Development of a SQUID System Using Field Reversal for Rapidly Detecting Bacteria

Abstract

Pathogen identification usually requires growth of the pathogen by culture, which requires considerable time and manipulation by an experienced operator, leading to delays in diagnosis and treatment. We have investigated pathogen detection using a highly sensitive HTS-SQUID and magnetic markers and have developed a rapid and simple pathogen detection method. The magnetic markers, magnetic nanoparticles coated with detecting antibodies, bind to the target substance (antigen). The magnetic signal of the bound markers is measured with the highly sensitive SQUID. A remarkable feature of the magnetic assay is the disappearance of the magnetic signal from the unbound markers due to Brownian rotation. This makes it possible to detect the magnetic signal of the bound magnetic markers without removal of the unbound markers. In practice, however, the residual field around the SQUID generates an undesired magnetic signal from the unbound markers as a result of biased Brownian rotation. We developed a field reversal method-a measurement scheme-that eliminates the magnetic signal from the unbound markers. A difference signal is obtained by subtracting the magnetic signals measured by applying a magnetization field in two directions. The validity of this method was demonstrated experimentally using polymer beads as simulated bacteria. Its feasibility was demonstrated by the detection of Candida albicans, a pathogenic fungus. A magnetic signal of 3 m8 0 was detected from a sample containing 300 cells of Candida albicans. The detection limit was estimated from the system noise level of 0.5 m8 0 to be about one hundred cells of Candida albicans, indicating that this method has high sensitivity. These results show that magnetic assay using a highly sensitive SQUID can provide rapid and simple pathogen testing without culture.

Improved Liquid-Phase Detection of Biological Targets Based on Magnetic Markers and High-Critical-Temperature Superconducting Quantum Interference Device

“…A blank signal was also generated by these markers. To eliminate the signal from the precipitated markers, we applied a field-reversal-measurement method [21], [22]. That is, in the measurement system shown in Fig.…”

Liquid-phase immunoassay utilizing binding reactions between magnetic markers and targets in the presence of a magnetic field

“…agnetic immunoassay techniques that use the Brownian relaxation of magnetic markers in suspension have been developed for the liquidphase detection of biological targets. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] The binding reaction between targets and magnetic markers has been detected by several methods, such as AC and DC susceptibility, [1][2][3][4][5][6][7][8] magnetic relaxation, [9][10][11][12][13][14] and remanence measurements. [15][16][17] The choice of detection method depends on the magnetic properties of the markers.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] The binding reaction between targets and magnetic markers has been detected by several methods, such as AC and DC susceptibility, [1][2][3][4][5][6][7][8] magnetic relaxation, [9][10][11][12][13][14] and remanence measurements. [15][16][17] The choice of detection method depends on the magnetic properties of the markers. By this method, we can magnetically distinguish bound markers from unbound (free) markers, eliminating the need for a time-consuming washing process for marker separation.…”

“…We have been developing a liquid-phase detection method employing large polymer beads that immobilize the bound markers. [15][16][17] In this method, biological targets are fixed on the surface of large polymer beads with sizes typically on the micrometer order. When the magnetic markers bind to their targets, the Brownian relaxation time is determined by the size of the polymer bead, which becomes much longer than that for free markers.…”

“…In this study, we developed a new method that can significantly decrease the aggregation of free markers due to the magnetization field. Unlike in previous studies, [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16][17] the binding reaction between magnetic markers and targets was performed in the presence of a reaction field of approximately B re = 1.5 mT. This field acts to prevent the Brownian rotation of markers during the binding reaction.…”