Graduate student researches how bacteria survive on produce
Published April 15, 2014
Interesting, smart and creative.
Those might sound like adjectives you’d find in a dating profile. But for Deepti Tyagi, they are the words she uses to describe bacteria.
Tyagi says she’s been fascinated by bacterial pathogens since eighth grade, which helped pave the way to her current stint as a microbiology graduate student at NDSU. She’s especially interested in the microbe’s ability to survive and adapt.
This also helps explain her research on E.coli O157:H7 and Salmonella. Namely, she’s studying how pre-harvest environmental conditions like humidity, time of harvest and temperature may influence these dangerous foodborne pathogens’ ability to survive on lettuce. The second phase of her work involves examining what takes place at the genetic level to help these bacteria survive on produce.
“These pathogens might be developing a tolerance for decontamination treatments like chlorine washes and even the gastric acidity of our stomachs, thereby leading to their presence in the food chain,” she says. “The quote, ‘What doesn’t kill you, makes you stronger,’ could be true in this situation.”
Dietitians may applaud our nation’s increased consumption of fresh fruits and vegetables, but food-safety experts have noticed a corresponding increase in tainted produce outbreaks. While the common wisdom has linked these outbreaks to improperly processed meat, produce – a minimally processed food – accounts for 25 percent of all foodborne outbreaks, according to Ohio research scientists.
The Centers for Disease Control estimated that an E.coli outbreak on spinach in 2006 resulted in more than 276 consumer illnesses and several deaths. Food-borne illnesses are responsible for 128,000 hospitalizations and 3,000 deaths per year. Besides serious gastrointestinal distress, E.coli and Salmonella can cause conditions as serious as renal failure, reactive arthritis and septicemia, or bacteria entering the blood.
From a purely economic standpoint, contaminated food costs producers $51 billion a year, according to researchers from Ohio State University.
E.coli and Salmonella originate in the guts of warm-blooded animals, such as cattle, pigs and deer. The pathogens are transferred to crops by factors like animals physically passing through fields, contaminated irrigation water or run-off from animal sheds. Once contamination occurs, the tenacious pathogens can survive for days or even weeks.
Tyagi conducted her research by growing crops of loose-leaf lettuce in a greenhouse, mimicking growing conditions of Southern California, where much of the nation’s lettuce is grown. She sprayed the plants with two strains each of E.coli and Salmonella, then subjected them to stressors like different temperatures, UV exposure and humidity levels. The lettuce was harvested in March and June on anywhere from the day of contamination to the fifth day after contamination.
What she found was remarkable: Although pathogen levels dropped over time, some survived five days after harvest.
She also found that the Salmonella survived better than E.coli after growing in high humidity. In general, Salmonella was hardier and survived better than E.coli over the five-day period.
As the second phase of her study, Tyagi carefully rinsed the harvested lettuce to extract ribonucleic acid from the pathogens. RNA, which is present in all living cells, acts as a messenger to carry instructions from DNA for controlling the synthesis of proteins. Because RNA is extremely delicate, Tyagi needed to collect it via a painstaking and time-consuming process. “You take care of RNA like you take care of a diamond,” she says, laughing.
The extracted RNA samples were sent to Cornell University’s Life Science Lab for sequencing. The sequencing work will enable Tyagi to determine what happens at the genetic level to help E.coli and Salmonella survive on lettuce, possibly building resistance to decontamination in the process.
Tyagi hopes to graduate next December and either continue conducting research or pursue a doctorate. In the meantime, she is excited to see the complete results of such an exhaustive study. “I know it’s going to be really helpful,” she says. “I want to give my efforts to whatever we can do for the betterment of human health.”