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Bacteriophages could offer an excellent weapon against pathogenic bacteria—they can exhibit exquisite host preference and could also prove valuable for dealing with antibiotic-resistant strains. National Institutes of Health researcher Sankar Adhya has been exploring the use of these viruses for antibacterial therapeutics, but recently came to realize that even the best therapies require a good diagnostic tool first. “Detecting [bacteria] earlier, when the bacterial counts are less, is invaluable for therapies,” says Adhya.

And so his group set about developing rapid and sensitive diagnostic assays. Bacteriophages remained the tool of choice, and Adhya's team also took advantage of the highly conserved bacterial biotinylation metabolic pathway. They engineered T7 phage in which the coat protein was tagged to allow processing by bacterial biotinylation enzymes. When these phage encounter target bacteria, infection will lead to the production of phage that are at least partially biotinylated before host lysis. These phage can then be detected by fluorescence microscopy using streptavidin-conjugated quantum dots.

The method tested successfully both in the laboratory and in the real world—using T7, Adhya's team found that they could detect as few as 10 Escherichia coli cells intermixed with other nontarget bacteria, and they were able to detect roughly 20 E. coli cells in a milliliter of Potomac River water. The sensitivity and speed of the assay (roughly an hour from start to finish) compared favorably with existing assays—a standard E. coli detection system took nearly 24 hours to detect coliform cells in river water.

Adhya is now looking into improving the assay's sensitivity by developing methods to improve the efficiency of phage biotinylation. In general, however, he believes the method should be broadly applicable because of the wide variety of bacteriophage types with different host preferences, even though engineering the phage could be difficult if genome data are lacking for a given strain. In the end, Adhya hopes that the system's usefulness will grow as more people adapt it for themselves: “We're [using this assay] with the model organisms we work with in our lab, but if people use it, I'm sure they'll succeed... with other bacteria.”