Thymol tolerance in Escherichia coli induces morphological, metabolic and genetic changes

Fatemah Al-Kandari,Arnoud H M Van Vliet,Martin J Woodward,Rabeah Al-Temaimi

doi:10.1186/s12866-019-1663-8

Abstract

BackgroundThymol is a phenolic compound used for its wide spectrum antimicrobial activity. There is a limited understanding of the antimicrobial mechanisms underlying thymol activity. To investigate this, E. coli strain JM109 was exposed to thymol at sub-lethal concentrations and after 16 rounds of exposure, isolates with a 2-fold increased minimal inhibitory concentration (MIC) were recovered (JM109-Thyr). The phenotype was stable after multiple sub-cultures without thymol.ResultsCell morphology studies by scanning electron microscopy (SEM) suggest that thymol renders bacterial cell membranes permeable and disrupts cellular integrity. 1H Nuclear magnetic resonance (NMR) data showed an increase in lactate and the lactic acid family amino acids in the wild type and JM109-Thyr in the presence of thymol, indicating a shift from aerobic respiration to fermentation. Sequencing of JM109-Thyr defined multiple mutations including a stop mutation in the acrR gene resulting in a truncation of the repressor of the AcrAB efflux pump. AcrAB is a multiprotein complex traversing the cytoplasmic and outer membrane, and is involved in antibiotic clearance.ConclusionsOur data suggests that thymol tolerance in E. coli induces morphological, metabolic and genetic changes to adapt to thymol antimicrobial activity.

Highlights

Thymol is a phenolic compound used for its wide spectrum antimicrobial activity
Tolerance to thymol was shown to be stable as demonstrated by repeated minimal inhibitory concentration (MIC) tests in seven repeated subculture in LB broth without thymol
After testing for stability, JM109-Thyr clone culture was plated onto an NA plate and isolated colonies were used for subsequent experiments to assess JM109-Thyr mechanism of resistance to thymol

Summary

Introduction

Thymol is a phenolic compound used for its wide spectrum antimicrobial activity. There is a limited understanding of the antimicrobial mechanisms underlying thymol activity. The antimicrobial activity of many essential oils (EOs) such as thymol and carvacrol has been widely demonstrated [1, 2] and is assigned to a number of small terpenoid and phenolic compounds [3]. Thymol (C10H14O) is a monoterpenoid phenol extracted from thyme (Thymus vulgaris) as well as other plants. Thymol has been shown to have a wide range of potential applications in pharmaceuticals and therapeutics due to its effective anti-inflammatory, anti-oxidant, and anti-hyperlipidemic properties [4]. In the agriculture and food industry thymol has shown potential insecticidal and antimicrobial properties [5, 6].

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