Antibiotic resistance has put humans in an escalating 'arms race' with infectious bacteria, as scientists try to develop new antibiotics faster than the bacteria can evolve new resistance strategies. But now, researchers have a new strategy that may give them a leg up in the race -- reproducing in the lab the natural evolution of the bacterial enzymes that confer resistance.
A team of scientists in Argentina and Mexico identified mutations that increased the efficiency of a bacterial enzyme that renders penicillin and cephalosporin antibiotics useless. The results could lead to more effective enzyme inhibitors by giving drug designers a sneak peek at the next generation of resistance.
Alejandro Vila, a Howard Hughes Medical Institute international research scholar, and colleagues at the University of Rosario's Institute of Molecular and Cellular Biology in Argentina and at the Biotechnology Institute of the National Autonomous University of Mexico report their findings in the early online edition of the Proceedings of the National Academy of Sciences the week of September 19, 2005.
Staying one step ahead of resistance with new antibiotics and treatments for infections is a huge challenge because bacteria evolve quickly to evade them. When the scientists introduced random mutations into the gene for a bacterial resistance enzyme and grew the bacteria on increasing concentrations of antibiotics, it took only a few days of test tube evolution to increase drug resistance. Eventually, they found four mutations in the evolved enzyme that allowed the bacteria to survive on a drug dose 64 times higher than the dose that kills bacteria hosting the un-evolved enzyme.
"We were mimicking what is going on in the doctor's clinic -- putting selection pressure on the enzyme by giving higher doses of antibiotic," said Vila. "The only ones to survive will be those that have an enzyme that can work more efficiently."
The researchers conducted their experiments using a drug called cephalexin, a member of the widely-prescribed cephalosporin class of antibiotics. These drugs and the penicillins, which share a common chemical backbone called the â-lactam ring, work by disrupting the bacterial cell wall. Bacteria have evolved enzymes called â-lactamases, which chop the â-lactam ring in half, inactivating the drugs. An inhibitor for one type of lactamase has already been marketed as part of a 'package drug' with amoxicillin to fight resistance.
But the lactamase enzyme that Vila's group studied is in a different class that is causing an emerging problem around the world. This class, the metallo-â-lactamases, is more threatening, said Vila, because it is effective against a broader spectrum of antibiotics, such as carbapenems. However, it also represents a younger set of enzymes that are still evolving, and that enabled the scientists to observe that evolution in fast-forward.