Portable Device Assays Genetic Detection of Pathogens

Polymerase chain reaction (PCR)-amplified mycobacterial genes are sequence-specifically captured on microspheres, labeled by magnetic nanoprobes and detected by nuclear magnetic resonance.

Tests of the device on samples from patients known to have TB and from healthy controls identified all positive samples with no false positives in less than three hours. Existing diagnostic procedures can take weeks to provide results and can miss up to 40% of infected patients. Results were even stronger for patients infected with both TB and human immunodeficiency virus (HIV), probably because infection with both pathogens leads to high levels of the TB bacteria, and specialized nucleic acid probes developed by the team were able to distinguish treatment-resistant bacterial strains.

The system detected DNA from the Mycobacterium tuberculosis bacteria in small sputum samples. After DNA is extracted from the sample, any of the target sequence that is present is amplified using a standard procedure, then captured by polymer beads containing complementary nucleic acid sequences, and labeled with magnetic nanoparticles with sequences that bind to other portions of the target DNA. The miniature NMR coil incorporated into the device, which is about the size of a standard laboratory slide, detects any TB bacterial DNA present in the sample.

The investigators also developed both a universal nucleic acid probe that detects a ribosomal ribonucleic acid (rRNA) region common to many bacterial species and a set of probes that target sequences specific to 13 clinically important pathogens, including Streptococcus pneumoniae, Escherichia coli, and methicillin-resistant Staphylococcus aureus (MRSA). The device was sensitive enough to detect as few as one or two bacteria in a 10 mL blood sample and to accurately estimate bacterial load. Testing the system on blood samples from patients with known infections accurately identified the particular bacterial species in less than two hours and also detected two species that had not been identified with standard culture techniques.

Ralph Weissleder, MD, PhD, cosenior author of the study said, “The magnetic interactions that pathogen detection is based on are very reliable, regardless of the quality of the sample, meaning that extensive purification, which would be difficult in resource-limited setting, is not necessary. The ability to diagnose TB in a matter of hours could allow testing and treatment decisions within the same clinic visit, which can be crucial to controlling the spread of TB in developing countries." The study was published on April 23, 2013, in the journal Nature Communications.

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