Scientists have amassed a wealth of genetic information about microbes that cause disease. But whether in the clinic or the field, doctors often lack quick diagnostic tests to identify a microbe by its genetic fingerprint. Now researchers have designed a microfluidic chip that could provide an all-in-one, sensitive method to detect specific viruses or bacteria in patient samples without the need for specialized training or expensive equipment (J. Am. Chem. Soc., DOI: 10.1021/ja203981w).
To pinpoint specific microbes in a complex clinical sample, such as a patient's throat swab, an all-in-one diagnostic chip has to perform three different steps, says H. Tom Soh of the University of California, Santa Barbara. The chip must purify the viruses or bacteria from the rest of the sample, make copies of the microbes' signature nucleic acid sequences using the polymerase chain reaction (PCR), and then report the presence of those sequences. In 2009, graduate student Brian Scott Ferguson focused on the last two problems. He developed a microfluidic device that consisted of one chamber for carrying out PCR reactions and another that produces an electrical signal when it detects the DNA sequence of interest (. Chem., DOI: 10.1021/ac900923e).
But clinical samples are a complex soup of biomolecules, and some of them can foul up the PCR step: Nucleases can chew up target DNA sequences, while other proteins can disable the PCR enzymes. In the new study, Soh and his colleagues purified the microbes by selectively holding on to the biomolecules they wanted to yze, while washing away the rest.
They do it with antibody-coated magnetic particles. The researchers add the beads to a clinical sample before dropping the mixture onto the chip. The antibodies grab the microbe of interest in the sample. External magnets hold the beads in one of the chip's chambers while the researchers wash away the contaminants. The device then can start the PCR step with only the target microbe present.
The researchers tested the device with samples swabbed from Ferguson's own throat. They spiked the samples with known amounts of influenza virus. With the new chip, the team could detect the virus in less than 4 hours from samples that had as few as 10 viral particles.
Charles Chiu, director of the University of California, San Francisco's Viral Diagnostics and Discovery Center, says the device shows potential to be useful in clinics or the field. But he says the chip still needs rigorous testing before doctors could use it. In particular, he says, the researchers have to show that the chip can be used repeatedly and that it can detect microbes in samples from actual sick patients.
