Magnetic nanoparticle-antibody conjugates for the separation of Escherichia coli O157:H7 in ground beef.

The immunomagnetic separation with magnetic nanoparticle-antibody conjugates (MNCs) was investigated and evaluated for the detection of Escherichia coli O157:H7 in ground beef samples. MNCs were prepared by immobilizing biotin-labeled polyclonal goat anti-E. coli antibodies onto streptavidin-coated magnetic nanoparticles. For bacterial separation, MNCs were mixed with inoculated ground beef samples, then nanoparticle-antibody-E. coli O157:H7 complexes were separated from food matrix with a magnet, washed, and surface plated for microbial enumeration. The capture efficiency was determined by plating cells bound to nanoparticles and unbound cells in the supernatant onto sorbitol MacConkey agar. Key parameters, including the amount of nanoparticles and immunoreaction time, were optimized with different concentrations of E. coli O157:H7 in phosphate-buffered saline. MNCs presented a minimum capture efficiency of 94% for E. coli O157:H7 ranging from 1.6 x 10(1) to 7.2 x 10(7) CFU/ml with an immunoreaction time of 15 min without any enrichment. Capture of E. coli O157:H7 by MNCs did not interfere with other bacteria, including Salmonella enteritidis, Citrobacter freundii, and Listeria monocytogenes. The capture efficiency values of MNCs increased from 69 to 94.5% as E. coli O157:H7 decreased from 3.4 x 10(7) to 8.0 x 10(0) CFU/ml in the ground beef samples prepared with minimal steps (without filtration and centrifugation). An enrichment of 6 h was done for 8.0 x 10(0) and 8.0 x 10(1) CFU/ml of E. coli O157:H7 in ground beef to increase the number of cells in the sample to a detectable level. The results also indicated that capture efficiencies of MNCs for E. coli O157:H7 with and without mechanical mixing during immunoreaction were not significantly different (P > 0.05). Compared with microbeads based immunomagnetic separation, the magnetic nanoparticles showed their advantages in terms of higher capture efficiency, no need for mechanical mixing, and minimal sample preparation.