2.3. Listeria monocytogenes
L. monocytogenes is the Gram-positive bacterium responsible
for the infectious disease listeriosis. It is the most virulent
microorganism and the third leading cause of death among
foodborne bacterial pathogens. For detection of L. monocytogenes,
Grossman et al [33] developed an interesting principle
based on monitoring the binding rate between antibodylinked
MNPs and bacteria using a high-transition temperature
superconducting quantum interference device (SQUID).
Superparamagnetic nanoparticles with a size of 50 nm coated
with antibody were added to the L. monocytogenes sample,
followed by applying pulsed magnetic field to align the magnetic
dipole moments. While free nanoparticles quickly
randomize by Brownian rotation, the nanoparticles bound to
L. monocytogenes could undergo Neel relaxation and gradually
dissipate magnetic flux for measurement by SQUID. A detection
limit of 5.6 106 and 230 L. monocytogenes cells were obtained
in a sample of 20 mL and 1 nL, respectively [33]. A similarmagnetic relaxation technique was also used for detection of
mycobacterial species, which will be described in the
following section.