Genetic and pharmacological inactivation of peptidoglycan remodeling increases antibiotic susceptibility of vancomycin-resistant Enterococcus faecium

This study demonstrates that both genetic deletion and pharmacological inhibition of the peptidoglycan hydrolase SagA impair cell wall remodeling in vancomycin-resistant *Enterococcus faecium*, thereby restoring its susceptibility to vancomycin.

The Gist

The Problem: The Unbreakable Fortress

Imagine a bacterial infection caused by Vancomycin-Resistant Enterococcus faecium (VREfm). Think of this bacteria as a fortress with incredibly thick, reinforced walls.

For decades, doctors have used a "super-antibiotic" called Vancomycin to break these walls down. It works like a sledgehammer, smashing the bricks (peptidoglycan) that hold the fortress together. However, VREfm has evolved a clever trick: it constantly repairs its walls faster than the sledgehammer can break them. It also changes the mortar (the chemical structure of the wall) so the sledgehammer slips right off. This makes the bacteria nearly impossible to kill, leading to deadly infections.

The Discovery: The "Repair Crew"

The scientists in this study discovered a specific "repair crew" inside the bacteria called SagA.

• The Analogy: Imagine the bacteria is a house being built. The walls are being constructed, but they are messy and need constant trimming and smoothing. SagA is the foreman who runs around with a pair of scissors, cutting away the excess mortar and smoothing the bricks so the wall is perfect and strong.
• The Twist: The researchers realized that this "repair crew" is actually helping the bacteria survive the sledgehammer (Vancomycin). By keeping the walls perfectly smooth and organized, SagA makes it harder for the antibiotic to find a weak spot to hit.

The Solution: Two Ways to Stop the Repair Crew

The team found two ways to disable this repair crew, making the bacteria vulnerable again.

1. The Genetic "Fire" (Deleting the Crew)

First, they used genetic engineering to literally fire the SagA foreman. They created a version of the bacteria without the sagA gene.

• What happened: Without SagA, the bacterial walls became messy, lumpy, and poorly constructed. The bricks didn't fit together right.
• The Result: When they hit these messy walls with Vancomycin, the sledgehammer worked perfectly. The antibiotic could now stick to the rough surfaces and smash the bacteria. The bacteria couldn't repair themselves fast enough, and they died.

2. The Chemical "Glue" (The New Drug)

Firing the crew is great for a lab, but you can't genetically rewire a bacteria inside a sick patient. So, the scientists needed a chemical weapon to stop SagA in action.

They screened thousands of chemical compounds and found a special type called Sulfonyl Fluorides.

• The Analogy: Imagine the SagA foreman is holding a pair of scissors. The scientists found a chemical "super-glue" (a drug they named pghi-4) that, when it touches the scissors, instantly fuses the blades together. The scissors are now stuck and useless.
• The Result: When they added this glue-drug to the bacteria, the SagA foreman couldn't cut or smooth the walls anymore. The walls became messy and weak, just like in the genetic experiment.
• The Combo Attack: When they used the glue-drug alongside the Vancomycin sledgehammer, the bacteria were crushed. The drug stopped the repairs, and the antibiotic finished the job.

Why This Matters

This is a game-changer for two reasons:

1. It's a "Key" to Old Locks: Instead of trying to invent a brand new antibiotic (which takes 10+ years), they found a way to make our existing best antibiotic (Vancomycin) work again against resistant bacteria. It's like finding a way to make an old key fit a new, locked door.
2. It Works on Many Strains: They tested this on many different types of VREfm bacteria found in hospitals, and the "glue-drug" worked on almost all of them.

The Bottom Line

The bacteria were winning because they were too good at fixing their own walls. The scientists found a way to jam the repair tools (SagA). Once the repair tools are jammed, the bacteria's walls fall apart, and the old antibiotics can kill them again.

It's like realizing that to stop a master builder from building an impenetrable castle, you don't need a bigger hammer; you just need to glue their trowel shut.

Technical Summary

1. Problem Statement

Vancomycin-resistant Enterococcus faecium (VREfm) is a critical "ESKAPE" pathogen responsible for severe healthcare-associated infections with high mortality rates. VREfm strains often acquire resistance to last-resort antibiotics like linezolid, daptomycin, and tigecycline. The emergence of multidrug-resistant VREfm necessitates novel therapeutic strategies beyond the discovery of new antibiotics. Current approaches often focus on direct killing, but there is a need for antibiotic adjuvants—compounds that do not necessarily kill bacteria on their own but restore the efficacy of existing antibiotics by targeting bacterial vulnerabilities or resistance mechanisms.

2. Methodology

The study employed a multi-disciplinary approach combining genetics, structural biology, chemical biology, and in vivo infection models:

• Genetic Inactivation: The researchers generated isogenic deletion mutants ($\Delta$sagA) and chromosomal complementation strains ($\Delta$sagA::sagA) in the VREfm clinical isolate ERV165 (ST412, vanA genotype) using CRISPR-Cas12a-mediated recombineering. They also deleted other NlpC/P60 hydrolases (pgh2, pgh3, pgh4) to determine specificity.
• Phenotypic Characterization:
  • Growth & Morphology: Analyzed colony morphology, growth curves, and cell separation using Differential Interference Contrast (DIC) microscopy and Transmission Electron Microscopy (TEM).
  • Ultrastructure: Utilized Cryo-Electron Tomography (Cryo-ET) to measure cell wall and septum thickness.
  • Peptidoglycan Analysis: Performed LC-MS analysis of mutanolysin-digested peptidoglycan to quantify muropeptide composition (cross-linked vs. non-cross-linked).
  • Antibiotic Susceptibility: Determined Minimum Inhibitory Concentrations (MICs) for vancomycin and other antibiotics (ampicillin, daptomycin, ceftriaxone) in both BHI and Mueller Hinton Broth (MHB).
• Chemical Biology & Drug Discovery:
  • Screening: Conducted a high-throughput screen of a library of SuFEx-based (Sulfur(VI) Fluoride Exchange) sulfonyl fluorides using a competitive fluorescence polarization (FP) assay targeting the active site cysteine (C433) of SagA.
  • Mechanism Validation: Used gel-based competitive labeling, intact protein mass spectrometry (MS), and computational covalent docking to confirm the mode of action of lead inhibitors.
  • Chemoproteomics: Employed competitive cysteine-directed chemoproteomics to assess the selectivity of the lead inhibitor (pghi-4) against the VREfm proteome.
• In Vivo Models:
  • Mouse Peritonitis: Evaluated the therapeutic efficacy of genetic deletion and pharmacological inhibition combined with vancomycin in a mouse sepsis model.
  • Macrophage Infection: Assessed the ability of the combination therapy to clear intracellular VREfm in RAW264.7 and THP-1 macrophages.

3. Key Contributions

1. Identification of SagA as a Critical Target: Demonstrated that Secreted Antigen A (SagA), a highly conserved NlpC/P60 peptidoglycan hydrolase, is essential for peptidoglycan remodeling and cell separation in VREfm, distinct from other redundant hydrolases.
2. Discovery of First-in-Class Inhibitors: Identified and characterized $\beta$-chloro alkenyl sulfonyl fluorides (specifically pghi-4) as the first covalent inhibitors of NlpC/P60 peptidoglycan hydrolases.
3. Mechanism of Adjuvant Action: Elucidated that inhibiting SagA prevents the remodeling of peptidoglycan, leading to the accumulation of D-Ala-D-Ala-containing muropeptides (the native target of vancomycin), thereby overcoming the D-Ala-D-Lac modification that confers vancomycin resistance.
4. Therapeutic Validation: Proved that pharmacological inhibition of SagA acts as a potent adjuvant, significantly lowering the MIC of vancomycin and improving survival in VREfm-infected mice.

4. Key Results

• Genetic Inactivation ($\Delta$sagA):

  • Deletion of sagA in VREfm caused severe growth defects, impaired cell separation (clustering), and aberrant cell wall ultrastructure (thinner cell wall, thicker septa).
  • Vancomycin Susceptibility: The $\Delta$sagA mutant showed a 2-fold decrease in vancomycin MIC (from 256 to 128 $\mu$g/mL) compared to wild-type, despite retaining the vanA operon. This was not observed for other NlpC/P60 deletions.
  • In Vivo: In a mouse peritonitis model, vancomycin treatment successfully cleared the bacterial burden and reduced weight loss in mice infected with $\Delta$sagA, whereas it failed against wild-type VREfm.

• Pharmacological Inhibition (pghi-4):

  • Inhibition: pghi-4 covalently modifies the catalytic cysteine (C433) of SagA via SuFEx chemistry, forming a thiosulfonate adduct. It also inhibits PGH2, a homologous hydrolase.
  • Synergy: pghi-4 alone had minimal bactericidal activity but acted as a potent adjuvant. When combined with sub-inhibitory vancomycin, it reduced the vancomycin MIC by up to 8-fold in a concentration-dependent manner.
  • Mechanism: Treatment with pghi-4 + vancomycin increased the accumulation of D-Ala-D-Ala-containing peptidoglycan fragments and enhanced vancomycin binding (measured by Van-BODIPY fluorescence).
  • Selectivity: Chemoproteomics confirmed pghi-4 primarily targets SagA and PGH2 without significant off-target effects on other essential cysteine proteins.

• Clinical Relevance:

  • The adjuvant effect of pghi-4 was observed across genetically distinct VREfm clinical isolates (different sequence types and vanA/vanB genotypes).
  • The efficacy correlated with the intracellular expression levels of SagA in these strains.
  • In Vivo Efficacy: A two-dose regimen of pghi-4 + vancomycin significantly reduced bacterial burden in the spleen and liver and improved survival in mice with VREfm-induced sepsis, whereas monotherapies were ineffective.

5. Significance

This study provides a proof-of-concept that peptidoglycan remodeling enzymes are druggable targets for antibiotic adjuvants. By inhibiting SagA, the study effectively "re-sensitizes" vancomycin-resistant bacteria to vancomycin. This approach offers a promising strategy to extend the clinical lifespan of existing last-resort antibiotics against multidrug-resistant Gram-positive pathogens. The discovery of $\beta$-chloro alkenyl sulfonyl fluorides as a new class of covalent inhibitors opens avenues for developing broad-spectrum adjuvants targeting the NlpC/P60 family of hydrolases.