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An extraction-free method is described for the colorimetric determination of thymol (TY) and carvacrol (CA) isomers in essential oils by making use of the pH-dependent formation of gold nanoparticles (AuNPs). In solutions of pH 12, TY and CA form gold nanoparticles, while at pH ≤ 11 only CA does so. By taking advantage of this finding, two different approaches based on colorimetric assay (absorption at 550 nm) were developed: one at pH 12 for the determination of total CA and TY, and other at pH 9 and pH 12 for differential quantification of TY and CA. The former agrees with the well-established Folin-Ciocalteu method, and the latter provides a simple way for calculation of TY/CA ratio. The linear ranges are from 100 to 1000 μM at pH 9, and from 50 to 200 μM at pH 12. The limits of detection are 0.09 μM at pH 9, and 0.02 μM at pH 12. These features make this method simple, fast and reliable. Conceivably, it can be used to assess the quality of essential oils and may become a valuable alternative to more sophisticated, laborious and high time-consuming methods. Graphical abstract Schematic of the assay: At pH 12 (blue color), thymol and carvacrol form gold nanoparticles (Au), while at pH 9 (red color) only carvacrol does so. This finding resulted in a colorimetric method for the differential quantification of both compounds in essential oils.

Aim of study: To evaluate the antimicrobial activity of the hexanic extract (HE) of Achyrocline satureioides on Paenibacillus larvae - a gram-positive spore-forming bacillus that affects the larval stage and causes American Foulbrood (AFB) - and its oral and contact toxicity on larvae and adult honey bees. Area of study: A. satureioides plants were collected in Santa Monica (32° 05’ 29” S, 64° 36’ 54” W, Córdoba. Argentina). The larvae and adults of Apis mellifera were obtained from the experimental apiary of the University of Córdoba, Spain. Material and methods: P. larvae 9 was previously isolated and identified in the Laboratory of General Microbiology (Dept. of Microbiology, National University of Río Cuarto, Argentina). The HE was obtained by liquid-liquid extraction. The minimum inhibitory concentration (MIC) of HE was determined by a microdilution method. This concentration and 2 ½ MIC were used for in vitro toxicity tests. Oral toxicity was tested on larvae, feeding them with both concentrations of the HE, while on adult bees the HE was spread to determine contact toxicity. Main results: The HE showed antimicrobial activity, the MIC obtained was 0.4 μg/mL. The HE presented very low toxicity at the MIC and 2 ½ MIC, with survival percentages to be around 95% for adult bees and larvae. Research highlights: The results show that this extract could be used for the development of an alternative product for a safe and effective treatment of AFB.

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