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Topical antibiotics affect the microbial make up of skin long after application, while antiseptics have a smaller, less durable impact, according to a mouse study published in Antimicrobial Agents and Chemotherapy.

Like the gut, numerous microbes live on the skin. These typically coexist as a stable ecosystem, however, when disturbed, infection by dangerous microbes can occur. These are usually treated by topical antiseptics, to disinfect the skin prior to surgical procedures or after exposure to contaminated objects, or by topical antibiotics, to decolonize the skin of specific kinds of bacteria or for rashes or wounds.

This is the first research to how that long-term effects of anti-microbial drugs on the skin microbiome. Past studies have revealed that medication can alter the gut microbiome, but, until now, little was known about how topical treatments can impact the microbial health of skin.

The research team from the Perelman School of Medicine at the University of Pennsylvania treated the skin of hairless mice with a variety of antibiotics. These antibiotics changed the makeup of the microbial communities. Significantly, this change lasted for days after treatment stopped.

"The use of laboratory mice were key to our investigation because of the limitations presented by more standard, in vitro methods. Traditionally, the efficacy of antibiotics and antiseptics are studied using tests in which specific bacteria are exposed to varying concentrations of an antimicrobial drug in a test tube or on a petri dish. While this can give us a relatively good idea of the concentrations necessary to eliminate a particular pathogen, the skin is a very different environment than these in vitro systems," the study's lead author, Adam J. SanMiguel, Ph.D., a researcher at the University of Pennsylvania, explained to ALN.

Conversely, when a group of hairless mice were treated with antiseptics, the researchers found that there was little change to the make up of their skin microbiome. Compared to the control group, there was no clear difference in the relative number of individual bacteria strains.

"We were so surprised by our finding with antiseptics and the skin microbiome. Normally, these drugs are extremely effective when testing bacteria by traditional, in vitro tests. However, we found this response to be fairly muted when examining skin bacterial residents of mice in vivo. This suggests that something very different is happening at the skin surface compared to what would be seen in a test tube or petri dish, and is something we plan to follow-up on with future experiments as well," SanMiguel said.

Both antiseptics and antibiotic treatments removed a resident bacteria that protected the skin against S. aureus infection.

"Our work shows that the skin microbiome can be relatively stable in the face of certain acute stressors such as antisepsis, but that other interventions such as topical antibiotic treatment can elicit immediate and sustained changes to these same communities," SanMiguel concluded.

"While we focus on the ramifications of these disruptions in regard to colonization resistance and S. aureus susceptibility, the potential also exists that altered communities may interact with the skin in previously unknown ways. By studying the impact of external stressors such as antimicrobial treatment, we hope to better understand host-microbiome interactions, and the importance of the skin microbiome to cutaneous health and disease."

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