MCP: How now, chilled cow?
Many strains of E. coli would like nothing more than to have a nice home in your lower intestine, protecting the intestinal microbial community from harmful pathogens while giving back with vitamin K. However, one strain, E. coli 0157:H7, would like to take a wrecking ball to that community. It often finds its way in through contaminated produce or beef products. In a recent paper published in the journal of Molecular & Cellular Proteomics, researchers reported how chilling and desiccation of beef during meat processing affect the expression of proteins critical to the survival of E. coli 0157:H7. Their findings may help reduce cases of food poisoning through beef products.
Bovine-carcass processing is a messy, complicated business that currently lacks a foolproof method for preventing E. coli contamination. “We wanted to explore whether there were some vulnerabilities in the response of E. coli that we could exploit to improve intervention methods for killing off E. coli on the carcass,” says Thea King, the lead author on the MCP paper. King is a research scientist in the Food Safety & Stability group at the Commonwealth Scientific and Industrial Research Organisation, the federal agency for scientific research in Australia.
Each year, E. coli 0157:H7, which produces Shiga toxin, is responsible for about 36 percent of the estimated 265,000 cases of food poisoning in the U.S. While the symptoms of the infection typically include diarrhea, fever and abdominal pain, severe cases can involve dehydration, gastrointestinal bleeding and kidney failure.
Australia is the largest exporter of beef to the U. S. In 2015, the U.S. consumed about 12.4 million tons of beef, or enough to make 124 billion Big Macs. Of that amount, roughly 418,000 tons, or 4.2 billion Big Macs, came from Australia.
The MCP paper built on a 2014 article published in the International Journal of Food Microbiology by one of King’s co-authors at the University of Tasmania, Tom Ross. In that paper, Ross and his team examined the growth responses of E. coli under separate and combined conditions of chilling stress and low-osmotic stress.
When subjected to the combined chilling and desiccation process by King and colleagues, as outlined in the MCP paper, the E. coli exhibited a sharp adaptive growth arrest, in which the bacteria made several changes in protein regulation, followed by a regrowth phase. This was similar to what had been hypothesized based on the previous paper by Ross’ team.
During the growth arrest, King and her colleagues noticed a window of cell susceptibility: The bacteria were undergoing DNA damage, reductions in the consumption of carbon sources and a downregulation of molecular chaperones and proteins associated with the response to oxidative damage. This was noticed at the transcriptomic and proteomic levels. The data indicated that there was a disruption of crucial energy-generating processes.
At the same time, the analysis indicated that the E. coli were busy upregulating a number of stress-related proteins by way of regulons, groups of genes that activate together in response to an external stimulus. In addition to a highly expressed cold-shock protein, “one of the first changes that we saw is that E. coli activates the RpoE regulon, which might be an emergency response for cells to repair protein misfolding in the cell envelope,” says King.
King goes on to explain, “The most interesting thing that came out of this was that we found that under conditions of low water activity, E. coli seemed to lose its culturability.” If the bacteria can’t replicate, they can’t contaminate the cow carcass. However, “when E. coli were just exposed to chilled temperatures, we didn’t see this loss of culturability,” adds King.
King and colleagues plan to expand the study to examine the effects of oxidative damage in additional strains of E. coli as well as Salmonella. “We really want to explore this loss of culturability further,” King says, adding that the aim is to try to “exploit the physiology of E. coli under these conditions and try to benefit meat safety.”
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