Microbiome claims: to do or not to do?

There is an ongoing debate regarding microbiome terminology in the microbiology field.1 In this paper, we will be using the following definitions: (i) Microbiota is defined as the live (or active) microorganisms present on the body, and (ii) microbiome is the community of these microorganisms and the ones that are not considered alive (phages, viruses, plasmids, prions, viroids, and free DNA), their environment and interaction amongst each other and their host.1 An additional difference between these two terms can be explained by how we study them. Microbiota can be studied separately from their natural environment (in vitro studies) however, the environment of microbiome studies must include all members of the microbiological “community” within its natural habitat1 despite that our interest might be only one strain. The microbiome is frequently evaluated while studying the genome of the microorganisms of interest. 

The microbiome is first formed when the newborn’s skin first makes a contact with the mother. The newborn’s digestive tract and skin become colonised by the mother’s microorganisms which will later become part of its microbiome. This process is associated with the development and stimulation of the immune system.2 While studies show that the gut microbiome can influence the skin, in this paper, we will focus primarily on the relationship between skin microbiome and skin health. 

There are two types of microorganisms on the skin, the resident (commensal) and the transient. The commensal microorganisms reside on human skin throughout human life. Due to occasional perturbation, they might leave the skin, but will repopulate again. Meanwhile, the transient ones are not permanent, usually persisting for hours or days due to external conditions.3 The skin, along with these microorganisms, consists of a protective system against external aggressions. There is a balance in the composition of the skin’s components, called hydrolipidic balance, and the microbiome has its own balance that is established by a variety of healthy skin microorganisms networking with each other and the skin. When external factors promote alterations in any of these equilibria, dysbiosis occurs, making the skin more susceptible to the development of pathologies.4 Therefore, it is important to understand how the skin and its external conditions can affect the microbiome homeostasis and consequently promote alterations in the skin.