| Summary: | Systemic antibiotics are extensively used to control the papules and pustules of rosacea. Hence, it is crucial to understand their impact on the rosacea skin microbiota which is thought to be perturbed. The purpose of this study was to compare the makeup and diversity of the skin microbiota in rosacea before and after taking oral antibiotics. We also compared the skin microbiota at baseline according to age and rosacea severity. A longitudinal cohort study was performed on 12 rosacea patients with papules/pustules and no recent use of oral and topical antimicrobials/retinoids. Patients were prescribed oral doxycycline, 100 mg, twice daily for six weeks. Skin areas on the cheek and nose were sampled for 16S ribosomal RNA gene sequencing at baseline, and after six weeks of doxycycline treatment. Eleven females and one male aged 20−79 (median 51) with a median Investigator’s Global Assessment score of 3 (moderate) were enrolled. At baseline, <i>Staphylococcus epidermidis</i> was the most dominant species followed by <i>Cutibacterium acnes</i> (formerly <i>Propionibacterium acnes).</i> In the 60 Over-age group, the prevalence of <i>Cutibacterium acnes</i> was lower than that of the 60 & Under-age group. Rosacea severity increased with age and was associated with a decrease in the relative abundance of <i>Cutibacterium acnes</i> and an increase of <i>Snodgrassella alvi.</i> Across all subjects, antibiotic treatment reduced clinical rosacea grades and was associated with an increase in the relative abundance of <i>Weissella confusa</i> (<i>P =</i> 0.008, 95% CI 0.13% to 0.61%). Bacterial diversity (alpha diversity) was not significantly altered by antibiotics treatment. Principal coordinates analysis showed mild clustering of samples by patient (ANOSIM, Analysis of Similarity, <i>R</i> = 0.119, <i>P</i> = 0.16) and scant clustering with treatment (ANOSIM, <i>R</i> = 0.002; <i>P</i> = 0.5). In conclusion, we believe that rosacea has a unique age-dependent characteristic (i.e., severity). Although we were not able to pinpoint a causative microbiota, our study provides a glimpse into the skin microbiota in rosacea and its modulation by systemic antibiotics.
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