University of Toronto researchers have uncovered three genes in the Salmonella bacteria critical for it to cause disease and withstand antibiotic treatment and may hold the key to improved disease treatment.
The study was published today in the journal Molecular Cell and conducted in partnership with laboratories headed by Dr. Michael Ibba at Ohio State University and Dr. Ferric Fang at the University of Washington in Seattle. The team found that three bacterial genes called poxA, yjeK andefp work together to protect the bacterial cell from stresses it encounters during infection and antibiotic treatment. Mice infected with Salmonella strains lacking any one of these genes do not get sick. Even more crucial is the discovery that these Salmonella strains are also highly sensitive to treatment with a variety of antibiotics and disinfectants.
“Salmonella continues to be a major source of food poisoning in North America. In the past few years we have seen numerous recalls of food products including a major recall of peanut products just two years ago and a current outbreak in Canada resulting in the recall of headcheese.” said Professor William Navarre of U of T Faculty of Medicine’s Department of Molecular Genetics and lead author of the study.
“We now aim to develop drugs that can inactivate poxA, yjeK or efp. By preventing these bacteria from responding appropriately to stress, we predict we will be able to prevent bacterial disease and decrease their resistance to antibiotics. We’re excited by the fact these genes exist in other bacteria that cause disease including E. coli so our strategy may work in cases beyond Salmonella.”
The three genes are critical for bacterial resistance to chemical stress. “Despite their small size bacteria are actually quite sophisticated” said Navarre, “Salmonella and E. coli bacteria grown in a mild amount of disinfectant or antibiotics are able to make adjustments so that they can survive without too much of a problem. In order to do this they must be able to detect that they are in trouble and activate the genes necessary to make the appropriate changes to their metabolism and cell structure.
“We are finding that Salmonella lacking poxA, yjeK or efp are unable to cause disease because they cannot withstand attack from the various immunity factors present in the mouse. Many of those factors attack the membrane of the bacterial cell and we believe that without poxA, yjeK or efp that the Salmonella becomes easy prey because it can’t respond to the damage.”
The research team screened thousands of Salmonella bacteria for variants with altered resistance to certain antimicrobial compounds. Several of the identified variants had defects in either the poxA or yjeK genes. Using genetic engineering the researchers removed the poxA and yjeK genes from normal Salmonella cells. These newly constructed variants were also unable to cause disease in mice and were sensitive to a wide number of antibiotics. It was subsequently found that these two genes were working with the third gene, efp. The efp gene codes for a protein called “elongation factor P” or EF-P that allows other genes necessary for stress resistance to work properly. EF-P carries out its function by mimicking another molecule called tRNA, that plays a role in the synthesis of bacterial proteins.
The researchers, in collaboration with the University of Toronto Structural Genomics Group headed by Professor Aled Edwards and Professor Alexei Savchenko, also determined the molecular structure of the PoxA protein and found that it was strikingly similar to a “tRNA synthetase,” an enzyme that modifies tRNA. “It’s a beautiful case of molecular mimicry where a molecule that usually modifies a tRNA evolved to instead modify a protein that looks like a tRNA.”