Inhibition of multidrug-resistant Staphylococcus aureus by commensal bacterial species from the human nose

1. Problem Statement

• Antimicrobial Resistance (AMR): Methicillin-resistant Staphylococcus aureus (MRSA) is a major cause of mortality, with infections increasingly resistant to last-line antibiotics (e.g., vancomycin, daptomycin, teicoplanin).
• Colonization as a Risk Factor: The human nose is the primary ecological niche for S. aureus colonization, which precedes invasive infection.
• Knowledge Gaps:
  • It is unclear which specific commensal bacterial species or combinations are most effective at inhibiting MRSA.
  • Existing pairwise studies fail to capture complex community interactions.
  • It is unknown whether MRSA strains that have developed resistance to last-resort antibiotics retain susceptibility to inhibition by commensal bacteria, or if resistance mechanisms confer cross-resistance to microbial competition.
  • The evolutionary stability of commensal-mediated inhibition (i.e., can MRSA adapt to overcome it?) is not well characterized.

2. Methodology

The study employed a systematic in vitro approach using semi-solid media to mimic the spatial environment of the nasopharynx (34°C).

• Strains:
  • Target: MRSA strain JE2 (USA300 community-associated) and various clinical isolates.
  • Commensals: Seven prevalent nasal species: Staphylococcus aureus (healthy isolate), S. lugdunensis, Corynebacterium accolens, C. tuberculostearicum, C. propinquum, Dolosigranolum pigrum, and Cutibacterium acnes.
  • Resistant Variants:
    • Lab-adapted JE2 mutants resistant to daptomycin, teicoplanin, and vancomycin.
    • Clinical isolates (JKD series) with reduced susceptibility to vancomycin/teicoplanin.
• Experimental Design:
  • Combinatorial Screening: All 128 possible combinations of the 7 commensal species were tested for their ability to inhibit JE2 colonization.
  • Media Optimization: Experiments were conducted with and without 1% Tween 80 (a lipid source) to support lipophilic Corynebacterium species.
  • Colonisation Assay: Commensals were pre-cultivated for 48 hours to establish a resident lawn, followed by inoculation of a low-density MRSA inoculum. MRSA abundance was quantified after 24 hours via CFU counting.
  • Mechanism Investigation: Supernatant assays were used to test for growth promotion or inhibition via metabolite exchange.
  • Evolutionary Adaptation: Serial passaging experiments (up to 20 cycles) were conducted to test if MRSA could evolve resistance to commensal inhibition. Genomic sequencing (Whole Genome Sequencing) was performed on adapted isolates to identify mutations.

3. Key Contributions

• Combinatorial Screening: Provided a comprehensive map of how single species and complex mixtures of nasal commensals interact to suppress MRSA, moving beyond simple pairwise comparisons.
• Synergistic Identification: Identified a specific, non-staphylococcal combination (Corynebacterium spp. + D. pigrum) that achieves inhibition levels comparable to intra-specific competition.
• Resistance Robustness: Demonstrated that commensal inhibition is effective against MRSA strains with clinically relevant resistance to last-line antibiotics, suggesting microbiota-based therapies remain viable even when antibiotics fail.
• Evolutionary Stability: Showed that while MRSA can partially adapt to commensal pressure, it does not fully overcome inhibition, and adaptation comes with fitness costs (increased doubling time).

4. Key Results

A. Inhibition Efficacy

• Staphylococcal Competition: Intra-specific competition (healthy S. aureus vs. MRSA) and S. lugdunensis provided the strongest inhibition, reducing MRSA abundance by ~4.5 log10 CFU/mL.
• Non-Staphylococcal Synergy:
  • D. pigrum alone showed weak inhibition.
  • However, combining D. pigrum with lipophilic Corynebacterium species (C. tuberculostearicum and/or C. accolens) resulted in strong inhibition, comparable to Staphylococci.
  • Mechanism: D. pigrum supernatants promoted the growth of C. tuberculostearicum and C. accolens (likely via lipid metabolism or metabolite cross-feeding), creating a synergistic effect that enhanced MRSA suppression.
  • Nutrient Dependence: Supplementation with Tween 80 (fatty acids) significantly improved the growth of lipophilic Corynebacterium and enhanced their inhibitory capacity.

B. Impact of Antibiotic Resistance

• Lab-Adapted Mutants: MRSA strains adapted to daptomycin, teicoplanin, and vancomycin (carrying mutations in mprF, walKR, vraT, rpoB) were inhibited by commensals to the same degree as wild-type JE2.
• Clinical Isolates: Clinical isolates with reduced susceptibility to vancomycin/teicoplanin were also effectively inhibited. In some cases (e.g., Pair 4), resistant clinical isolates were inhibited more strongly than their susceptible counterparts when exposed to the C. tuberculostearicum + D. pigrum combination.
• Conclusion: Resistance to last-line antibiotics does not confer cross-resistance to commensal-mediated inhibition.

C. Temporal Stability and Adaptation

• Short-term vs. Long-term: In simple serial transfer (transferring the whole community), MRSA often overgrew commensals due to faster intrinsic growth. However, when commensals were pre-established as a lawn at each transfer, inhibition was sustained over multiple passages.
• Evolutionary Adaptation: After 20 cycles of co-culture with C. tuberculostearicum (alone or with D. pigrum), MRSA showed partial adaptation (reduced inhibition compared to ancestral strain) but was not fully resistant.
  • Genetic Basis: Adapted isolates harbored mutations in stress response (relA, relP) and metabolic pathways (thiD, thiW for thiamine).
  • Fitness Cost: Adapted strains exhibited increased doubling times (slower growth).
  • Specificity: Adaptation was specific to the treatment; strains adapted to C. tuberculostearicum did not show cross-adaptation to other commensals.

5. Significance

• Therapeutic Potential: The study validates the concept of "colonization resistance" using defined bacterial consortia. Specifically, a combination of Corynebacterium and D. pigrum offers a potent, non-antibiotic strategy to clear MRSA from the nose.
• Addressing AMR: The findings suggest that microbiota-based interventions are a viable alternative for patients infected with or colonized by multidrug-resistant MRSA, as the mechanism of inhibition (competition and niche exclusion) is distinct from antibiotic targets.
• Ecological Insight: The research highlights the importance of community complexity and metabolic cross-feeding (e.g., D. pigrum aiding Corynebacterium) in maintaining colonization resistance, suggesting that probiotic therapies may require specific multi-species formulations rather than single strains.
• Evolutionary Safety: The observation that MRSA adaptation to commensals is partial and incurs fitness costs suggests a lower risk of rapid therapeutic failure compared to antibiotic resistance evolution.