The scriptural dictum, "Love thy neighbor as thyself," conveys ahidden hint that some neighbors might not be all that lovable.
What about a neighbor that is thyself _ in part?
Such a one is the gram-negative bacterium Escherichia coli, a fellow-traveler in everyone's intestines _ and a prime cause of travelers'diarrhea, urinary tract infection, appendicitis and septic shock, amongother nasties.
But E. coli isn't all bad. In the gut it synthesizes vitamin K, a vitaldomino in the blood-clotting cascade. And of course, historically, E.coli blasted biotechnology off the launching pad into genetic-engineering orbit.
This ubiquitous microbe wreaks its intestinal havoc three ways: byspewing forth enteric toxin; by directly invading the gut's mucosallining; by sticking to the gut wall. Perhaps its least lovable, mostalarming, trait is that, in common with most other microbialpathogens, E. coli develops resistance to commercial antibiotics.
The tricky bug can generate a specific resistance-factor gene evenbefore it's exposed to the specific antibiotic. So E. coli, along with itsother chores, has become a model for testing ways and means offoiling bacterial resistance.
One of the bullet-shaped, micron-size bug's defense mechanismsagainst antibiotics is a major structural component in its outermostmembrane, a glycolipid called Lipid A. This substance, which isessential to bacterial growth, does its antibiotic-bouncing job in allGram-negative bacteria, not just E. coli. And it constitutes E. coli'sendotoxin.
Antibiotics in current use have not heretofore aimed at Lipid A.Rather, many of them target the cell wall underneath the oily Lipid Alayer of the bacterial membrane. Others go after protein synthesis andDNA replication.
At the Merck Research Laboratories, in Rahway, N.J., a task force ofmicrobiologists, medicinal chemists and biochemists, led bybiochemist Christian Raetz, trained its sights on E. coli's Lipid A as apossible target of vulnerability, for a new class of antibiotics.
Infiltrating E. Coli's Permeability Barrier
Raetz, who was Merck's vice president for basic biochemical andmicrobiological research, is senior author of a paper in today'sScience, which reports his team's results to date. Its title:"Antibacterial agents that inhibit Lipid A biosynthesis." He nowchairs biochemistry at Duke University, in Durham, N.C.
Bacteria change their spots, take on new properties, by rapid-firemutating. As one bacteriologist observed long ago, "They really haveonly one function: to reproduce their kind." At this assignment, E.coli is superbly efficient. It divides and multiplies every 20 minutes.
In that process, gene mutations are frequent, promiscuous andprotean.
Raetz and his crew at Merck discovered several defects in the geneticmachinery encoding Lipid A, which disorganized the outermembrane layer. Bacteria that suffer this disability are easy prey tofresh blood serum, which contains components. of the cell-killingcomplement system. And as a bonus, reduced Lipid A synthesisrendered Gram-negative bacteria highly susceptible to otherantibiotics in current use, such as erythromycin.
After designing a synthetic hydroxamic acid compound that inhibits akey enzyme in the second pathway of Lipia A synthesis, theyscreened more than 200 analogs of it, and came up with an even morepowerful inhibitor.
"One of the nice features of it," Raetz told BioWorld Today, "is thatwhen you begin to inhibit the synthesis, you not only gradually killthe cells, but before you do, you open up the bug's impermeabilitybarrier against antibiotics, and let other things in that normally don'tget in, such as erythromycin. It overcomes E. coli's intrinsic resistantto that antibiotic."
Their inhibitor proved to be on a par with current antibiotics in its E.coli-killing power. It killed off wild-type E. coli, as well as several(but not all) other Gram-negative bacteria, in vitro. What's more, itcured mice of a lethalintraperitoneal E. coli challenge.
"Since I left Merck," Raetz continued, "both they and we have triedto crystallize that inhibitor, because that's the push we both want totake: Figure out the structure of the targets, co-crystallize them withthe inhibitors we do have, then take the Vertex approach to makingthem better.
"You can find inhibitors of these enzymes," he observed, "that aremany orders of magnitude more potent than the ones we have now.So we're not nearly at the theoretical limit of this search. We're juststarting."
Ten More Years To Go
Looking down the road, Raetz went on: "Most drug discovery takes20 years, at least historically. We're about 10 years into this. So that,I think, is an accurate reflection of how long it's going to take toreally refine this one. We now have all of the enzymes identified andcloned.
"What we haven't done," he concluded, "that would be fun to do, issome sort of combinatorial chemistry in conjunction with thestandard chemistry and enzymology that I know how to do. So if youcould find somebody who can do that, I'd love to collaborate withthem."
In an editorial accompanying the Science paper, Finnishbacteriologist Martti Vaara, University of Helsinki, commented thatsuch reports "stimulate the search for promising new antibiotics . ..and are part of the drug evolution that balances the evolution of drugresistance."
In a press statement, Merck spokesperson Eileen Undercoffer notedthe team's conclusion "that compounds exhibiting this mode of actioncould be developed to treat some bacterial infections, and to reducethe amount of toxin released from bacteria when other antibiotics areused as the primary therapy." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.