Having formed the mixed model glycocalyces, their binding to the bacterial toxins was then quantified by QCM-D (Figures and). Figures and show the net negative frequency shifts for binding of STxB and CTB, respectively, as a function of the target glycan concentration in the model glycocalyx. For the StxB experiments, use of the (HA-g-Gb3)-H6 with (Ni 2+ -NTA)3-presenting SLB allowed access to the highest Gb3 concentrations. Each lectin was studied at two concentrations that differed by a factor of five, within an order of magnitude of the Kd, values obtained in Figure . As expected, lectin binding increased monotonically with the concentrations of both the target glycan and the lectin. Most interestingly, the dependence of the binding response on the concentration of target glycans was rather strong. Data in Figure larger than one indicates a super-linear increase in the rate of the relative change in lectin binding as a function of the rate of the relative change in target glycan concentration. Crude fits with power laws across the full spectrum of target glycan concentrations reveal mean α values significantly larger than 1 for both CTB concentrations tested, and also for 0.4 μM STxB pentamer (Figure -C; dashed lines). For 0.7 μM CTB pentamer, for example, α = 3.3 implies that a 2-fold change in Le x concentration entails a 2 3.3  10-fold change in toxin binding. Such a super-linear dependence of multivalent binding on ligand concentration has been termed 'superselectivity', and our data thus demonstrate that AB5 toxins recognize their target glycans superselectively.