Immuno-magnetic bead mass transport and capture efficiency at low target cell densities in phosphate-buffered saline

Anti-salmonella immuno-magnetic bead (IMB) capture efficiency (E) was determined by varying both IMB levels ([IMB]) and mixing time (tau(MIX)) and enumerating captured Salmonella enteritidis cells. We observed that E varied with tau(MIX) as a Pseudo-first order process with a rate constant (kappa) of 0.028 +/- 0.001 min(-1) and an infinite tau(MIX) asymptote of 0.97 (97% capture). Thus, even at low target cell densities ([S. E.] less than or equal to 70 CFU mL(-1)), nearly 100% of the bacteria were captured as tau(MIX) approached 2 h. We hypothesize that kappa is the product of both IMB mass transport (gamma) and IMB concentration ([IMB]) terms. Thus, gamma represents the total volume which each IMB samples per unit time of mixing. This idea appears reasonable as gamma({3.2 +/- 0.2} x 10(-9) mL min(-1) IMB-1), determined from E-based observations at various [IMB] levels (similar to10(6) -10(8) IMB mL(-1)) and fixed tau(MIX) (30 min), was nearly identical to gamma ((3.5 +/- 0.1) x 10(-9) mL min(-1) IMB-1) derived kinetically (tau(MIX) = 0-120 min; [IMB] 8 X 10(6) IMBs mL(-1)). Estimating a mass transport term (gamma(CALC)) founded on classical dynamics we obtained a value 5.3 X 10(-10) mL min(-1) IMB-1 whereupon gamma(CALC) and gamma would have completely agreed were the IMB radius (r(IMB) = 1.4 mum) 0.8 PM larger. The apparent discrepancy in r(IMB) might be explained by a greater effective (hydrodynamic) r(IMB) which could result from some combination of IMB swelling and brownian motion. These results argue that IMB-based target cell capture is modulated by IMB mass transport and collision probability.