## 1 Introduction  
Hydroxytyrosol (3,4-dihydroxyphenylethanol, HXT) is one of the most abundant phenolic compounds in olive oils [1]. The biological effects of HXT have been investigated considerably as they seem to account for some of the health benefits associated with the consumption of olive oil.  
These potential benefits have been mainly associated with the antioxidant capacity of this phenolic compound [2, 3]. However, a known biological effect of HXT involves the antimicrobial activity displayed against a wide range of microorganisms, including bacteria [2]. The antimicrobial effects of HXT suggest that this compound may generate reactive oxygen species (ROS) given that antimicrobials usually share the production of these molecular species as mechanism to kill bacteria [4]. Supporting this hypothesis, other plant phenolic compounds that display antimicrobial capacity, including tannic acid [5, 6] or p-comamic acid [7], also generate ROS and have been observed to elicit antioxidant responses in _Lactobacillus plantarum_, a bacterium species associated with the olive and olive food products.  
On account of its antimicrobial properties, HXT could influence host health status by shaping gut microbial communities given the recognized role of gut microbiota in host health. Lactobacillus are successfully adapted to olive and mammalian gut habitats and display higher tolerance to phenolic compounds than other bacterial groups [8]. Increasing the insights into the molecular mechanisms by which HXT exert selective pressures on lactobacilli could advance the understanding of transformation changes induced by this olive phenolic compound in the gut or olive microbiota. For example, HXT could influence the structure of the microbiota of the small intestine, which is the site where fats, including olive oil, are absorbed and where _Lactobacillus_ species constitute a major bacterial population [9]. Thus, shifts triggered by HXT towards gut microbial groups such as Lactobacillus, which are regarded as safe, may contribute to improve the host health status.  
_Lactobacillus plantarum_ is a suitable microorganism to characterize the molecular responses to HXT. This bacterium is present in the olive and olive oil microbiota, is also a member of the olive orchard microbiome and colonizes the gastrointestinal (GI)-tract from animals. In addition, _L. plantarum_ can be only inhibited at high HXT concentrations [10]. Increasing the insights into the effects of HXT on gene transcription may help to elucidate in more detail the molecular mechanisms underlying the effects of HXT at genome scale in whole biological systems, such as _L. plantarum_, which are currently fragmentary [2]. These insights can provide a knowledge base for using HXT to, for instance, improve the antioxidant potential of lactobacilli, better manage the gut or olive microbiomes or refine the use of olive oil as a vehicle for probiotic microorganisms. From a fundamental viewpoint this knowledge may help to better understand the antimicrobial effects of HXT and the mechanisms of bacterial adaptation to this compound. In the same vein, this knowledge would expand the current catalog of transcriptional datasets that report the gene and gene-circuits used by this microorganism to adapt to other plant phenolic compounds [5; 6; 7; 11; 12; 13; 14] as well as to plant niches [15; 16] including olive oil [10]. In view of the above mentioned, the objective of this work is to study the contribution of differential gene expression in the adaptation to HXT, the main olive oil phenolic compound, as characterized by whole genome profiling in _L. plantarum_ WCFS1.