The Mode of Action of Cyclic Monoterpenes (-)-Limonene and (+)-α-Pinene on Bacterial Cells

A broad spectrum of volatile organic compounds' (VOCs') biological activities has attracted significant scientific interest, but their mechanisms of action remain little understood. The mechanism of action of two VOCs-the cyclic monoterpenes (-)-limonene and (+)-alpha-pinene-on bacteria was studied in this work. We used genetically engineered Escherichia coli bioluminescent strains harboring stress-responsive promoters (responsive to oxidative stress, DNA damage, SOS response, protein damage, heatshock, membrane damage) fused to the luxCDABE genes of Photorhabdus luminescens. We showed that (-)-limonene induces the PkatG and PsoxS promoters due to the formation of reactive oxygen species and, as a result, causes damage to DNA (SOSresponse), proteins (heat shock), and membrane (increases its permeability). The experimental data indicate that the action of (-)-limonene at high concentrations and prolonged incubation time makes degrading processes in cells irreversible. The effect of (+)-alpha-pinene is much weaker: it induces only heat shock in the bacteria. Moreover, we showed for the first time that (-)-limonene completely inhibits the DnaKJE-ClpB bichaperone-dependent refolding of heat-inactivated bacterial luciferase in both E. coli wild type and mutant Delta ibpB strains. (+)-alpha-Pinene partially inhibits refolding only in Delta ibpB mutant strain.

Automated summary

This study investigated the mechanisms of action of two volatile organic compounds on bacteria and found that (-)-limonene induces oxidative stress leading to DNA, protein, and membrane damage, while (+)-alpha-pinene only induces heat shock in bacteria. Additionally, it was shown that (-)-limonene completely inhibits the refolding process of heat-inactivated bacterial luciferase, while (+)-alpha-pinene partially inhibits this process in a mutant strain.