UV irradiation trial #1

Five-week-old SPF-class female ICR mice (n = 38, body weight = 23 g) were used for this trial. The mice were divided into eight groups: a blank control group (n = 3) that received no treatment, negative control group (n = 5), and experimental groups A–E (n = 5 for each group) that received bacterial therapy with different candidates (A: Arthrobacter gandavensis; B: Bacillus psychrosaccharolyticus; C: Pantoea eucrina; D: Paenibacillus amylolyticus; E: Paenibacillus terrae) (Supplementary Table S3). After a preparation period, the area on the back of each mouse, measuring 2 cm × 3 cm, was depilated and smeared with the corresponding bacterial suspension for 3 days.

Before the trial, the mice were pre-feed with SPF pellet feed for one week. All feeding utensils were sterilized twice a week, and the ambient temperature was controlled at 20–25°C. Each mouse was housed in an individual cage with ad libitum access to food and water. After the mice were adaptively fed for a week, and bacterial colonization of each mouse was checked by bacterial culture of the back. Further details regarding the grouping and treatment conditions can be found in Supplementary Table S3. All mice were exposed to the same combination-rays light, and the changes in the skin morphology of each group were observed and recorded daily throughout the trial. Two types of UV light (UVA, UVB) boxes were used to irradiate the depilated mice.

A UV facility comprises a UVA lamp (40 W, with a wavelength range of 320 to 400 nm and a peak wavelength of 365 nm, from Philips, Germany) and a UVB lamp (20 W, with a wavelength range of 290 to 320 nm and a peak wavelength of 297 nm, also from Philips, Germany) positioned side by side. The distance between the lamps and the backs of the mice was set at 30 to 40 cm, and the irradiation intensity was measured using a UV-radiometer (UVAB/ST-513, SENTRY, Taiwan, China). We followed the subacute light injury model used in animal experiments (Wei et al., 2002) and optimized it based on our specific experimental conditions. After preheating the tubes for 20 min, the mouse cages were placed under the lamp box. During the trial, the position of the mouse cages was rotated to ensure equal irradiation dose for all mice. In the first week of irradiation, we performed daily UV irradiation, with a daily UVA dose of 3.96 J/cm² and a UVB dose of 252 mJ/cm². During the second and third weeks of irradiation, we performed UV irradiation every other day for a duration of 6 h, with UVA and UVB doses of 10.8 J/cm² and 1.08 J/cm², respectively. After each irradiation, we applied the corresponding bacterial suspension on the depilated area of the mice’s backs. Overall, the total amount of UVA irradiation in this trial was 114.12 J/cm², and for UVB, it was 8.964 J/cm².

At the end of the trial, the mice were euthanized by cervical dislocation. The skin on their backs was immediately peeled off and immersed in a 4% formaldehyde solution, and stored at 4°C. The skin was then cut into 5 μm thickness sections, stained with H&E (hematoxylin–eosin staining), and observed under a microscope to record the histopathological changes (Figures 1A–D).

Mice trial #1: pathological changes of skin and sections, and scoring of skin pathological changes. (A–D) As followed, no obvious pathological changes in mouse skin; mild lesions, a small amount of inflammatory cell infiltration; moderate lesions, epidermal thickening, inflammatory cell infiltration and severe lesions, exposed subcutaneous tissue, and a large number of inflammatory cell infiltration. (E) Scores of pathological changes in skin sections of mice in each group. Blank (blank control group): without UV and smear bacteria treatment; Negative (negative control group): only UV treatment without smear bacteria suspension treatment; Group A–E: separately smear the suspension of Arthrobacter gandavensis, Bacillus psychrosaccharolyticus, Pantoea eucrina, Paenibacillus amylolyticus, and Paenibacillus terrae.