# Role of Nutribiotics in Skin Care
###### Abstract
The current definition of probiotics does not include inactivated or dead cells; therefore, the concept of postbiotic has emerged more recently, referring to the use of dead or inactivated cells (non-viable microorganisms), cell extracts, or metabolites of these microorganisms that can provide favorable effects on human health, observing that the action of probiotics depends fundamentally on their metabolites, rather than on living organisms [12; 13]. Later on, the concept of para-probiotics emerged, consisting of inactivated, dead, or non-viable microbial cells of intact or broken probiotics containing cellular components of probiotic cells after lysis [14]. Thus, in the recent ten years, several studies  
Figure 1: Concept of prebiotics, probiotics, synbiotics, para-probiotics, and postbiotics (adapted from [PERSON] et al., 2023) [10]. have been conducted using inactivated or heat-killed probiotics [15]; however, the terms 'postbiotics', 'para-probiotics', and 'inactivated probiotics' have been used indistinctively in multiple research studies [16; 17; 18; 19; 20]. Postbiotics include the metabolites generated by the microbiota, such as exopolysaccharide, short-chain fatty acids (SCFAs), cell wall fragments, enzymes/proteins, and other metabolites [21], but also structural ones, such as teichoic acids, peptides, and plasmalogens, or ones based on their basic composition (proteins, carbohydrates, lipids, vitamins, etc.) [22]. In this context, several postbiotics have been shown to improve gut health by reinforcing the gut barrier, reducing inflammation, and promoting antimicrobial activity against gut pathogens [23]. According to the most recurrent definition, para-probiotics, also known as non-viable probiotics, inactivated probiotics, tydmallized probiotics, or ghost probiotics, are \"non-viable microbial cells (either intact or broken), or crude cell extracts, which, when administered (orally or topically) in adequate amounts, confer a benefit on the human or animal consumer\" [7; 24]. A recent review by [PERSON] et al. [25] focused on the ability of different types of para-probiotics and postbiotics to modulate the immune system. The most used strains to develop as para-probiotics are _Lactobacillus_ and _Bifidobacterium_ strains. The postbiotic components that modulate the biological reactions include lipoteichoic acids, bacteriocins, SCFAs, peptidoglycan, and exopolysaccharide. Some studies showed that prescribing live probiotic cells to people with weakened immune systems increases inflammatory responses. In such cases, a combination of dead cells can be a good alternative. Thus, the use of killed or inactive probiotics created a new field and various scientists tried to come up with new terms to describe the mentioned cases [26]. Additionally, [PERSON] et al. [9] described the techniques to obtain para-probiotics, which include thermal treatments, sonication, ionizing radiation, high pressure, ultraviolet rays, and pH modification. In terms of efficacy, [PERSON] et al. [6] revised the bioactivities, health-promoting effects, and applications, among other issues, related to post and para-probiotics, referring that in vitro and in vivo studies have shown that some postbiotics and para-probiotics exhibit bioactivities that are immunomodulatory, anti-proliferative, anti-inflammatory, antimicrobial, and antioxidant. The authors postulated that these bioactivities could be involved in the observed health-improving effects, both in clinical trials and in humans; however, more investigations are needed, as the mechanisms of action and the signaling pathways involved have not been fully elucidated. They concluded that para-probiotics and postbiotics are of great interest for the development of nutraceutical products due to their potential for improving health [6].