Which bacteria dominate over other bacteria?

Human ecological community

The chemical, physical and immunological barrier functions of the skin are well known. According to current data, the microbiome takes on another protective function and forms the microbial barrier. The environmental signals do not reach the skin directly, but first hit a diverse, living microbe layer that interacts with external influences - such as chemicals, drugs, creams or "foreign" microbes.

With the help of colonization resistance - this means that all available niches on the skin are almost already occupied - the existing microbial community prevents newly arriving microbes from spreading. Pathogens naturally occurring in the microbiome such as Staphylococcus (S.) aureus or opportunistic pathogens such as S. epidermidis also keep the microbiome in check with a finely balanced balance. Propionibacterium acnes and S. epidermidis, for example, are involved in the growth control of S. pyogenes and S. aureus (10). To put it bluntly: the more diverse, the healthier the skin flora.

If there is an imbalance (dysbiosis), for example through repeated topical application of an antibiotic, individual species (bacteria or fungi) can gain the upper hand. External influences such as washing with soap and hand or skin disinfection change the skin microbiome only temporarily; it regenerates quickly.

It is becoming increasingly clear that the microbiome and its metabolic products are in close contact with the immune system. This is because the bacteria of the microbiome also occur in deeper layers of the skin, for example in the epidermis, dermis and adipose tissue (11). There are specialized cells such as Langerhans cells or dendritic cells that have receptors that can initiate an immune response (1). Studies show that S. epidermidis controls the activation of dendritic cells and T lymphocytes in the skin to ward off invading pathogens (12, 13). The microbiome does not only seem to play a key role in cutaneous immunity, but also coordinates this immune response (14).

Overall, the functions of the skin are presented in a different light today. Instead of a simple mechanical barrier to the outside world, the skin is now an important immunological organ that interacts with the various components of the barrier.

Changes in atopic dermatitis

Atopic dermatitis (AD) is a common chronic inflammatory skin condition that affects children in particular. The very dry, inflamed skin is due to a disrupted skin barrier, in the development of which various genes are involved.