Wednesday, May 29, 2013
By building a model of a human colon and studying its engineered waste, a UCR engineer has discovered that digested E. coli is able to survive in groundwater much longer than previously thought. "Pathogens could potentially linger longer and over a long period of time travel greater distances in the groundwater," says Ian Marcus, the first author on a paper published this week in Applied and Environmental Microbiology.
Marcus built his fake feces machine to answer a question that simpler (and perhaps more civilized) experiments weren't able to address: What happens to pathogenic bacteria along the path from gut to groundwater?
Specifically, Marcus wanted to track a nasty strain of E. coli entering the microbe-rich human digestive tract, then getting flushed into a septic tank, and eventually seeping back into groundwater. "We were really looking downstream from the colon," he says.
The recent PhD graduate had chosen a pretty gross topic to study. A dangerous one too, considering how sick humans get from ingesting E. coli. We're not talking about just any old strain of E. coli here, especially not the beneficial kinds swimming around in our guts right now, helping us break down food and produce vitamins. The strain that interested Marcus was E. coli O157:H7, responsible for causing recurring food safety panics and almost always followed by the word "outbreak" in newspaper headlines.
Obviously, human experiments were out of the question. Marcus jokes, "We wanted to try this with some undergrads, but they were like, 'No.'"
Instead, like a gastroenterological MacGyver, Marcus fashioned himself a contraption that mimics how human colons break down E. coli-laced food. He "fed" his colon three times a day, pumping it full of sugars, salts, and proteins modelled on a typical Western diet. He also emptied its waste three times a day, depositing the artificial poop into a desktop septic tank. The tank smelled just as bad as you'd think.
"People were avoiding my part of the lab," Marcus says. Despite the noxious odor, Marcus kept at it for six weeks. Finally, he exposed samples from the septic tank to a solution modeled on groundwater.
Marcus saw that the bacteria wasn't travelling through the groundwater very quickly. You might think that sounds positive. Immobilized E. coli is better than E. coli spreading out every which way, right?
"There's a caveat," says Marcus. "The bacteria within this microbial community had higher propensity to form biofilms. The problem with biofilms is that they can linger in the environment for quite a long time." By morphing into sticky biofilms, the digested E. coli was better at clinging to objects floating in the groundwater, meaning that disease-causing bacteria could persist long after public health officials believe an outbreak has subsided.
Around 20 percent of households in the United States use septic tanks. A nauseating percentage of these tanks are leaky, sometimes polluting community groundwater with sewage. Such leakage is one of many possible culprits behind recent E. coli outbreaks in states like Missouri.
Marcus warns that even his complex human waste model isn't real-world proof of E. coli lingering in your neighborhood's lakes and streams. However, he notes that his colon-to-groundwater system represents E. coli interacting with its environment much better than previous experiments, which typically observe bacteria in stripped-down lab settings.
"The real takeaway is if you want to study environmental organisms, you have to study it in a system as opposed to a single isolate," he says. Sometimes that system involves a contraption that produces fake crap.