Phage induction analysis Cell-DNA was extracted using a protocol described by Walsh et al.  modified to include a 20% Chelex® (BioRad Laboratories; Hercules, USA) solution instead of 5%. 20 ng of DNA was added to the sea real-time PCR assay, see above. Phage DNA was purified using zinc chloride as previously described by Santos  without previous DNase or RNase treatments. 200 ng of DNA was added to the sea real-time PCR assay, see above. Induction of the bacteriophage using MC (Duchefa Biochemie, Haarlem, the Netherlands) was performed according to Resch et al. . S. aureus overnight culture (0.2 ml) was added to 30 ml of fresh broth in 250 ml Erlenmeyer flasks. When cultures were
in the mid-exponential phase of growth, MC was added to a final Selleck PI3K inhibitor concentration of 0.5 μg/ml or 5 μg/ml, followed by continued incubation for 3 h. SEA concentrations, viable cell counts, and viable virus particles were determined. Cultures without addition of MC were used as controls. The phage plaque assay was performed as described by France and Markham . Supernatants from S. aureus cultures were spotted onto agar and the plates were then incubated at least overnight. S. aureus RN450 and RN4220 were used as receiver strains. The relative sea gene copy number was calculated using Decitabine concentration equation 1. The relative phage copy number was calculated using
the nominator part of equation 1. ELISA A modified protocol was developed for ELISA analysis of SEA using affinity-purified sheep polyclonal antibodies based on Poli et al. . A microtiter plate (Immulon® 2HB polystyrene, Flat Bottom Microtiter® Plates, 96 wells solid; Thermo Electron Corporation;
Waltham, MA) was coated with 100 μl/well of a solution containing 2 μg/ml SEA affinity-purified antibody (Toxin Technology, Inc.; Sarasota, FL) in coating buffer (0.1 M sodium carbonate, P-type ATPase pH 9.6, Merck) and left at 37°C overnight. All sites were blocked with 185 μl blocking buffer (SuperBlock Blocking Buffer in PBS, pH 7.4, Pierce, Rockford, IL) for one hour at 37°C and at least one hour at 4°C. The plate was washed four times with washing buffer (0.05% Tween 20, BioRad Inc., in 10 mM PBS, Sigma-Aldrich, St Louis, MO). Standards or culture supernatants were loaded onto the plate (100 μl/well) at appropriate dilutions and incubated for 90 min at 37°C. As SEA standard, highly purified SEA staphylococcal enterotoxin from Toxin Technology Inc. (Sarasota, FL), was used. The plate was washed and the biotinylated antibody (Toxin Technology, Inc.), diluted 2000 × in assay buffer (50 mM PBS, 0.01% bovine serum albumin, Sigma-Aldrich, 0.1% Tween 20, 0.01% Thimerosal, Sigma-Aldrich, 1% milk powder, Semper, Sundbyberg, Sweden) was added (100 μl/well). The plate was incubated for one hour at 37°C and washed. NeutrAvidin™-linked alkaline phosphatase (ImmunoPure NeutrAvidin™, alkaline phosphatase conjugated, 0.