Robert Pore's Ag Blog

WEST LAFAYETTE, Ind. — Researchers have shown that a new low-cost system to quickly identify bacteria by analyzing scattered laser light can accurately distinguish between different strains of E. coli, a potentially valuable way to screen the food supply.

The technique, which works by passing a laser beam through bacterial colonies growing on a nutrient medium, also promises to have future applications in medicine and homeland security, identifying dangerous organisms far more quickly and at much lower cost than conventional technologies, said E. Daniel Hirleman, a professor and William E. and Florence E. Perry Head of Purdue's School of Mechanical Engineering.

Laser light passing through and around the colony is redirected by the bacteria and produces a scattering pattern. This light-scatter pattern is recorded and analyzed to identify the types of bacteria growing in colonies.

"We have learned that slight genetic differences between strains of E. coli create subtle differences in the micro- and macrostructure of the respective colonies," Hirleman said. "Our scattering instrument, in effect, amplifies these slight differences to produce remarkably different scattering phenomena."

The light-scattering project was initiated by Hirleman, working with Arun K. Bhunia, a professor of food microbiology in the Department of Food Science, and other researchers, including J. Paul Robinson, a professor in the Weldon School of Biomedical Engineering.

Hirleman has specialized in research to develop new types of sensors that work by analyzing light scattering off objects for applications such as detecting impurities on silicon wafers in computer chip manufacturing and measuring the size and speed of burning fuel droplets in jet engines.

A major motivation for the bacteria-detection research is to reduce the time it takes to identify harmful organisms in food processing. The industry generally collects food samples or swabs, places them first in a nutrient broth and then on a plate coated with solid nutrient to allow the bacterial growth to reach detectable levels. E. coli bacterial cultures take about 18-24 hours to grow. Then, subsequent biochemical analyses and other time-consuming and expensive techniques, such as polymerase chain reaction, take four to seven days to complete the bacteria identification.

The light-scattering method works immediately after the colony is grown.

"Within a second after the colony is full grown we can identify by its scattering fingerprint a certain strain of E. coli, for example," Hirleman said. "Or we might see a new scattering fingerprint and only be able to say that something is growing on the same growth medium as E. coli. We've never seen it before, but there is something here. That means we are warned within seconds instead of days."

A mass-produced system based on the technology would consist of inexpensive, off-the-shelf hardware, such as computers, red lasers and low-resolution digital cameras available at consumer electronics stores, and likely would cost less than $10,000. Instruments used for conventional methods can exceed 10 times that cost, Hirleman said.

Another advantage is that the light-scattering fingerprints of bacteria can be added to a library for quick reference in future outbreaks of food-borne pathogens.

One conventional approach requires sophisticated methods to "label" the bacterium with antibodies that attach a fluorescent dye to the target.

"But that means you have to use a designer antibody specifically suited for the strain of E. coli in question," Hirleman said. "What if food is contaminated with, say, a new strain of E. coli? You won't see it because the label will not attach to it."

The light-scattering technique, however, would enable researchers to detect bacterial contaminants they were not specifically looking for, making it less likely to miss an unsuspected culprit.

"Our team has done experiments where we've spiked ground beef and spinach with known pathogens, found those but also found another bacterium that was not in our library," Hirleman said. "So then we went back to the colony and did further analyses to find out what it was."

The researchers have studied growth characteristics of bacterial colonies and the corresponding evolution of the scattering patterns for a wide variety of pathogens relevant to food safety, developing a mathematical model that predicts the light-scattering signatures for given pathogens.

"We have found that different strains generally have unique forward scattering fingerprints, and those fingerprints can be used for rapid detection and classification of the bacteria," Hirleman said. "The light-scattering method has high sensitivity and speed and shows great promise for identifying bacterial colonies of a wide range of organisms relevant to infectious disease, homeland security and food safety."

This research was supported through a cooperative agreement with the Agricultural Research Service of the U.S. Department of Agriculture and Purdue's Center for Food Safety Engineering.

A provisional patent has been filed for the data-processing technique, and a full patent is pending on the underlying light-scattering technology.

TEMPE, Ariz. -- Raindrops can wreak havoc on Earth. They just do it on a microscopic scale. At that scale, raindrops hitting bare ground have nearly the force of a hammer hitting a mound of dirt. What happens when the water hits the soil is the micro-ballistic effect of displaced soil splattering around in all directions. In arid regions, such as central Arizona, this is an important process that shapes the landscape.

While it sounds elemental, it has only been recently that researchers, including one from Arizona State University, have studied these effects up close and in freeze frame.

The research team is led by Mark Schmeeckle, an assistant professor in ASU's School of Geographical Sciences and David Furbish, professor of earth and environmental studies at Vanderbilt Univ., Nashville, Tenn. They focused on the effect of raindrops hitting bare soil in a series of controlled experiments that included high-speed photography to capture the soil splattering process and its aftermath.

What they found were some violent confrontations, as water hit bare soil causing splatter effects of the soil. They also found that momentum plays a key role in slope erosion and gravity has a muted effect.

Schmeeckle and Furbish were joined by Katherine Hamner, Miriam Borosund and Simon Mudd, all of Vanderbilt, in the project. They report their findings in the current issue of the Journal of Geophysical Research (Jan. 16, 2007) in "Rain splash of dry sand revealed by high speed imaging and sticky paper splash targets."

In the experiments, the researchers mounted a 20-foot long PVC pipe vertically and attached a syringe at the top of the pipe. The distance was great enough where raindrops, coming from the syringe, could achieve terminal velocity (the fastest speed they can fall through still air). The pipe blocks air currents from deflecting the drops.


"Without it we wouldn't be able to hit our target," Schmeeckle said.

The drops were aimed at a sand target 2.5 cm in diameter by 2 cm deep set flush to a surrounding surface covered with sticky paper. Depending on the syringe needle size, the researchers could adjust drop size from 0.5 mm to 5 mm. A 5 mm raindrop traveling at terminal velocity would hit the sand target at a relative force of 20 mph.

When a drop hit the target, a high-speed camera operating at 500 frames per second recorded the dynamic interactions between the water and the sand. In addition, sand grains ejected by each impact stuck to the surrounding paper where they hit, allowing the researchers to precisely plot their positions.

The researchers did several experiments simulating raindrops hitting sand on flat surfaces.

"The raindrops splashed particles in all directions, resembling ballistic trajectories of particles going up and out and then down," Schmeeckle said.

Then they angled the target to five inclinations (10, 15, 20, 25 and 30 degrees). With the target tilted, Schmeeckle said the researchers dispelled a 50-year old misconception about how rain splash transport works.

"We found that when the raindrop hits, very few particles actually move up slope and most of the particles move down slope," he said. "It kind of bulldozes in the down slope direction and you get a large ejection of particles moving down slope."

But gravity takes a back seat to momentum as the driver of this phenomenon.

"It's the momentum," Schmeeckle said. "As the raindrop comes in, it already has downward momentum and that momentum gets transferred to the down slope momentum of the soil particles."

This experimental result could have a big impact on soil erosion and add to the knowledge engineers use to devise systems to prevent such erosion on hills and mountains.

"The discovery is important for soil health," said Schmeeckle, who primarily studies sediment movement in rivers. "In semi arid and arid regions like ours, where there is not a lot of vegetation on hills, raindrops directly and dramatically affect soil as they hit.

"A lot of material transport from hill slopes will eventually make it into the river systems," he added. "This study will lead to a much better understanding of the processes of how soil is eroded and transported on hill slopes."

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