Ultraviolet Irradiation of Skin Alters the Faecal Microbiome Independently of Vitamin D in Mice

Abstract

Reduced sunlight exposure has been associated with an increased incidence of Crohn's disease and ulcerative colitis. The effect of ultraviolet radiation (UVR) on the faecal microbiome and susceptibility to colitis has not been explored. C57Bl/6 female mice were fed three different vitamin D-containing diets for 24 days before half of the mice in each group were UV-irradiated (1 kJ/m²) for each of four days, followed by twice-weekly irradiation of shaved dorsal skin for 35 days. Faecal DNA was extracted and high-throughput sequencing of the 16S RNA gene performed. UV irradiation of skin was associated with a significant change in the beta-diversity of faeces compared to nonirradiated mice, independently of vitamin D. Specifically, members of phylum Firmicutes, including Coprococcus, were enriched, whereas members of phylum Bacteroidetes, such as Bacteroidales, were depleted. Expression of colonic CYP27B1 increased by four-fold and IL1β decreased by five-fold, suggesting a UVR-induced anti-inflammatory effect. UV-irradiated mice, however, were not protected against colitis induced by dextran sodium sulfate (DSS), although distinct faecal microbiome differences were documented post-DSS between UV-irradiated and nonirradiated mice. Thus, skin exposure to UVR alters the faecal microbiome, and further investigations to explore the implications of this in health and disease are warranted.

Keywords: inflammatory bowel disease; microbiome; ultraviolet radiation; vitamin D.

Figure 1

Alpha diversity of faecal samples. Faecal pellets were collected from UV-irradiated and nonirradiated mice that had been given either a high-dose (D++), standard-dose (D+), or no vitamin D (D−) diet. Mice were first established on diets for 24 days (days 0–24), then half were UV-irradiated daily with 1 kJ/m² UVR for 4 consecutive days (days 24 to 28), followed by biweekly exposures. Samples were collected after 6 UV treatments (day 35) and at the end of follow-up (protocol day 63). After faecal DNA was extracted, the V3–V4 segment of 16S rRNA was sequenced using the Illumina MiSeq. Alpha diversity is represented by Chao1. n = 5/group. UVR = ultraviolet radiation.

Figure 2

Faecal microbial composition of UV-irradiated vs nonirradiated mice. The effect of UV irradiation of skin on faecal microbial composition was examined using two-factor PERMANOVA controlling for vitamin D group. Linear discrimination analysis effect size (LDA score) was used to determine significant differences in relative abundance of individual taxa with UV treatment. Only taxa where a significant change (p < 0.05) was observed are illustrated. n = 44/group.

Figure 3

Colon CYP27B1, vitamin D receptor, cathelcidin, and IL-1β gene expression. At day 35, mice were sacrificed and colonic tissue harvested to determine gene expression in the distal (A–D) and proximal (E–H) colon from mice acquiring vitamin D exclusively through diet (D+UVR−) or UV irradiation (D−UVR+). Data are expressed as fold-change with the D+UVR− group as the control. mRNA gene expression by qPCR was calculated using the 2^−ΔΔCT method with TATA-box-binding protein as the housekeeping gene, n = 3/group. Values are expressed as mean ± SEM. * p < 0.05, ** p < 0.01.

Figure 4

Effect of UV irradiation of skin on dextran sodium sulfate (DSS) colitis. After mice were established on respective diets for 28 days and half of each group received 4 daily doses of 1 kJ/m² UVR, mice were treated orally with DSS 3% for 6 days. Colitis was measured by (A) colonoscopy calculating the murine endoscopic index of severity (MEICS), with n = 35/group for day 34, n = 10/group for day 42, and n = 5/group for assessments at days 49 and 63; (B) percentage weight loss; p-value comparison was for UVR vs no UVR within the same vitamin D diet group; and (C) histological severity score at day 35 and (D) day 42, n = 7–10/group. Values are shown as mean ± SEM. * p < 0.05, *** p < 0.001.

Figure 5

The effect of UV irradiation of skin versus dietary vitamin D on microbial composition of faecal samples post-DSS. Linear discrimination analysis effect size (LDA score) was determined for evaluation of significant differences in the relative abundance of individual taxa in faeces from DSS mice (day 35) that acquired vitamin D following skin exposure to UVR (D−UVR+) versus those mice acquiring vitamin D through diet alone (D+UVR−). Only taxa where a significant change (p < 0.05) was observed are illustrated; n = 5/group.