New mechanism of action may beat MRSA

Washington - "We now understand at the molecular level how MRSA (methicillin-resistant staph aureas) resistance manifests itself, " Shahriar Mobashery, Ph.D., told the 230th National Meeting and Exposition of the American Chemical Society here.

The University of Notre Dame biochemical researcher called Staphylococcus aureus "extremely diabolic" in terms of its ability to generate resistance to antibiotics.

Resistance to methicillin, a second-generation penicillin, appeared just two years after the drug was introduced in 1959 and MRSA has become resistant to all known beta-lactams. Now MRSA accounts for about a third of all nosocomial infections and is spreading rapidly in the community.

Each staph bacterium normally carries four penicillin-binding proteins (PBPs), so named because they are the sites to which penicillin binds to kill the organism.

"When PBP is inhibited by penicillin, the cell wall cannot be cross linked," Dr. Mobashery explains in an interview with Dermatology Times. "There is high osmotic pressure inside bacteria, and bacteria cannot regulate its own osmotic pressure. So in the absence of a cross-linked cell wall, the organism swells and blows up."

S. aureus gained its resistance to methicillin by taking on another gene in the 1960s; the leading theory is that it probably jumped from another staph species. That gene produces the new protein PBP 2a, which performs the same functions of others in the family, plus it is resistant to antibiotics.

"So with MRSA, even though the normal PBPs lose their function in the presence of beta-lactams, PBP-2a kicks in and does the job for them. The cross-linking takes place and the organism is happy."

Dr. Mobashery's lab showed in 2001 that for the cross-linking step by PBPs, the cell wall occupies 1,000 cubic angstroms of space and that antibiotics block access to this. So how did PBP 2a overcome the presence of beta-lactams and still carry out its physiological functions?

Crystallization of the protein about two years ago by the group led by Natalie C.J. Strynadka, Ph.D., associate professor, The University of British Columbia, offered some clues.

The lab synthesized three analogs of cell wall fragments and added them to a solution containing the beta-lactam nitrocefin.

"What happens is that the cell wall binds with PBP 2a, and that triggers an opening of the active site."

Findings of study

Representatives from Sandoz, a division of Novartis, asked Dr. Mobashery to run his cell wall fragment assay on three novel cephalosporin compounds that they were developing as possible new antibiotics.

He found that the three compounds "have a unique mechanism of action, something that is not seen with the other beta-lactams. They bind at the allosteric site, an event that triggers the active site opening, which allows the drug to sneak in. It's a unique mechanism."

Not content

He wasn't content to simply evaluate those compounds by themselves.

Dr. Mobashery says it is clear at the molecular level that the conformational change that makes the active site available to the drug occurs, but, "What we really need is the details at the atomic level. That means x-ray crystallography of these structures as they undergo their conformational changes."

He hopes that those structures can be determined from the cell wall fragments that they have generated, as they are bound to the allosteric sites. That type of information may help to tweak compounds to enhance their therapeutic potential by better activating the conformational change and allowing a more targeted antibiotic to enter.