A team of scientists is genetically engineering plants to produce antimicrobial proteins known as colicins, which can then be extracted and applied to contaminated meat and produce to kill E. coli bacteria. In a study published earlier this month in the Proceedings of the National Academy of Sciences, the team engineered tobacco, leafy beets, spinach, chicory and lettuce to produce colicins. The research team, led by scientists from two German biotech companies, Nomad Bioscience and Icon Genetics, found that certain plants, such as tobacco, can yield high levels of active colicins. They also identified a mixture of two colicins that can efficiently kill all major disease-causing strains of E. coli. “All of the food outbreaks that have been recorded in the last 15 years or so could have been controlled very well by a combination of just two colicins, applied at very low concentrations,” said Yuri Gleba, CEO of Nomad Bioscience and one of the authors of the study. He added that colicins are 50 times more active against bacteria than normal antibiotics. Most E. coli in food is found in contaminated beef and pork, but an increasing number of infections have been linked to organic produce, which is typically treated with animal manure in place of chemical fertilizers. According to the World Health Organization, up to 10 percent of E. coli infections could lead to life-threatening disease. Colicins are proteins naturally produced by E. coli strains to kill or inhibit the growth of competing E. coli strains. The proteins are extremely toxic — so toxic, in fact, that engineering microbes to produce colicins usually ends up killing the hosts. To get around that problem, the study’s authors decided to engineer plants since colicins are not as toxic to plant cells. They were successful at getting various plants to express large amounts of 12 different types of colicins, all of which were compositionally identical to colicins found naturally in E. coli. In their study, Gleba and his colleagues sprayed E. coli-laced pork steaks with a mixture of two types of colicins, at 4 milligrams of colicin per kilogram of meat, and found significant reductions in E. coli after just an hour. The team also hired a third party to do an economic analysis of their process and found that the method competes with decontamination methods, such as acid washes and heat processing, currently favored by the meat industry. “Normally the meat industry’s ready to spend two to five cents to treat one pound of meat,” said Gleba. “Our costs are lower than that.” Furthermore, Gleba believes their process is superior because, unlike heat and acid treatments, colicins don’t affect the quality and taste of the meat. The scientists plan to seek approval from the U.S. Food and Drug Administration (FDA) for the process to be considered GRAS (or “generally recognized as safe”). Francisco Diez-Gonzalez, a food safety microbiologist at the University of Minnesota, warned that FDA’s approval could take a long time. “To get approval for a food additive, you have to present a lot of toxicology studies and effectiveness studies,” he said. “I think it would be difficult to make the case that these colicins should qualify for GRAS because they’re not typically part of the human diet.” Other types of bacteria besides E. coli can be killed with colicins, and research team members are planning to extend their process to these next. “We are already studying colicins for the control of Salmonella,” Gleba said.
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