencing of clinical isolates

  • Establish databases for sharing sequence data and antimicrobial susceptibility profiles

  • Develop early warning systems for detecting novel resistance patterns

• Collaborative research initiatives:

  • Form international consortia focused on S. paratyphi B

  • Establish biobanks of well-characterized clinical isolates

  • Coordinate funding for target-based drug discovery efforts

• Methodological standardization:

  • Develop consensus protocols for susceptibility testing

  • Standardize metadata collection for clinical and environmental isolates

  • Establish common bioinformatic pipelines for sequence analysis

• Interdisciplinary approaches:

  • Integrate structural biology, medicinal chemistry, and clinical microbiology

  • Apply metabolomic techniques that have proven successful in differentiating Salmonella infections

  • Combine epidemiological data with molecular characterization

• Translational research priorities:

  • Develop rapid diagnostic tools for XDR strain identification

  • Establish alternative treatment protocols for XDR infections

  • Investigate combination therapies targeting multiple bacterial systems