Microbes, as a target
OUR world is teeming with tiny bacteria, most of them utterly safe for us to swallow. A few, however, are anything but -- such as E. coli O157:H7, at the root of the recent spinach illness outbreak; or salmonella, which is the reason we are told to avoid raw eggs; or Listeria monocytogenes, a troublesome microbe with a fondness for growing on soft cheese and luncheon meats.
“Washing produce with water is one of the most effective methods of removing unwanted bacteria,” says food microbiologist Scott Martin of the University of Illinois at Urbana-Champaign. “The problem is you can’t remove all of them.”
To improve another line of defense, microbiologists and bioengineers are working hard to develop biosensors that can detect and identify different species of bacteria in food.
Some analyze whole colonies of millions of bacterial cells, while others look for just one rogue bug or molecules from that bug. They hope to make the assays for nasty microbes faster than current technology, which takes at least a day. They are developing these sensors for food safety labs, and working with food safety companies to make them portable so they can be taken into the field.
And some see no reason why these technologies couldn’t one day be cheap and easy enough to use in the home.
One new identification technique has been developed by Arun Bhunia, professor of food microbiology at Purdue University in West Lafayette, Ind. His method relies on the way laser light beams bounce off different bacteria.
In August, he reported in the journal Biosensors and Bioelectronics that different types of bacteria have laser light shadows that are as unique as fingerprints -- glowing diffuse under some circumstances, or forming rings under another, depending on the species.
To determine whether he could use laser light to identify Listeria monocytogenes, first he grew the pathogen in a petri dish until the colonies were the size of a pinhead. He also grew dishes of harmless listeria species. When he shined red laser light through the colonies, each species of listeria produced a distinct pattern on a light detector on the other side.
Bhunia says he can identify species with greater than 90% accuracy and can also identify other pathogens, including E. coli and salmonella, using the same method. The test is reasonably rapid: Fast-growing bacteria such as E. coli can form the right-sized colony in about 12 to 20 hours, while the slower listeria takes 24 to 30 hours.
His group is now testing food such as hot dogs and bologna to verify the ability to distinguish harmless from pathogenic bacteria in a real-world setting.
Neat as this approach is, it has a drawback: To use it, one must wait for the cells to grow.
To get around that, Raj Mutharasan, professor of chemical and biological engineering at Drexel University in Philadelphia, has developed a way to detect individual bacteria in food samples. His method uses immune molecules called antibodies that can recognize, and bind to, specific structures on the surface of bacteria, allowing different species to be distinguished. These specific antibodies are attached to a material that vibrates.
The method, published in July in Biosensors and Bioelectronics, tests food samples, such as beef or vegetables, that have been ground up in water. The sensor (in this case, designed to detect E coli O157:H7) is dipped into the sample. Should such a bacterium be present in the sample and stick to the antibodies on the device, the sheer weight of even one tiny microbe changes the vibration’s frequency -- and this change is picked up by the equipment.
Using this method, Mutharasan says he has detected E. coli at concentrations as low as 1 cell per milliliter, or one bug on a gram of meat. The test can take as little as 10 minutes, he adds. Now he’s working to develop prototypes that can be taken to an agricultural field and used by growers or set up in a laboratory.
“We hope it can be made easy to use by ordinary technicians rather than highly skilled ones,” he says.
Other scientists are trying to come up with microbe-identification devices that have the ease and convenience of pregnancy test strips. Cornell University bioengineer Antje Baeumner and her colleagues have developed a method that identifies pathogens by virtue of unique sequences of a certain type of genetic material, called rRNA, that they carry.
In the test, a small strip is loaded with short molecules that will bind to only the rRNA of the pathogen for which the test is designed. The strip is dipped into liquid samples taken from food. If the pathogen is there in the sample, bits of its rRNA attach to the strip. A red dye is also present, causing a red band to show up on the strip if the pathogen’s present in sufficient quantity.
The test can take as little as half an hour to detect bacteria present in large amounts, , Baeumner says. It would take longer (say, four hours) in cases where just a few bacteria are present.
Her team has designed strips for different pathogens, allowing the same food sample to be tested for various disease-causing bacteria. She is collaborating with a New York company to commercialize the technique, as well as using the method to identify a waterborne parasite, Cryptosporidium.
During the next year, she plans to team up with water-treatment plants in New York to test for the parasite.
Such technologies have yet to hit large-scale agriculture, but if they do, there’s a hope that faster, easier tests would raise the incentive to test food more often and cut down on problems, experts say.
But technology, while it can help, will never render food 100% safe. Says University of Illinois’ Martin: “We have to recognize that there will always be a small risk to eating raw food.”
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Ultrasound gives ‘em a blast
One reason that food-borne bacteria are difficult to remove is that the bugs often gang together to form biofilms, layers of tightly linked microbes that protect the individual cells and help them stick to a surface.
Scott Martin, a food microbiologist at the University of Illinois at Urbana-Champaign, is exploring the use of ultrasound to disrupt these biofilms in apple cider.
Contamination can be a problem in cider: At least six outbreaks of E. coli O157:H7 have been linked to it since 1996.
The drink tastes better if it is not heated, but because of the risks, the Food and Drug Administration requires cider beverages to be heated two-thirds of the way to boiling, which kills 99.99% of the microbes. If not, the manufacturer must slap on a warning label about the drink potentially harboring harmful bacteria.
In his studies, Martin exposed some cider samples to sound waves (beyond the range of human hearing), and others to heat. He found that warming the samples less thoroughly -- halfway to boiling for 20 minutes -- killed only a few bacteria.
But if he also zapped them with ultrasound, 99.99% of the microbes were killed.
Using ultrasound, Martin says, might allow cider makers to cut back on heating, preserving the cider’s flavor without compromising safety.
-- Mary Beckman
