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

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 mPhi₀ was detected from a sample containing 300 cells of Candida albicans. The detection limit was estimated from the system noise level of 0.5 mPhi₀ 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.