Immunofluorescent microscopy
Overnight cultures of GAS were collected, washed and resuspended in PBS to an OD540 of 0.1. Ten microlitres of the bacterial dilution was then smeared across polylysine microscope slides (Esco, USA), air-dried at room temperature, and fixed for 10 min with 3% paraformaldehyde (Sigma, USA) in PBS. The slides were subsequently washed in PBS (10 min) and blocked with 5% skim milk/PBS-Tween 20 for 30 min in a humidified chamber at 37°C. The slides were then rinsed in PBS, incubated with non-specific human IgG (Serotech, UK) diluted 1 in 200 with 0.5% skim milk/PBS-Tween 20, for 1 h at 37°C in a humidified chamber. A 2-ml aliquot of the primary antibody samples diluted 1 in 200 with 0.5% skim milk/PBS-Tween 20 was added to each slide and incubated at 4°C overnight in a humidified chamber. Slides were then washed with two changes of PBS for 15 min and covered with 1 ml of a 1:100 dilution of fluorescein isothiocyanate (FITC)-conjugated F(ab′)2 goat anti-mouse IgG (Silenus), and incubated in the dark at room temperature for 2 h in a humidified chamber. The slides were then washed three times in PBS for 30 min, before mounting with Vectashield (anti-fade) (Vector Laboratories, USA). Slides were examined using an Axioskop fluorescent microscope at a detection wavelength of 520 nm.