A new, rapid and low-cost method for detecting bacteria in a water or food samples has been developed by researchers in America where microbial contamination is considered the number one food safety concern.
The two-step method – one optical, one chemical – has been conceived by researchers from the University of Massachusetts Amherst, and once commercially available, should be useful to cooks using fresh fruits and vegetables and aid workers in the field responding to natural disaster, the team say.
Most people around the world cook their vegetables before eating, but here in the U.S. more and more people like to eat these foods raw. This gave us the idea that a quick test that can be done at home would be a good idea. Microbial contamination is an important research topic right now. It has been a problem for a long time, but it is now the number one concern for food safety in the U.S.
Lili He, Analytical Chemist
Together with food science researcher Lynne McLandsborough and their students, Lili He designed a sensitive and reliable bacteria-detecting chip that can test whether fresh spinach or apple juice for example, carry a bacterial load. Used with a light microscope for optical detection, the chip relies on what He describes as a "capture molecule," 3-mercaptophenylboronic acid (3-MBPA), which attracts and binds to any bacteria. The chemical detection method, known as "surface-enhanced Raman spectroscopy" (SERS), relies on silver nanoparticles. Both techniques - reported in Food Microbiology and Analytical Methods - are now in the patenting process.
The first step in the new bacteria test is to collect samples of water, juice or mashed vegetable leaf and then place the chemical-based detection chip in the sample. A straightforward smart phone app that visually detects bacteria in samples containing the chip, was developed by a summer high school student in He's food science laboratory. The optical detection method has been adapted for possible home use with a smart phone microscope adapter widely available online.
"This is just the beginning of the work," says He, who hopes to receive more funding to continue this practical application.
Currently, the standard method for culturing bacteria from food samples, known as an aerobic plate count (APC) takes two days, He explains. APC is used as an indicator of bacterial populations on a sample and works on the assumption that each cell will form a visible colony when mixed with agar containing the appropriate nutrients.
There are some others that are faster, but they are not very sensitive or reliable because ingredients in the food can interfere with them. We show in our most recent paper that our method is both sensitive and reliable and it can give you results in less than two hours.
Lili He, Analytical Chemist
To overcome the problem of food interference, the researchers designed their chip to attract only bacteria but not sugars, fats and proteins in food or dirt. With a high-pH buffer, the food compounds can be washed away , leaving only bacteria for visual counting with the smart phone microscope and app.
The method can detect as little as 100 bacteria cells per 1 milliliter of solution, compared to a sensitivity of 10,000 cells for other rapid methods.
In their study, He and colleagues examined "the advantages and constraints of this assay over the conventional APC method and further developed methods for detection in real environmental and food matrices," the authors explain. "This study advanced the SERS technique for real applications in environment and food matrices,"