Mouse model of S. aureus skin infection.

Skin infection experiments were done as described before (47). S. aureus strain USA300/MRSA was used for infection. In brief, the backs of sex-matched and age-matched (8-week to 12-week) C57BL/6 WT or C57BL/6 Ella/Hyal1 mice were shaved and hair removed by chemical depilation (Nair), then injected subcutaneously with 100 μl of a midlogarithmic growth phase of S. aureus (2 × 10⁶ CFU of bacteria) in PBS. Mice were sacrificed after day 3, and an 8-mm skin punch biopsy comprising the center of the injection site was harvested. Infected skin surrounding the infection center (6–8 mm) void of center abscess was carefully dissected out for RNA extraction or CFU determination. Skin biopsies were homogenized in 1 ml TRIzol (Life Technologies Corporation) (for RNA) or PBS (for CFU counting) with 2-mm zirconia beads in a Mini-Beadbeater-16 (BioSpec Products). To count CFU, homogenized skin samples were serially diluted, plated onto Tryptic Soy Agar (Sigma-Aldrich), and enumerated after 18 hours to quantify the CFU per gram of tissue. For some experiments, PEGPH20 (1 mg/kg) was injected intravenously starting from 1 day before infection once, and the vehicle (10 μmol/L histidine, 130 μmol/L NaCl, at pH 6.5) was used as a control.