Terpineol enhances antibiotic effectiveness and reduces resistance in clinically relevant bacteria

<h2>Abstract</h2><h3>Background and aim</h3> The synergistic interaction between natural compounds and existing antibiotics (ABXs) offers a promising strategy to enhance ABX efficacy while mitigating resistance. This study evaluated the potential of terpineol (TER), a monocyclic monoterpene of plant origin with antimicrobial properties, as an enhancer of antibiotic activity against 14 clinically and veterinary relevant bacterial strains, including ESKAPE pathogens identified by WHO as urgent targets. <h3>Experimental procedure</h3> The antimicrobial activity of TER and its synergistic effects with 13 commonly used antibiotics were assessed using the checkerboard microdilution method and isobologram representation, with growth kinetics and time-kill curves generated over 24 h. <h3>Key results</h3> TER showed antimicrobial activity against 14 tested bacteria. The lowest MIC was 0.25 mg/mL for <i>Acinetobacter baumannii</i> (WHO priority 1). TER was effective (MIC = 0.5 mg/mL) against various Gram-negative and positive bacteria, exhibiting bactericidal activity in all cases. TER formed additive and synergistic relationships with 11 antibiotics. Six combinations demonstrated synergy (FICi≤0.5), 24 showed additivity (0.5<FICi≤1), and 13 exhibited indifference (1<FICi<4), with no antagonism (FICi≥4). The six TER/ABX synergies resulted in 100 % inhibition of bacterial growth, reducing MICs by up to 87.5 %. For the first time, this study demonstrated that TER minimally generates microbial resistance compared to commercial antibiotics. Additionally, TER acted as an Antimicrobial Resistance Modifying Agent, reducing resistance to four ABXs (gentamycin, florfenicol, chloramphenicol, tetracycline) in both Gram-positive and negative bacteria. <h3>Conclusions and implications</h3> Given its safety profile endorsed by EPA and FDA, TER emerges as a promising candidate for natural antibiotic enhancement, preventing the development of resistance.