Using Positively Charged Magnetic Nanoparticles to Capture Bacteria at Ultralow Concentration

Detecting bacteria at low concentrations without time-consuming culture processes would allow rapid diagnoses. Since electrostatic attraction exists between negatively charged bacterial cells and positively charged magnetic nanoparticles (NP+), capture of bacteria holds great promise towards achieving this goal. Here, we present a rapid and highly efficient approach to capture Escherichia coli, which was used as a model for gram-negative bacteria. Capture of E. coli at very low concentrations of 10 and 100 CFU/mL using NP+ is rapidly and efficiently achieved within 1 h. Moreover, the capture efficiency of NP+ was over 90% by analyzing the number of bacterial colonies on the plate. Optical and transmission electron microscopy confirmed the bacterial capture abilities of electrically charged nanoparticles (NPs). In contrast, negatively charged magnetic nanoparticles (NP-) did not show affinities towards E. coli. These results showed that bacterial cells, such as E. coli, carry a negative charge. Unlike a ligand-dependent capture system, our designed NP+ has potentials to capture a broad range of bacteria via electrostatic attractions.