Rapid COVID-19 tests are on the rise, delivering results to more people faster, and scientists need a simple, fail-safe way to know that these tests are working correctly and the results can be trusted. The nanoparticles that play the role of Novel Coronavirus’s passing may be just the thing.
The coronavirus-like nanoparticles, developed by nanoengineers at the University of California, San Diego, will act as a so-called positive control for COVID-19 tests. A positive control is a sample that always tests positive. They are run and analyzed with patient samples to verify that COVID-19 tests are consistently working as intended.
The positive controls developed at UC San Diego have several advantages over those currently used for COVID-19 testing: they do not require refrigeration; They are easy to make; They can be included throughout the testing process, just like patient samples; Because they do not come from actual virus samples from COVID-19 patients, they do not pose an infection risk to those running the tests.
Researchers led by Nicole Steinmetz, a professor of nanoengineering at the University of California, San Diego, published their work in the journal Biomacromolecules. This work builds on an earlier version of the positive control developed by Steinmetz LABS for the RT-PCR test, which is the gold standard for COVID-19 testing. The positive control in the new study could be used not only for the RT-PCR test, but also for a cheaper, simpler and faster test called the RT-LAMP test, which can be done on the spot and provide results in about an hour.
Having a robust tool to ensure these tests run accurately, especially for low-tech diagnostic tests like RT-LAMP, is critical, Steinmetz said. It can help rapid, large-scale testing of COVID-19 in under-resourced, underserved areas and other places where advanced testing equipment, specialized reagents and trained professionals are not available.
The new positive controls are basically tiny viral coats made of plant viruses or bacteria that contain fragments of coronavirus RNA. The RNA fragments included the binding sites of the two primers used in PCR and LAMP tests. This design creates an all-in-one comparison that can be used for any of these tests, making it very versatile.
The team developed two types of positive controls. One is made from nanoparticles of plant viruses. To make them, researchers infected cowpea plants with cowpea spotted virus in the lab and extracted the virus from the plants. After that, researchers removed the virus’s RNA and replaced it with a custom RNA template that contained the specific but noninfectious sequence of the Novel Coronavirus. The resulting nanoparticles are made up of sequences of coronavirus RNA wrapped in the shell of the plant virus.
Another positive control was made from thallus nanoparticles. It involves a similar formula. The researchers infected E. coli with a custom-made plasmid DN ring that contained a specific sequence fragment of Novel Coronavirus, which is specifically non-infectious, as well as a gene encoding a bacteriophage surface protein called Qbeta. This process allows the bacteria to produce nanoparticles, which are made up of sequences of coronavirus RNA wrapped in mycelium shells.
Plant viruses and mycelium shells are what make these positive controls so strong. They protect coronavirus RNA fragments from being destroyed at higher temperatures, and tests have shown that they can be stored for up to 40C (104F) for up to a week. The shell also protects RNA in the first step of PCR and LAMP testing, which involves breaking down the cells in the sample — by releasing their genetic material with enzymes or heat for testing.
These protections are not present in the positive controls currently used for COVID-19 testing. That’s why existing controls either need to be refrigerated or have to be added later in the test, meaning scientists won’t know if something went wrong in the first step.