Development of a continuous bioreactor to maintain stable nasal microbiomes from swab specimens and synthetic communities

This study demonstrates that a continuous bioreactor, optimized for specific environmental conditions and inoculated with either nasal swab specimens or synthetic microbial communities, can maintain stable, reproducible nasal microbiomes for over a month, providing a robust model for investigating nasal ecology and developing targeted *Staphylococcus aureus* decolonization strategies.

The Gist

Imagine your nose isn't just a hole for breathing; it's a bustling, tiny city. Inside this city, millions of microscopic residents (bacteria) live together. Most of them are friendly neighbors who keep the peace, but sometimes, an aggressive bully named Staphylococcus aureus (or "S. aureus") moves in. This bully can cause serious infections, especially in hospitals.

Currently, doctors try to kick this bully out using "nuclear options"—strong antibiotics. But these antibiotics are like a carpet bomb; they kill the bully, but they also wipe out all the friendly neighbors, leaving the city empty and vulnerable to new invaders.

The Problem with Previous Models
Scientists have tried to study this nose-city in two main ways, but both had flaws:

1. Petri Dishes (The "Static Zoo"): These are like keeping animals in a cage with no food coming in and no waste going out. The bully bacteria usually take over the whole cage because they are the toughest, making it impossible to study how the whole community interacts.
2. Lab Animals (The "Imposter"): Using mice or other animals is like trying to understand New York City by studying a hamster village. The biology is different, and the results don't always translate to humans.

The Solution: The "Nose in a Jar"
The researchers in this paper built a continuous bioreactor. Think of this as a high-tech, self-cleaning aquarium for your nose bacteria.

Instead of a static cage, this system is a flowing river.

• Fresh Food: They constantly pump in a special liquid (Synthetic Nasal Medium) that mimics the nutrients found in a real human nose.
• Waste Removal: They constantly drain out old liquid, preventing waste from building up and keeping the environment fresh.
• Stable Conditions: They carefully control the temperature (like a cozy 30°C) and acidity (pH), just like inside a real nose.

What They Discovered
The team ran a series of experiments to see if they could keep this "nose city" alive and stable for weeks.

1. Finding the Sweet Spot: They tried different settings (temperature, food types, flow rates). They found that if they kept the "river" flowing at a specific speed with the right food mix, the bacteria didn't just survive; they thrived in a balanced community for over a month.
2. The Bully Didn't Win: In the old "static cage" (batch mode), the bully S. aureus took over 97% of the city. But in their new "flowing river" (continuous mode), the bully only took up about 15–20% of the space. The friendly neighbors (like S. epidermidis) held their ground and kept the bully in check.
3. Resilience: When they temporarily changed the acidity of the water (a stress test), the community got shaken up, but it bounced back to normal within two days. This proved the system is tough and realistic.
4. The "Synthetic City" Experiment: To test specific strategies, they built a "Synthetic Community" (SynCom)—a pre-made team of 9 specific bacteria from one volunteer. They added the bully to this team.
   • Result A: When they added a bully that was "friends" with the existing team (from the same person), the bully took over.
   • Result B: When they added a bully that was a "stranger" (from a different person), the existing team rejected it, and the bully died out.

Why This Matters
This "Nose in a Jar" is a game-changer for two reasons:

• Better Science: It gives scientists a realistic, stable model to study how nose bacteria interact without needing animals or messy petri dishes.
• New Treatments: Instead of using antibiotics that kill everything, we might be able to use probiotics (good bacteria) or specific nutrients to "starve" the bully or strengthen the friendly neighbors. For example, they found that if the friendly bacteria are hungry for a specific nutrient (tyrosine), the bully can't survive. This opens the door to targeted, safe treatments that don't destroy the whole microbiome.

In a Nutshell
The researchers built a sophisticated, flowing ecosystem that mimics the human nose. They proved that by keeping the environment stable and flowing, we can maintain a healthy balance of bacteria where the "bad guys" don't take over. This new tool could help us develop smarter, gentler ways to stop dangerous infections without wrecking our natural defenses.

Technical Summary

1. Problem Statement

The nasal microbiome plays a critical role in human health, acting as a barrier against pathogens like Staphylococcus aureus. S. aureus is a major opportunistic pathogen that colonizes the noses of ~30% of the population, posing severe risks for invasive infections in immunocompromised and hospitalized patients.

• Current Limitations:
  • Antibiotics: Standard decolonization treatments (e.g., mupirocin) are broad-spectrum, damaging the resident microbiome and fostering antibiotic resistance.
  • Existing Models:
    • In-vitro cell cultures (batch systems): Lack environmental complexity, have fixed nutrient supplies, poor pH control, and often result in the dominance of a single species (usually S. aureus), failing to mimic the stable, diverse nasal ecosystem.
    • Animal models: Exhibit physiological differences from humans, suffer from low reproducibility, and raise ethical concerns.
• Gap: There is a lack of a physiologically relevant, continuous culture system capable of maintaining stable, complex human nasal microbiomes over extended periods to study microbial interactions and develop targeted decolonization strategies.

2. Methodology

The authors employed a "Design-Build-Test-Learn" cycle, integrating top-down (using natural swab specimens) and bottom-up (using synthetic communities) approaches.

• Bioreactor System:

  • Setup: Six 250-mL glass chemostats operated in continuous mode with magnetic stirring (250 rpm) to ensure homogeneity and microaerobic conditions.
  • Control Parameters: Temperature (25–35°C), pH (5.5–7.5), and oxygen levels were actively controlled.
  • Medium: Synthetic Nasal Medium (SNM) mimicking nasal secretions, with varying concentrations of glucose, amino acids, and organic acids.
  • Operation: Continuous feeding with dilution rates (D) ranging from 0.5 to 1.5 d⁻¹.

• Experimental Design (5 Experiments):

  1. Optimization: Inoculated with a nasal swab from Volunteer 1. Tested five variables: Operating mode (Batch vs. Continuous), Dilution Rate, Temperature, pH, and Medium Composition (SNM 1, 3, 10).
  2. Reproducibility: Inoculated six bioreactors with six different swab specimens from the same volunteer (collected over two days) to test consistency under optimized conditions.
  3. Perturbation: Subjected a stable bioreactor to a temporary pH shift (6.5 → 5.5 → 6.5) to test resilience.
  4. Stability: Inoculated six bioreactors with swabs from six different volunteers to assess the system's ability to maintain diverse, stable communities.
  5. Decolonization (Bottom-up): Inoculated with a Synthetic Microbial Community (SynCom) derived from Volunteer 1 (9 strains, excluding native S. aureus). After stabilization, two different S. aureus strains were added:
     • Sa C: Isolated from the same volunteer (SynCom C).
     • Sa A: Isolated from a different volunteer.

• Analytical Techniques:

  • Microbiology: Optical Density (OD600), qPCR (for total bacteria and S. aureus quantification).
  • Genomics: Amplicon sequencing of the full 16S-ITS-23S rRNA operon (~4,300 bp) using Oxford Nanopore technology for high taxonomic resolution.
  • Metabolomics: Non-targeted metabolomics via UHPLC-Q-Exactive HF mass spectrometry.

3. Key Contributions

• First Continuous Nasal Bioreactor: Developed the first continuous bioreactor system specifically designed to cultivate stable human nasal microbiomes, overcoming the limitations of batch cultures.
• Optimized Operating Conditions: Identified a specific set of conditions that prevent S. aureus dominance and maintain diversity: Continuous mode, Dilution rate of 1 d⁻¹, 30°C, pH 6.5, and SNM 3.
• SynCom Integration: Successfully demonstrated the use of defined Synthetic Microbial Communities (SynComs) within a continuous flow system to test specific colonization and decolonization hypotheses.
• Strain-Specific Colonization Insights: Revealed that S. aureus colonization success is highly dependent on the specific strain and the pre-existing community composition, particularly regarding tyrosine metabolism.

4. Key Results

• Optimization:
  • Batch vs. Continuous: Batch cultures led to S. aureus dominance (>97%), whereas continuous cultures maintained a diverse community (~82% S. epidermidis, ~15% S. aureus, ~3% S. capitis).
  • Parameters: Growth was optimal at 30°C and pH 6.5. A dilution rate of 1.5 d⁻¹ caused washout, while 0.5 d⁻¹ yielded lower biomass. SNM 3 provided the best balance of growth and diversity.
• Reproducibility: Six bioreactors inoculated with different swabs from the same volunteer converged to nearly identical stable communities (high correlation in species abundance), demonstrating high system reproducibility.
• Resilience: When pH was perturbed (dropped to 5.5), S. aureus temporarily increased, but the community rapidly recovered to its original state within two days, showing strong resilience.
• Stability Across Volunteers: The system successfully maintained stable microbiomes for up to 36 days using swabs from six different volunteers. While the final dominant species varied (e.g., Staphylococcus dominance in some, Klebsiella/Citrobacter in others), all reached a stable steady state.
• Metabolomics: Metabolic profiles mirrored bacterial community structures. Communities clustered into "Staphylococcal-dominant" and "Staphylococcal-absent" groups, linking specific metabolite signatures to community composition.
• Decolonization Strategy:
  • Sa C (Same Volunteer): When added to SynCom C, Sa C proliferated and dominated (52–63%), displacing other commensals.
  • Sa A (Different Volunteer): When added to SynCom C, Sa A failed to colonize and remained undetectable.
  • Mechanism: This suggests that S. aureus colonization is strain-specific and influenced by community-derived factors (e.g., tyrosine auxotrophy/prototrophy). The SynCom C community effectively excluded the foreign S. aureus strain.

5. Significance

• Superior Model System: This continuous bioreactor offers a more physiologically relevant alternative to animal models and batch cultures, allowing for long-term (36+ days) study of nasal ecology.
• De-risking Decolonization: The system enables the testing of non-antibiotic decolonization strategies (e.g., probiotics, metabolic interventions) in a controlled, human-relevant environment before clinical trials.
• Mechanistic Insights: The study highlights that successful S. aureus decolonization may involve manipulating the community to exclude specific strains based on metabolic dependencies (e.g., tyrosine availability) rather than broad-spectrum killing.
• Future Applications: The platform is scalable for high-throughput screening of drugs, probiotics, and antimicrobial resistance mechanisms, bridging the gap between in-vitro studies and clinical reality.

Limitations noted by authors: The system cannot yet cultivate all nasal bacteria (e.g., strict anaerobes were washed out), lacks human host cells (epithelial layer), and requires further integration with multi-omics and organ-on-chip technologies for full physiological relevance.