A sensor that can discriminate between closely related bacteria has been developed by material scientists in the US.

The piezoelectric cantilever can detect anthrax among its close relatives

Wan Shih and colleagues at Drexel University in Philadelphia immobilised a rabbit antibody to Bacillus anthracis (anthrax) on a piezoelectromicrocantilever sensor (PEMS). They then exposed the sensor to suspensions of anthrax bacterial spores and its close relatives B. thuringiensis, B. cereus and B. subtilis. Because the chemical structures in the spore coats of these bacteria are very similar, they all bind to the anthrax antibody to some extent. However, the binding interactions of the 'close relatives' are weaker than that of anthrax itself.

PEMS uses antibodies, which have high affinity and specificity for different microbes, attached to tiny cantilevers. 'Cantilevers can be thought of as tiny diving boards made from piezoelectric material that can detect very small particles, like deadly Bacillus anthracis spores, by vibrating,' explained Wei-Heng Shih, a member of the research team. This combination of specific antibody and sensitivity, provided by the piezoelectric cantilevers, enables the sensor to accurately detect any micro-organism (depending on the antibody used) in real time, with no added chemicals, radioactive tags or external manipulation. 'The sensor can detect its preferred microbe in the presence of a million of its closely related cousins. It can detect a needle in a haystack!' said Wei-Heng Shih.

To test if PEMS could preferentially detect anthrax spores, the team flowed suspensions of the different bacteria across the sensor. The team observed that, at first, increasing the flow speed of the bacterial suspensions across the sensor led to increased binding of all four species because the sensor surface was exposed to more spores. But at higher flow rates, preferential anthrax-antibody binding was increased and binding to close relatives was suppressed.

'This result is significant because it means that flow can be used to enhance the specificity of antibody-based detection,' said Wan Shih. 'With the right antibodies or receptors our sensor can detect air or food-borne pathogens like anthrax or E. coli and blood or urinary biomarkers for human diseases like cancer or AIDS. And it will make detection possible in minutes instead of the days it takes now,' she added.

Janet Crombie