Bacterial interactions with receptor constructs. Bacterial lysates (c. 2 ¥ 10 7 bacteria) were applied to nitrocellulose strips, air-dried and non-specific binding sites blocked using 3% BSA in PBST for 1 h at room temperature. Strips were overlaid with either native or mutated soluble NA1B-Fc diluted in 1% BSA in PBST at required concentrations for 1 h at room temperature. In most experiments, excess (1-3 mg ml -1 ) of the receptor was used. In competition studies, a range of concentrations (0.008-0.5 mg ml -1 ) was employed. Following washing to remove unbound NA1B-Fc, receptor binding was detected using anti-human-Fc alkaline phosphatase conjugate and substrates as described above. For quantification, densitometric analyses of the developed immunoblots were carried out using NIH Scion Image software. In most cases, multiple estimations were carried out and means and SE of each determination have been reported.