‘Nanobot’ viruses tag and round up bacteria in food and water

NEW ORLEANS — Viruses engineered into “nanobots” can find and separate bacteria from food or water.

These viruses, called bacteriophages or just phages, naturally latch onto bacteria to infect them (SN: 7/12/03, p. 26). By tweaking the phages’ DNA and decking them out with magnetic nanoparticles, researchers created a tool that could both corral bacteria and force them to reveal themselves. These modifications can boost the sensitivity and speed of rooting out bacteria in tainted food or water, the researchers reported March 20 at the annual meeting of the American Chemical Society.

“You’re taking the power of what evolution has done … to bind bacteria, and then we’re just helping that out a little bit,” said Sam Nugen, a food and biosystems engineer who leads the team designing these phages at Cornell University.

Competing technologies for detecting bacteria use antibodies, the product of an immune response. But these are expensive to produce and work best in a narrow temperature and pH range. In contrast, phages “exist everywhere,” making them potentially more broadly useful as bacteria hunters, Nugen said. “They’ve had to evolve to bind well in much broader conditions than antibodies.”

Phages identify and grab bacteria using proteins on their leglike tail fibers, which form a strong bond with compounds on the bacterial cell surface. To infect the cell, the phage injects its genetic material. This hijacks the cell, forcing its machinery to produce phage clones.

Nugen and collaborators programmed phages to tag E. coli bacteria. The team’s engineered phages contained extra DNA that told the bacteria to make an easily detectable enzyme. When the infection caused the bacterial cells to rupture and release the new phages, a chemical reaction involving the enzyme produced a measurable signal: light, color or an electric current. For example, the phages exposed E. coli in milk and orange juice by turning the liquids red or pink.

The researchers also loaded the phages with nanoparticles with a magnetic iron and cobalt core. Once the phages latched onto the bacteria, researchers could use a magnet to round the bacteria up even before the bacteria ruptured and announced their presence. This allowed the researchers to detect low concentrations of bacteria: less than 10 E. coli cells in half a cup of water. Conventional methods grow the bacteria into colonies to find them, which can take up to two days. But using the phages, Nugen and his colleagues skipped this step and found the cells within a few hours.

Using phages for magnetic separation would be “really nice for food and environmental samples because they tend to be really dirty,” said Michael Wiederoder, a bioengineer at the U.S. Army Natick Soldier Research, Development and Engineering Center in Massachusetts, who was not involved in the research. The salt, sugar and fats in food can slow the reactions of antibody-based tests, he said.

Also, the phages infect only bacteria that can reproduce, allowing testers to tell the difference between live cells and those killed by antibiotics, heat or chlorine. With food, “whether the bacteria are alive or dead is the difference between you getting sick and not,” Wiederoder said.

The nanobots could also prove useful for blood or other human samples. There, phages would provide a way to find resistant bacteria left alive after a course of antibiotics.

The next challenge: tinkering with the phages to tune which bacteria they go after. In nature, phages prey on specific species. But in food, it may be helpful to detect several common offenders, like E. coli, Salmonella and Listeria, or, alternatively, to have greater discrimination to find only the pathogenic E. coli and leave the rest.