Essential oils are complex mixtures of hydrocarbons and their oxygenated derivatives arising from two different isoprenoid pathways. Essential oils are produced by glandular trichomes and other secretory structures, specialized secretory tissues mainly diffused onto the surface of plant organs, particularly flowers and leaves, thus exerting a pivotal ecological role in plant. In addition, essential oils have been used, since ancient times, in many different traditional healing systems all over the world, because of their biological activities. Many preclinical studies have documented antimicrobial, antioxidant, anti-inflammatory and anticancer activities of essential oils in a number of cell and animal models, also elucidating their mechanism of action and pharmacological targets, though the paucity of in human studies limits the potential of essential oils as effective and safe phytotherapeutic agents. More well-designed clinical trials are needed in order to ascertain the real efficacy and safety of these plant products.

The human gastrointestinal tract is inhabited by a vast number of microorganisms that are called as the microbiota. Each individual harbors a unique gut microbial composition, this composition evolves throughout the host's lifetime and it is easily affected by internal or external changes. It has been shown that gut microbiota plays a crucial role in host's health and as this complex community has the ability to interact with each other and with the host's immune system, the presence or absence of some major species can affect the homeostasis. Diet can be considered as one of the pivotal factors in modulating the functionality, integrity, and composition of the gut microbiota as the gastrointestinal tract is the first organ exposed to components of the diet. In this review, we have focused on the effects of polyphenols, key compounds of a healthy diet with several biological activities, on the gut microbial composition, their biotransformation by the gut microbiota, and the effect of their reciprocal interactions in human health and disease.