Faster cheaper TB diagnosis
A new cost-efficient on-the-spot diagnostic test that can detect both drug-sensitive and drug-resistant tuberculosis (TB) directly from patient samples has been developed by a collaborative group at the Center for Systems Biology (CSB). The test uses nuclear magnetic resonance to detect magnetically tagged genetic material extracted from sputum samples. The test is not only accurate, specific and fast but also portable and cheap; thus represents a promising solution to the problem of TB diagnosis in developing countries. The system is described in the May 2013 issue of Nature Communications.
TB is a significant public health threat and economic burden, particularly in developing countries. While early diagnosis and treatment are paramount for controlling spread of the disease, current diagnostic tests suffer from many shortcomings, primarily in terms of cost, labor and sensitivity. Consequently, TB diagnosis has continued to remain suboptimal.
Now, in a collaborative effort between investigators from MGH, Harvard Medical School, Harvard School of Public Health, and the Broad Institute of Harvard and MIT, an all-in-one, fast, sensitive, cheap, simple and portable TB test may soon be a reality.
Based on a previously designed diagnostic assay that relies on nuclear magnetic resonance, the test can detect the presence of TB directly in sputum samples.
“Originally we tried to detect whole bacteria,” explains Monty Liong, a research fellow at the Center and first author of the paper, “but we couldn’t find specific antibodies against the bacteria.” The researchers thus switched gears and designed a test that captures and detects TB-related genetic material.
The test involves injecting small amounts of sputum into a specially designed microfluidic chip. There, the bacterial DNA is amplified and captured onto polymer beads via complementary ‘capture’ DNA strands that bind to a specific end of the bacterial DNA. The captured DNA molecules on the beads are then mixed with complementary ‘probe’ DNAs, which bind to the other end of the bacterial DNA stands to form a ‘sandwich’. According to Liong, this sandwich is what makes the assay so sensitive. The probe DNAs are also bound to magnetic particles, which are consequently detected by magnetic resonance.
Importantly, by using DNA for TB detection, the investigators discovered that they could also detect drug resistant TB bacteria caused by a single nucleotide mutation. “This was very exciting,” says Hakho Lee, Assistant Professor at MGH and a senior author on the paper. “Using our system, we can not only universally detect the presence of TB but we can also identify whether the same patient will be resistant to conventional treatment.”
The research team are currently working on optimizing and miniaturizing the system further to be more easily deployed for use in resource-limited countries.
“Most developed countries have advanced systems to characterize TB bacteria,” explains Liong. “But this is not where TB is most relevant. It’s most relevant in lower resource settings. That is why having a simple assay that anyone can do is very crucial.”
After injecting sputum samples into the microfluidic device, target (tubercular) DNA is replicated (amplified), captured onto beads via ‘capture’ DNA strands, and magnetically labeled via ‘probe’ DNA strands. Tubercular DNA molecules are thus ‘sandwiched’ between a bead and a magnetic particle, which renders them highly detectable by nuclear magnetic resonance.
Written by Yvonna Fisher-Jeffes, PhD
Liong M, Hoang AN, Chung J, Gural N, Ford CB, Min C, Shah RR, Ahmad R, Fernandez-Suarez M, Fortune SM, Toner M, Lee H, Weissleder R
Magnetic barcode assay for genetic detection of pathogens
Nat Commun. 2013;4:1752 – PMID: 23612293 – PMCID: PMC3635151
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FirstPost – Now a portable device to detect tuberculosis, other infections (pdf)