Lysosomal activation in bladder epithelium enhances intracellular antibiotic clearance of uropathogenic Escherichia coli

This study demonstrates that the oral bacterial lysate OM-89 enhances intracellular clearance of uropathogenic *E. coli* in bladder epithelium by promoting lysosomal acidification and protease activity, thereby synergizing with antibiotics to reduce bacterial regrowth and offering a promising host-directed strategy for managing recurrent urinary tract infections.

The Gist

Imagine your bladder as a high-security castle. The walls are made of a special layer of cells called the urothelium. Normally, these walls are tough, but sometimes a sneaky invader called UPEC (a type of E. coli bacteria) can sneak inside the bricks of the wall itself.

Once inside the bricks, the bacteria hide in little "safe rooms" (intracellular niches). Here's the problem: when you take antibiotics, the medicine flows through the castle's moat (your urine) and hits the outside of the walls, but it struggles to get deep inside the bricks to kill the hidden bacteria. The bacteria wait for the medicine to wear off, then they burst out, causing the infection to come back. This is why urinary tract infections (UTIs) often keep coming back.

This paper investigates a clever solution: a therapy called OM-89 (sold as Uro-Vaxom), which is basically a "training manual" made from dead bacteria that you swallow. The researchers wanted to know: How does this training manual help the castle walls fight back?

Here is the breakdown of their discovery, using some everyday analogies:

1. The Problem: The "Safe House"

Think of the bladder cells as a fortress. When UPEC bacteria invade, they don't just sit on the surface; they hide inside the cells. Inside the cell, they find a "safe house" (a compartment that isn't acidic or destructive).

• The Antibiotic Issue: Most antibiotics are like rain that washes over the castle walls. They can't easily penetrate the bricks to reach the safe houses inside.
• The Bacteria's Trick: The bacteria are smart; they can turn off the cell's internal "trash compactors" (lysosomes) so they don't get eaten.

2. The Solution: OM-89 as a "Bodyguard Trainer"

The researchers found that OM-89 doesn't kill the bacteria directly. Instead, it goes into the bladder cells and acts like a personal trainer for the cell's immune system. It wakes up the cell and tells it, "Hey, we have intruders! Get ready!"

3. The Magic Mechanism: Turning on the "Trash Compactors"

The study discovered that OM-89 does three specific things to the bladder cells:

• It builds more "Trash Cans" (Lysosomes): The cell starts making more lysosomes. Think of these as the cell's internal garbage disposals or trash compactors.
• It makes the "Trash Cans" acidic: Normally, a trash compactor needs to be very acidic (like a chemical bath) to dissolve garbage. The bacteria usually try to keep the trash can neutral so they survive. OM-89 forces the trash can to become super acidic again.
• It opens the "Delivery Door": This is the most surprising part. OM-89 also changes the cell's doorways. It makes it much easier for the antibiotic "rain" to get inside the bricks.

4. The Result: A Double-Whammy Attack

When you combine OM-89 with standard antibiotics, you get a powerful one-two punch:

1. The Cell's Own Weapon: Because the "trash cans" are now acidic and active, the bacteria hiding inside get digested by the cell itself.
2. The Medicine's Boost: Because the cell's "doors" are wide open, the antibiotic can finally get inside the bricks where the bacteria are hiding and finish the job.

5. The "Regrowth" Prevention

The study showed that without OM-89, even after antibiotics kill the visible bacteria, the few survivors hiding in the walls would wake up later and cause a new infection (regrowth). But with OM-89, the bacteria are cleared out so thoroughly that they can't come back. It's like not just putting out the fire, but also removing all the embers so the fire can't restart.

The Big Picture

This research is a game-changer because it changes how we view the bladder.

• Old View: The bladder is just a passive container that gets infected.
• New View: The bladder is an active fighter.

By using OM-89, we aren't just trying to kill the bacteria with stronger poison; we are boosting the bladder's own natural defenses. It turns the bladder wall from a passive hiding spot for bacteria into an active, aggressive zone that hunts down and destroys them.

In short: OM-89 teaches the bladder cells to stop being a hiding spot for germs and start being a fortress that actively digests them, making your antibiotics work much better. This could mean fewer UTIs, shorter treatments, and less need for heavy antibiotics in the future.

Technical Summary

1. Problem Statement

Recurrent urinary tract infections (rUTIs) are a significant clinical burden, primarily driven by Uropathogenic Escherichia coli (UPEC). UPEC possesses a unique virulence mechanism allowing it to invade bladder epithelial cells and establish protected intracellular niches (Intracellular Bacterial Communities and quiescent reservoirs). These niches shield bacteria from both host immune responses and systemic antibiotics, which often fail to accumulate effectively within host cells. Consequently, standard antibiotic therapies frequently fail to eradicate the infection completely, leading to relapse and the acceleration of antimicrobial resistance. While OM-89 (Uro-Vaxom®), a bacterial lysate derived from 18 E. coli strains, has been clinically used for decades to prevent rUTIs, its precise cellular mechanism of action remains poorly understood, particularly regarding its interaction with the bladder epithelium itself rather than just systemic immune modulation.

2. Methodology

The study employed a multi-faceted approach using both murine and human models to dissect the mechanism of OM-89:

• In Vitro Models:
  • Mouse Bladder Organoids (mBOs): Generated from C57BL/6 mice, used for 3D infection dynamics and microinjection assays.
  • Human Bladder Organoids (hBOs): Derived from primary human bladder epithelial cells (H-6215), differentiated into stratified urothelium (basal, intermediate, and umbrella cells) to recapitulate human tissue architecture.
  • Differentiated Monolayers: Mouse and human bladder epithelial cells cultured to form differentiated layers expressing specific cytokeratins (CK5, CK13, CK20) and uroplakins.
• Bacterial Strains: Used well-characterized UPEC strains (CFT073, UTI89, J96) and primary clinical isolates (721536-18, 721402-18), all engineered to stably express sfGFP for real-time fluorescence tracking.
• Treatment Regimens: OM-89 was applied via three protocols: pre-application (72h before infection), co-application (during antibiotic treatment), and continuous application (72h pre-infection + throughout).
• Assays:
  • Microinjection & Time-Lapse Microscopy: Direct injection of bacteria into organoids followed by live imaging to quantify bacterial growth, antibiotic killing, and post-treatment regrowth.
  • Transcriptomics (RNA-seq): Performed on infected organoids to identify gene expression changes and pathway enrichment (GSEA).
  • Immunofluorescence & Confocal Microscopy: Quantification of vesicle markers (Rab11A, Rab27B, Rab7, Lamp1, p62, LC3b), lysosomal pH (LysoSensor), and protease activity (Cathepsin L).
  • Antibiotic Uptake: Quantification of intracellular accumulation of fluorescently labeled ampicillin and gentamicin.
  • Pharmacological Inhibition: Use of Bafilomycin A1 (BafA1) and Chloroquine (CQ) to block lysosomal acidification and test mechanistic dependency.

3. Key Contributions

• Mechanistic Elucidation of OM-89: The study identifies that OM-89 acts directly on the bladder epithelium to remodel endo-lysosomal networks, rather than solely relying on systemic immune activation.
• Host-Directed Therapy Validation: It demonstrates that pharmacological reinforcement of epithelial intrinsic antimicrobial pathways (specifically lysosomal function) can overcome the limitations of current antibiotics against intracellular pathogens.
• Cross-Species Conservation: The findings are validated in both murine and human organoid systems, establishing translational relevance for clinical application.
• Synergy with Antibiotics: The paper provides evidence that OM-89 enhances the efficacy of multiple antibiotic classes (β-lactams, fosfomycin, TMP-SMX) by increasing intracellular drug accumulation.

4. Key Results

A. Enhanced Bacterial Clearance and Reduced Regrowth

• OM-89 treatment significantly reduced UPEC regrowth following antibiotic withdrawal in both mouse and human organoids.
• This effect was most pronounced when OM-89 was present during antibiotic treatment (co-application or continuous application).
• OM-89 did not reduce initial bacterial loads during the first 4 hours of infection, indicating it does not have direct bactericidal activity but rather enhances the host's ability to clear bacteria during antibiotic therapy.

B. Remodeling of Lysosomal and Autophagic Pathways

• Transcriptomics: RNA-seq revealed that OM-89 treatment during infection strongly upregulated lysosomal gene signatures (KEGG_LYSOSOME) and autophagy-related pathways, distinct from the inflammatory response seen in untreated infections.
• Vesicle Dynamics: OM-89 increased the number and size of Rab7-positive late endosomes and Lamp1-positive lysosomes.
• Autophagic Flux: In mouse cells, OM-89 restored autophagic flux (accumulation of LC3b-II upon BafA1 treatment) which is typically stalled by UPEC. In human cells, while autophagic flux restoration was not observed, lysosomal expansion and acidification were conserved.

C. Lysosomal Acidification and Protease Activation

• OM-89 treatment led to a significant decrease in intracellular pH (increased acidification) of vesicles.
• It increased the activity of Cathepsin L (a lysosomal protease) and promoted the colocalization of intracellular UPEC with Cathepsin L-positive compartments.
• Mechanistic Proof: Pharmacological inhibition of acidification using BafA1 or Chloroquine abolished the protective effect of OM-89, restoring bacterial regrowth to control levels. This confirms that lysosomal acidification is essential for OM-89-mediated clearance.

D. Enhanced Intracellular Antibiotic Accumulation

• OM-89 treatment significantly increased the intracellular uptake of fluorescently labeled ampicillin and gentamicin in bladder epithelial cells.
• This uptake was observed across different antibiotic classes and was most prominent in terminally differentiated umbrella cells (CK20+).
• The increased antibiotic accumulation correlated with enhanced bacterial killing and reduced regrowth, even at lower antibiotic concentrations (1x MIC).

E. Broad Spectrum Efficacy

• The protective effect of OM-89 was observed across diverse UPEC strains, including phylogroup B2 (CFT073, UTI89, J96) and phylogroups A and B1 (primary clinical isolates).
• It was effective in combination with Ampicillin, Fosfomycin, and Trimethoprim/Sulfamethoxazole.

5. Significance

This study redefines the bladder epithelium from a passive reservoir of infection to an active, targetable antimicrobial compartment. The findings suggest that:

1. Clinical Mechanism: The decades-long clinical success of OM-89 (Uro-Vaxom®) is likely driven by its ability to "prime" the bladder epithelium, enhancing lysosomal degradation and intracellular antibiotic delivery.
2. Therapeutic Strategy: It proposes a host-directed therapy approach where modulating epithelial cell-intrinsic pathways (specifically lysosomal acidification) can synergize with existing antibiotics to eradicate intracellular bacterial reservoirs.
3. Antimicrobial Resistance: By improving the efficacy of existing antibiotics against intracellular niches, this strategy could reduce treatment failure rates, shorten treatment courses, and mitigate the development of antimicrobial resistance.
4. Translational Potential: The conservation of these mechanisms in human organoids supports the immediate clinical relevance of these findings for managing recurrent UTIs in humans.