Materials and Reagents

agr-positive S. aureus strains
Bacto^TM Tryptic Soy Broth (BD, catalog number: 211825 )
Lysostaphin (Wako Chemicals USA, catalog number: 120-04313 )
15% sodium dodecyl sulfate (SDS)-polyacrylamide gel
N-Cyclohexyl-3-aminopropanesulfonic acid (CAPS) (Dojindo Molecular Technologies, catalog number: 343-08321 )
Methanol
Immobilon-P (EMD Millipore, catalog number: IPVH304F0 )
Tris(hydroxymethyl)aminometane (Nacalai Tesque, catalog number: 35434-34 )
EDTA^.2Na (Dojindo Molecular Technologies, catalog number: 345-01865 )
Hydrochloric acid
Coomassie Plus Protein Assay Reagent (Thermo Fisher Scientific, catalog number: 23236 )
Easy Blocker (GeneTex, catalog number: GTX425858 )
Anti-AgrA IgG that was made in our laboratory (Kaito et al., 2013)
Anti-rabbit IgG conjugated with alkaline phosphatase (Promega Corporation, catalog number: S3731 )
Nitro blue tetrazolium/5-bromo-4-chloro-3’-indolyphosphate (NBT/BCIP) (Roche Diagnostics, catalog number: 1681451 )
PVDF membrane
TE buffer (see Recipes)
Lysis buffer (see Recipes)
Staining buffer (see Recipes)
10x SDS sample buffer (see Recipes)
10x TBS (see Recipes)
TBST (see Recipes)

Equipment

Sonicator (Branson, model: Sonifier 450) with double stepped microtip (3 mm) (Branson, part number: 101-063-212 )
Plastic container
Centrifuge machine
Electrophoresis apparatus
Power supply
Wet/Tank Blotting Systems (Bio-Rad Laboratories)

Software

Image J (1.45 s, NIH)

Procedure

Pick up single colony of S. aureus into 5 ml of fresh tryptic soy broth (TSB) and aerobically culture it for 15 h at 37 °C with shaking at 150 rpm.
Inoculate 50 µl overnight culture of S. aureus into 5 ml (1:100 dilution) of fresh TSB and aerobically culture it for 24 h or 15 h at 37 °C with shaking at 150 rpm. A₆₀₀ of starting and end will be approximately 0.06-0.1 and 6-10, respectively.
Collect 650 µl S. aureus cell culture by centrifugation at 10,000 x g for 1 min at 4 °C and discard the supernatant. Resuspend the cell pellet in 195 µl of TE buffer by vigorous vortexing. Add 5 µl of lysostaphin (1mg/ml) to the cell resuspension and incubate the resuspension at 37 °C for 30 min without agitation.
Sonicate the sample on ice (Branson Sonifier 450; double stepped microtip; duty cycle, constant; output control, 1; time, 15 sec), and centrifuge it at 10,000 x g for 10 min at 4 °C.
Note: This step is essential to remove the most of protein A that is associated with cell wall.
Measure the amount of protein in the supernatant by the Bradford method using Coomassie Plus Protein Assay Reagent according to the manufacture’s protocol. Equalize the protein concentration of different samples by adding TE buffer. Mix the protein samples with 3x SDS sample buffer.
Boil protein samples for 5 min and electrophorese the samples in 15% sodium dodecyl sulfate (SDS)-polyacrylamide gel at 100 V for 3 h.
Note: For details of SDS-polyacrylamide gel electrophoresis, please see Kaito et al. (2013).
Dip the Gel in a buffer (10 mM CAPS, 20% methanol) for 10 min. Treat PVDF membrane with methanol for a few seconds and dip it in a buffer (10 mM CAPS, 20% methanol). Set the gel, membrane, and buffer (10 mM CAPS, 20% methanol) using Wet/Tank Blotting Systems and transfer the proteins from the gel to a PVDF membrane at 150 mA for 3 h at 4 °C.
Treat the membrane with 10 ml blocking buffer (TBST containing 5% Easy Blocker) in a plastic container at room temperature with gentle agitation for 1 h.
Note: The use of Easy Blocker is essential to decrease the reactivity of IgG against protein A in the samples.
Treat the membrane with 10 ml blocking buffer containing 1:1,000 anti-AgrA IgG in a plastic container at room temperature with gentle agitation for 1 h.
Note: For using antibodies, the appropriate concentration should be determined by serial titration.
After washing with 20 ml TBST for three times of 5 min agitation, treat the membrane with 10 ml blocking buffer containing 1:2,000 anti-rabbit IgG conjugated with alkaline phosphatase in a plastic container at room temperature with gentle agitation for 1 h.
After washing with 20 ml TBST for three times of 5 min agitation, treat the membrane with a staining buffer without agitation for 5 min.
Note: For detection of proteins by alkaline phosphatase, please see details in Kaito et al (2013).
After bands appear, dip the membrane in TE buffer supplemented with 10 mM EDTA to stop overstaining and then dry the membrane on paper towel.
Measure the band intensity by densitometry scanning (Image J 1.45 s, NIH).

Recipes

TE buffer
10 mM Tris-HCl (pH 8.0)
1 mM EDTA (pH 8.0)
Staining buffer
100 mM Tris-HCl (pH 9.5)
100 mM NaCl
50 mM MgCl₂
2% NBT/BCIP
3x SDS sample buffer
1.5 ml 1 M Tris-HCl (pH 6.8)
3 ml 20% SDS
3 ml Glycerol
500 µl 1% bromo phenol blue
1.8 ml 2-mercaptoethanol
10x TBS
24.2 g Tris
87.6 g NaCl
1 L milliQ
Adjust pH to 7.6 by adding hydrochloric acid
TBST
125 ml 10x TBS
1,168 ml milliQ
7.5 ml 20% Tween20

Acknowledgments

This protocol was adapted from the original work (Kaito et al., 2013) to provide the detailed procedures. This work was supported by Grants-in-Aid for Scientific Research (23249009, 24590519). This work was supported in part by the Naito Foundation, the Program for Promotion of Fundamental Studies in Health Sciences of the National Institute of Biomedical Innovation (NIBIO), and the Genome Pharmaceutical Institute.

References

Gallagher, S. R. and Emily A. Wiley, E. M. (2008). Current protocols essential laboratory techniques. John Wiley & Sons, Inc.
Kaito, C., Saito, Y., Ikuo, M., Omae, Y., Mao, H., Nagano, G., Fujiyuki, T., Numata, S., Han, X., Obata, K., Hasegawa, S., Yamaguchi, H., Inokuchi, K., Ito, T., Hiramatsu, K. and Sekimizu, K. (2013). Mobile genetic element SCCmec-encoded psm-mec RNA suppresses translation of agrA and attenuates MRSA virulence. PLoS Pathog 9(4): e1003269.