The speed and accuracy of pathogen diagnosis can be improved with homogeneous magnetic separation
Microbiological culture is a common diagnostic method for the detection of infectious disease. However, it is not a perfect tool. Results are sometimes inconclusive, and long growth time means that a microorganism may not be detected quickly enough to prevent a disease outbreak. Microbial culture requires sterile media containing proper growth serum to selectively boost the growth of a microorganism. The diagnostic culture may fail if the media is contaminated or if an inappropriate nutrient cocktail is used. Moreover, results can be inconclusive if multiple microorganisms are present in a single culture. Similarly, one microorganism may out-compete and mask the detection of another. All of these problems could lead to an inaccurate or incomplete clinical diagnosis, and could adversely affect public health by delaying treatment. The speed and accuracy of pathogen diagnosis can be improved with biomagnetic separation. This technique offers tailored identification and isolation of commonly suspected pathogens in patient samples. The major benefits of technology are sharp selectivity, quick isolation time, improved cell viability, and high isolate concentrations. The rapid nature of the technique is especially valuable for clinical laboratories interested in quick pathogen detection for the maintenance of public health.
The general idea
Microorganisms can be specifically identified by unique surface ligands. Magnetic beads are coated with antibodies that bind to the ligands to make microorganism-bead conjugates. The superparamagnetic beads become magnetized when placed into a magnetic field. Then they move along a magnetic field gradient. This property allows the microorganism-bead conjugates to be isolated from a bulk solution. If all of the parameters of the technique are engineered properly the collected microorganisms will remain viable for further characterization by culture or qPCR. There are two main parameters to optimize when performing magnetic separation. First is the choice of magnetic bead including size, coating, and surface antibody. The second is choosing a magnetic separation system that provides a homogeneous magnetic force throughout the sample to improve microorganism collection rates and viability.
Antibody-bound magnetic microbeads
Immunomagnetic separation is a technique whereby antibody-coated magnetic microbeads selectively bind to a microorganism of interest and magnetically isolate it from bulk solution. Magnetic microbeads range from 0.5 to 500 micrometers, on the scale of bacterium, and magnetic nanoparticles range from 5 to 500 nanometers, on the scale of a virus. They consist of a core of superparamagnetic material, commonly magnetite. The core is not innately magnetic, but becomes magnetized when placed into a magnetic field. This quality allows them to be manipulated with a magnetic field. The beads are coated with a biocompatible material that allows antibodies or other peptides to be attached to the surface. The coatings also render the magnetic particles themselves biologically inert. Therefore, they don’t damage the microbes during the separation process. By selecting an appropriate antibody to the microbe of interest, the beads are tailored to selectively isolate microbes from a heterogeneous sample. One or more beads will bind to the surface of the microorganism, and this conjugate can then be isolated from the solution with the use of an external magnetic field.
Homogeneous magnetic separation systems
Choosing a separation system to isolate microorganism-bead conjugates from solution is not a decision to be made lightly. System choice is important for obtaining an enriched population of viable undamaged cells. Poorly designed systems can lead to cell death due to uneven magnetic forces that break the cell membrane. When a magnet is placed outside of a tube it creates a magnetic field gradient throughout the solution. The magnetic force on the beads is strongest close to the magnet and decreases inversely with distance away from the magnet. In a magnetic field gradient the microorganism-bead conjugates move along the gradient toward the magnet. If the magnetic forces far from the magnet are too weak the conjugates will move slowly or not at all. If the magnetic force close to magnet is too strong then the cell membrane of the cells nearest to the magnet may break. Therefore, it is important to use a properly scaled separation system to create a homogeneous magnetic gradient throughout the sample. In homogeneous magnetic separation all immunocaptured microbes experience the same conditions regardless of the distance between them and the magnetic source. This gentler separation technique produces an enriched population with a high percentage of viable microorganisms for analysis.
Putting it to the test
Immunomagnetic separation has been shown to be more reliable than traditional culture methods in the detection of microorganisms. A recent study compared the reliability of the traditional culture system to the reliability of immunomagnetic separation for the detection of Legionella bacterium in water samples. Infection with Legionella requires treatment with antibiotics. Left untreated it can be very dangerous and complications include lung failure and death. It is spread through contamination of fresh water. Therefore, the rapid and accurate detection of the bacteria in water samples is crucial to maintenance of public heath. In the study, a total of 65 water samples were evaluated for presence of Legionella bacteria. The study found that traditional culture diagnosis technique was inconclusive in 36.9% of samples tested, but the magnetic separation method did not have any inconclusive results [1]. The immunomagnetic separation method is a good technology for fast and reliable diagnosis and detection of microorganisms.
Obtaining a homogeneous magnetic separation system
SEPMAG is a leading manufacturer of commercial systems engineered to maintain cell viability and achieve the highest bead recovery possible. SEPMAG systems are also carefully designed to allow proper scaling to ensure a homogeneous gradient for magnetic separation across a range of volumes. More information about choosing a proper magnetic bead size, coating, or antibody for a specific application can be found at SEPMAG. Multiple references available for those seeking an in-depth explanation of the details of homogeneous magnetic separation. SEPMAG has the experience and the technology to improve the speed and reliability of microbe detection in a variety of samples.
Reference
- Comparing methods of determining Legionella spp. In complex mater matrices. Diaz-Flores, Alvaro et. al. BMC microbiology 2015 15:91.