Differential mechanism of Escherichia coli Inactivation by (+)-limonene as a function of cell physiological state and drug's concentration

PLoS One. 2014 Apr 4;9(4):e94072. doi: 10.1371/journal.pone.0094072. eCollection 2014.

Abstract

(+)-limonene is a lipophilic antimicrobial compound, extracted from citrus fruits' essential oils, that is used as a flavouring agent and organic solvent by the food industry. A recent study has proposed a common and controversial mechanism of cell death for bactericidal antibiotics, in which hydroxyl radicals ultimately inactivated cells. Our objective was to determine whether the mechanism of Escherichia coli MG1655 inactivation by (+)-limonene follows that of bactericidal antibiotics. A treatment with 2,000 μL/L (+)-limonene inactivated 4 log10 cycles of exponentially growing E. coli cells in 3 hours. On one hand, an increase of cell survival in the ΔacnB mutant (deficient in a TCA cycle enzyme), or in the presence of 2,2'-dipyridyl (inhibitor of Fenton reaction by iron chelation), thiourea, or cysteamine (hydroxyl radical scavengers) was observed. Moreover, the ΔrecA mutant (deficient in an enzyme involved in SOS response to DNA damage) was more sensitive to (+)-limonene. Thus, this indirect evidence indicates that the mechanism of exponentially growing E. coli cells inactivation by 2,000 μL/L (+)-limonene is due to the TCA cycle and Fenton-mediated hydroxyl radical formation that caused oxidative DNA damage, as observed for bactericidal drugs. However, several differences have been observed between the proposed mechanism for bactericidal drugs and for (+)-limonene. In this regard, our results demonstrated that E. coli inactivation was influenced by its physiological state and the drug's concentration: experiments with stationary-phase cells or 4,000 μL/L (+)-limonene uncovered a different mechanism of cell death, likely unrelated to hydroxyl radicals. Our research has also shown that drug's concentration is an important factor influencing the mechanism of bacterial inactivation by antibiotics, such as kanamycin. These results might help in improving and spreading the use of (+)-limonene as an antimicrobial compound, and in clarifying the controversy about the mechanism of inactivation by bactericidal antibiotics.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Citric Acid Cycle / drug effects
  • Cyclohexenes / pharmacology*
  • Dose-Response Relationship, Drug
  • Escherichia coli / drug effects*
  • Escherichia coli / physiology*
  • Free Radical Scavengers / pharmacology
  • Hydroxyl Radical / antagonists & inhibitors
  • Hydroxyl Radical / metabolism
  • Iron / metabolism
  • Kanamycin / pharmacology
  • Limonene
  • Terpenes / pharmacology*

Substances

  • Cyclohexenes
  • Free Radical Scavengers
  • Terpenes
  • Hydroxyl Radical
  • Kanamycin
  • Limonene
  • Iron

Grants and funding

This study was financially supported by the CICYT (Project AGL2012-32165), European Social Fund and Aragonese Departamento de Ciencia, Tecnología y Universidad; and Spanish Ministerio de Educación, Cultura y Deporte that provided B. Chueca with a grant to carry out this investigation. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.