Antibiofilm and resistance-modulating properties of tt-farnesol against Streptococcus equi subsp. equi and zooepidemicus.

Overview

• This study evaluated the antibacterial, antibiofilm, and antibiotic resistance-modulating effects of trans-trans-farnesol (tt-farnesol) on two subspecies of Streptococcus equi, which are bacteria associated with horse health.
• The findings demonstrated that tt-farnesol effectively kills the bacteria, inhibits their ability to form biofilms, and restores sensitivity to the antibiotic tetracycline by modulating resistance mechanisms.

Background and Objectives

• Pathogens studied: Streptococcus equi subsp. equi and Streptococcus equi subsp. zooepidemicus, bacteria that can be pathogenic or commensal in horses.
• Importance: These bacteria affect equine health and have economic implications.
• Main goal: To investigate the antibacterial activity of tt-farnesol, including its impact on biofilms and antibiotic resistance modulation.

Methods

• Antibacterial assays: Determined minimum inhibitory concentration (MIC) and minimum bactericidal concentration (MBC) values to measure the killing effect of tt-farnesol on the bacterial strains.
• Biofilm studies: Evaluated the ability of tt-farnesol to inhibit biofilm formation by measuring biomass reduction at different concentrations.
• Resistance modulation: Tested subinhibitory concentrations of tt-farnesol for their effect on tetracycline resistance, assessing whether they could restore sensitivity.
• Mechanistic assays:
  • Membrane damage assessment to understand the antibacterial mechanism.
  • Oxidative stress evaluation to determine if the killing effect involved reactive oxygen species.
  • Rhodamine 6G assay to study the modulation of efflux pumps, which are often involved in antibiotic resistance.
• In silico studies: Molecular docking to evaluate potential binding affinity between tt-farnesol and key bacterial proteins including:
  • 1YUB (a methylase related to bacterial processes)
  • 6PON (a protein involved in biofilm formation)
  • 3OP1 (an efflux pump protein)

Key Findings

• Antibacterial activity:
  • tt-farnesol had bactericidal effects against all tested strains.
  • The MIC and MBC values ranged between 4 and 8 μg/mL, indicating effective killing at relatively low concentrations.
  • The compound caused significant membrane damage but did not rely on inducing oxidative stress.
• Biofilm inhibition:
  • tt-farnesol strongly inhibited the formation of biofilms.
  • Reduction of biofilm biomass by up to 58.9% was observed at 1 μg/mL, showing its effectiveness at sub-MIC levels.
• Resistance modulation:
  • Subinhibitory doses (0.5 and 1 μg/mL) of tt-farnesol restored sensitivity to the antibiotic tetracycline in resistant bacterial strains.
  • The rhodamine 6G assay suggested that tt-farnesol may inhibit bacterial efflux pumps, which often expel antibiotics, contributing to resistance.
• In silico interaction:
  • tt-farnesol showed predicted affinity for bacterial proteins that are involved in methylation, biofilm formation, and drug efflux—supporting its multi-targeted mechanism.

Implications and Conclusion

• tt-farnesol shows promise as a novel antibacterial agent against Streptococcus equi strains that affect horses.
• Its ability to inhibit biofilms is particularly important since biofilms protect bacteria from antibiotics and immune responses.
• The capacity to modulate antibiotic resistance and restore antibiotic effectiveness points to a potential role as an adjuvant in antimicrobial therapy.
• This is the first study to demonstrate the therapeutic potential of tt-farnesol against these specific resistant equine pathogens, which could have economic and animal health benefits.
• Future work may explore clinical applications and further mechanistic studies to optimize its usage.