Fig. 2: Human nasal mucus–derived S. epidermidis suppressed the viral spread and replication in IAV-infected mice.

A Schematic of the mouse model experimental design for intranasal S. epidermidis inoculation and IAV infection. The mice (N = 20, group 1 (n = 5): S. epidermidis (−) IAV (−), group2 (n = 5): S. epidermidis (+) IAV (−), group 3 (n = 5): S. epidermidis (−) IAV (+), group 4 (n = 5): S. epidermidis (+) IAV (+)) were used in these experiments. The mice were inoculated with S. epidermidis (3.2 × 10⁶ CFU/30 µl PBS) and infected with IAV (2,130 pfu/30 ul PBS) at the indicated time points. The changes in (B) the mean body weight (analyzed by repeated measure two-way ANOVA) and survival rate (C) of IAV-infected mice (analyzed by Kaplan–Meier with log-rank test) were compared according to the inoculation of S. epidermidis. 20 mice were used and ten were infected with IAV (2,130 pfu). IAV-infected mice with depletion of the nasal microbiota exhibited over 30% decrease in mean body weight until eight days but the mean body weight of nasal microbiota-depleted B6 mice that were treated with S. epidermidis before IAV infection exceeded 20 g until 8 days after infection. Lung samples from uninfected mice and mice that survived up to 7 dpi were collected. D B6 mice were inoculated with human nasal mucus–derived S. epidermidis before IAV infection, and H&E-stained micrographs were also generated from lung sections obtained at 7 dpi (Scale bar 100 uM). Micrographs shown are representative of lung sections from five mice and were used to assess inflammation and tissue damage and to calculate a histological score. B6 mice were inoculated with human nasal S. epidermidis before IAV infection, and IAV PA mRNA levels in the mouse lung tissue 9 (E) and nasal mucosa (F) were assessed at 7 dpi. Levels of IAV NP were monitored in lung tissue (G) and nasal mucosa (H) using western blot analysis, and representative results are shown. I Viral titers were also measured in the BAL fluid of IAV-infected mice following S. epidermidis inoculation. In the next independent experiments, ten mice were used to measure the colonies and FemA mRNA levels of S. epidermidis according to IAV infection. J S. epidermidis CFUs were determined at 7 dpi in the NAL and BAL fluid of IAV-infected mice following S. epidermidis inoculation. K FemA mRNA levels of S. epidermidis were measured in the nasal mucosa and lung tissue of IAV-infected mice at 7 dpi following S. epidermidis inoculation. Real-time PCR, plaque assays, colony count, and ELISA results are analyzed by Mann–Whitney U-test and presented as mean ± SD values from three independent experiments. *p < 0.05 vs. mice infected with IAV alone.