Summary: A trivalent lanthanide ion, erbium (Er), has been used in combination with a magnetic separation technique to isolate seven bacterial species from suspensions in 0·9% saline. Erbium has an exceptionally high atomic magnetic moment of 9·3 Bohr magnetons, and following addition as ErCl (final concentration 5 mM) to bacterial suspensions, it imparts the magnetic moment to the bacterial cells by ionic binding to the cell surface. Strains of and were obtained from the Quality Control Depository of The Cleveland Clinic Foundation, Cleveland, Ohio, USA as suspensions in 0·9% NaCl, in concentrations ranging from 10 to 10 c.f.u. ml. Bacteria were separated from solution inside a capillary flow cell exposed to a highly non-homogeneous magnetic field (maximum field intensity was 0·4 T) and quantified by a light scattering method. The quantity of cellular deposition in the magnetic field was correlated with the initial concentration of cells in the suspension, expressed in c.f.u. ml, and sample volume (1·5 and 3·0 ml), sample pH (prior to ErCl addition), affinity to Gram stain (negative vs positive) and species. Magnetic deposition was observed for concentrations as low as 10 and 10 c.f.u. ml, and a significant correlation between average scattered light intensity and initial cell concentration (correlation coefficient ≥ 0·98) was established in the range 10 to 10 c.f.u. ml for all but one species (). Magnetic deposition increased with increasing pH from 7·0 to 10·0 which is consistent with the prevailing view on the mechanism of the lanthanide ion binding to bacterial cells (ionic, to the cell surface). No significant difference in magnetic deposition was observed between individual strains, or between Gram-positive and -negative bacteria. Magnetic isolation of cells may find application in rapid total cell count determination, such as rapid urine screening.


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