Elucidating the Impact of Bacterial Composition and Diversity on Skin Homeostasis Using a Human 3D In Vitro Skin Model

The importance of the skin microbiome in human health has become increasingly evident. Despite considerable advancements, complex host–microbe interactions remain largely unexplored, primarily due to a paucity of models mimicking human skin and maintaining bacterial diversity in vitro. Therefore, we assessed the role of bacterial diversity for skin–microbiota co-cultivation in vitro by co-culturing reconstructed human full-thickness skin models with one of seven defined bacterial communities for seven days. The most diverse ten-strain community (BAP.10) comprised the three main skin phyla Bacillota, Actinomycetota, and Pseudomonadota. Lower diversity was obtained by the inoculation with two and one phyla combinations only. Inoculations with higher bacterial diversity were maintained at physiological bacterial counts and resulted in a higher functional similarity to human skin microbiomes, with BAP.10 showing the closest resemblance. In contrast, colonization with a single phylum led to either absence of viable bacteria or overgrowth of a single species. Overgrowth was associated with impaired skin integrity and cytotoxicity. Our data demonstrate the importance of bacterial diversity and the presence of certain genera for human-relevant skin co-culture models, which can be used for future in vitro studies elucidating skin–microbiome interactions.