FIG. 2.

CRP binding to piliated and nonpiliated N. meningitidis in the presence or absence of calcium. (a) Cells of the piliated strain C311 (solid bars) and nonpiliated mutant C311p− (open bars) were dried on microtiter plates and incubated with CRP-AP with 0.5 mM CaCl₂ (calcium rich) or 0.5 mM CaCl₂ with 10 mM EDTA (calcium depleted). Plates were washed thoroughly, and bound CRP-AP was detected by adding pNPP substrate and measuring the optical density at 405 nm with a reference wavelength of 490 nm. (b) The calcium dependency of CRP binding to the piliated strain was confirmed by repeating the experiment without added calcium alongside 10-fold dilutions of the standard 0.5 mM CaCl₂. Data are expressed as the mean absorbances from six replicate experiments plus standard errors of the mean (*, P < 0.05).