Isolation and characterization of Escherichia coli mutants exhibiting altered response to thymol

The antimicrobial mode of action of the plant essential oil thymol was studied with Escherichia coli. Random transposon-insertion mutants were screened for altered response to thymol. Of four mutants showing more sensitivity, three were found in rfaQ, whose product is involved in lipopolysaccharide biosynthesis, and the fourth in the quorum-sensing gene qseC. Mutants showing more resistance had mutations in genes whose products are involved in the degradation of short-lived regulatory and abnormal proteins (the lon gene), menaquinone biosynthesis (menA), an unknown function of a putative membrane protein (yagF), synthesis of a small hypothetical protein (an intergene region between the two small genes yiiE and yiiF), and the efflux pump of cadaverine and lysine (cadB). The antibacterial activities of carvacrol, menthol, and cymene, essential oils structurally similar to thymol, were also determined. Although the level of resistance toward thymol was conserved in the respective mutants qseC, menA, and cadB, knockout mutants displayed different levels of tolerance to carvacrol; inconsistencies in resistance levels were also noted in mutants challenged with menthol. Wild-type and mutant E. coli responded to thymol exposure with a massive potassium efflux that generally corresponded to the resistant rate. The verity of the loci accounting for E. coli response suggests a multitarget mode of the antimicrobial activity of thymol and multitolerance mechanisms.