Scattered Laser Light Identifies Bacteria

A new technique using light scattered by a laser takes less than five minutes to identify harmful organisms after they have been grown in a Petri dish.

The technique, called bacteria rapid detection using optical scattering technology, works by shining a laser through a Petri dish containing bacterial colonies growing in a nutrient medium. Particles of light, called photons, bounce off of the colony, and the pattern of scattered light is projected onto a screen behind the Petri dish. This light-scattered pattern is recorded with a digital camera and analyzed with software to identify the types of bacteria growing in colonies.

The system was developed by researchers at Purdue University (West Lafayette, IN, USA). The work was initiated by Arun Bhunia, a professor of food microbiology in the department of food science, and E. Daniel Hirleman, a professor and head of Purdue's School of Mechanical Engineering. A major motivation for the research was to reduce the time it takes for industry to identify harmful organisms in food processing. Details of the system appear in the July 2006 issue of the Journal of Biomedical Optics.

The scientists adapted a mathematical method created in 1934 by Dutch physicist Fritz Zernike, who created a set of mathematical "descriptors” subsequently called radial Zernike polynomials. These descriptors can be used to analyze how light-wave patterns are distorted after passing through lenses having complex flaws or aberrations.

Individual bacterial colonies growing in a Petri dish distort light passing through them, just as a lens changes light-wave patterns. Factors such as the shape of bacteria, their refractive indexes, the types of substances secreted by a particular bacterium, and the distance between individual bacteria in a colony all contribute to how a colony distorts light. The procedure identifies a bacterial colony by comparing an image of its scatter pattern against a template that contains 120 features described by Zernike polynomials.

The new system was used to identify six species of listeria, one of which is a dangerous food-borne pathogen for humans. Other types of bacterial colonies, including Salmonella, vibrio, Escherichia coli and bacillus were also accurately identified. It might also be possible to use the technique to identify staphylococci that are resistant to antibiotics.



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