Is this apple safe to eat? Did that course of antibiotics work? To answer these types of questions we often need to know how to find and count illness-causing bacteria. Several bacteria counting techniques already exist. However, these approaches are slow and sensitive to laboratory conditions.

Sam Nugen and his team from Cornell University are streamlining this process using a type of virus called phages, which attack bacteria. The phages’ DNA were altered so they would produce colorful proteins after they invaded a bacterial cell. By measuring the intensity of the color in a sample, the researchers could determine how much bacteria is present. The scientists also modified the phages to insert magnetic nanoparticles, very small pieces of metal, that function as tiny magnets.  By placing a bigger magnet outside the sample, the scientists could herd the bacterial cells and find them even in very low concentrations.

Nugen states that in the future he wants to look at designing phages to target specific types of bacteria or groups of bacteria that you might care about in food. Our expert, Anqi Zhang from the department of Chemistry and Chemical Biology, also notes that this technique will need to be individually calibrated to different samples in order to correctly measure the color change caused by the phages. For example, the resulting color will look different if the original sample is orange juice as opposed to milk. Older techniques, although they take longer, do not have to be individually calibrated to different types of samples.

Managing Correspondent: Emily Kerr

Awknowledgements: Many thanks to Anqi Zhang, a graduate student in Harvard’s department of Chemistry and Chemical Biology for his feedback on this article.

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