Quantum dots and a camera could detect killer bacteria

A combination of off-the-shelf quantum dot nanotechnology and a smartphone camera soon could allow doctors to identify antibiotic-resistant bacteria in just 40 minutes, thanks to research from Australian scientists.

Staphylococcus aureus (golden staph) is a common form of bacterium that causes serious and sometimes fatal conditions such as pneumonia and heart valve infections. Of particular concern is a strain that does not respond to methicillin — the antibiotic of first resort — known as methicillin-resistant S. aureus (MRSA).

Recent reports estimate that 700,000 deaths globally could be attributed to antimicrobial resistance, such as methicillin resistance. Rapid identification of MRSA is thus essential for effective treatment, but this is a challenging process even within well-equipped hospitals.

“Rapid and simple ways of identifying the cause of infections and starting appropriate treatments are critical for treating patients effectively,” said Associate Professor Anwar Sunna, Head of the Sunna Lab at Macquarie University.

“This is true in routine clinical situations, but also in the emerging field of personalised medicine.”

Assoc Prof Sunna has now collaborated with colleagues from Macquarie University and UNSW Sydney on a proof-of-concept device that uses bacterial DNA to identify the presence of S. aureus positively in a patient sample — and to determine if it will respond to frontline antibiotics. Described in the journal Sensors and Actuators B: Chemical, the team’s approach identifies MRSA by using convective polymerase chain reaction (cPCR) — a derivative of a widely employed technique in which a small segment of DNA is copied thousands of times, creating multiple samples suitable for testing.

The colleagues subject these DNA copies to a process known as lateral flow immunoassay — a paper-based diagnostic tool used to confirm the presence or absence of a target biomarker. The researchers then use probes fitted with ultrasmall semiconductor particles known as quantum dots to detect two unique genes which confirm the presence of methicillin resistance in golden staph.

A chemical added at the PCR stage to the DNA tested makes the sample fluoresce when the genes are detected by the quantum dots — a reaction that can be captured easily using the camera on a mobile phone. The result is a simple and rapid method of detecting the presence of the bacterium, while simultaneously ruling first-line treatment in or out.

Although currently at proof-of-concept stage, the researchers say their system, which is powered by a simple battery, is suitable for rapid detection in different settings. Lead author Vinoth Kumar Rajendran, from Macquarie University, said, “We can see this being used easily not only in hospitals, but also in GP clinics and at patient bedsides.”

Image caption: Researchers Vinoth Kumar Rajendran and Anwar Sunna with their smartphone-enabled MRSA detector. Image credit: Sunna Lab.

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