E. coli Testing: Using Nanotechnology and Liquid Droplet Tests to Detect Pathogens

Imagine detecting pathogens in your favorite foods right in the grocery store using your mobile phone.

It might not be long before you can do this, thanks to some recent groundbreaking research at Pittsburg State University (PSU), in southeast Kansas.

Detection with Nanotechnology

Tuhina Banerjee, PhD, Santimukul Santra, PhD, and James McAfee, PhD, faculty members in the PSU Department of Chemistry, along with six students, have successfully married magnetic resonance imaging (MRI) technology and fluorescence emission to detect E. coli O157:H7 in milk and lake water. The result is hybrid nanosensors that are able to screen quickly for target pathogens.

According to Dr. Banerjee, the PSU nanosensors are composed of special iron oxide particles blended with an optical dye, plus antibodies that specifically latch onto E. coli O157:H7 cells.

“When mixed into a solution with bacterial colonies, the nanosensors swarm around their target organism’s outer membrane and adhere to any such pathogens that are present, while ignoring nontargeted cells, even other strains of E. coli, or heat inactivated O157:H7,” she explains. “This aggregation is detectable using magnetic resonance. Similar to MRI technology used in human medicine, the bacteria detection procedure launches a magnetic field through the sample. But instead of measuring water molecules, as is the action of medical MRIs, the detector picks up iron-rich nanosensor clumps.”

In the presence of a small amount of bacteria (low colony forming units, CFUs), the magneto-fluorescent nanosensors (MFnS) cluster around the bacterial cell, which inhibits their interaction with the surrounding water protons, thus increasing magnetic relaxation (T2) values, Dr. Banerjee elaborates. “However, as bacterial concentration increases (high CFUs), the clustering decreases,” she says. “This causes the T2 signal to become saturated and the ability to quantify bacterial concentration is lessened.”

One huge benefit of the PSU nanosensor technology, Dr. Banerjee says, is its capability to scan both very small amounts of a pathogen, as well as very large amounts. “Magnetic resonance has the ability to detect bacteria in small quantities, but not in large ones,” she points out. “With fluorescence, the opposite is true.”

Dr. Banerjee explains that the dye glows when the nanosensors cling to bacteria. “When there are just few colonies, then detection is mainly mediated by monitoring T2 change (magnetic relaxation) and so even a minimum amount of bacterial contaminant can be detected,” she notes. “But if there are many cells, the sample will light up. The brighter the glow, the more contaminated the sample. We can detect as low as 1 CFU using our magnetic nanosensors.

“We believe that by tweaking the antibodies, we can adapt the technique to detect a wide range of pathogens, including bacteria and viruses, in foods, beverages, water, and even human blood samples,” she continues. “We plan to test the technology on lettuce and juice next.”

Another selling point is that the PSU nanosensors are currently able to detect bacterial contamination in less than an hour. “This is much quicker than current gold standard techniques, including real-time polymerase chain reaction, which can take up to 24 hours for data collection, sample amplification, and results,” Dr. Banerjee notes.

With further experimentation, the time required to get test results is expected to be reduced to a few minutes or even seconds, Dr. Banerjee predicts. “Magnetic relaxation is a powerful technique and we start to see a light change within a few minutes when we incubate MFnS with bacterial CFUs,” she relates. “Soon we expect to do multiplex samples for other contaminants besides E. coli in the same sample, so pathogen testing will be even faster then. And if one organism is more prominent than the other, we will be able to detect the less prominent one.”