By David N. Leff
An off-duty British policeman was the first person in history to receive the first modern antibiotic, penicillin. He was scratched while pruning his roses, and the minor wound festered into a major staphylococcal septicemia infection that brought him near death. This happened in 1938, when there was no specific therapy for sepsis. But a team of young Oxford scientists had come across Alexander Fleming¿s report a decade earlier of a bacteria-killing fungus, Penicilium notatum, which he named penicillin.
The Oxford gang scraped together all the penicillin they could concoct, and saved the policeman¿s life ¿ for four days. But on the fifth day their supply of the novel antibiotic ran out, and their patient soon died.
But the new ¿miracle drug¿ saved thousands, if not millions, of lives a few years later during World War II. Its broad spectrum of lethal bacterial targets ranged from the pneumococcus, gonococcus, meningococcus and diphtheria bacillus to the bacilli of anthrax, tetanus and syphilis. However ¿ a very weighty ¿however¿ ¿ as microbiologist Ivo Boneca pointed out to BioWorld Today, ¿there were already identified strains of Staphylococcus aureus that were resistant to penicillin even before the new antibiotic was available commercially.¿
Boneca explained this zero time lag between innovation and bacterial resistance: ¿Most of the antibiotics in use today are naturally created antibacterials,¿ he pointed out. ¿They have been produced for millions and millions of years, so their potential for drug resistance already exists, because they are normally synthesized by bacteria or fungi with which they are competing.
¿Normally, those organisms designed their own antibiotics ¿ at least that¿s the current theory ¿ as a way of outcompeting their competitors for nutrients,¿ he continued. ¿So most of the resistance mechanisms are already present in nature. It¿s just a question of selecting and finding a good host that has medical impact for them to become resistant.¿
Boneca, now at the Pasteur Institute in Paris, is senior author of a paper in today¿s Science, dated Aug. 24, 2001. It¿s titled: ¿Selective cleavage of D-Ala-D-Lac by small molecules: Re-sensitizing resistant bacteria to vancomycin.¿ (At the time of writing, he was at the Rockefeller University in New York, and his co-author, molecular chemist Gabriela Chiosis, was then at Columbia University. She is now at Memorial Sloan-Kettering Cancer Center in New York.)
Bacterium¿s Wall Comes Tumbling Down
Vancomycin, extracted from the fungus Nocardia orientalis, is ominously known today as ¿the antibiotic of last resort¿ for treating lethal infections from bacteria such as Staph. Already, the closely related enterococci bacteria are showing increased resistance to vancomycin. This outlook raises the specter that the ubiquitous enterococcal intestinal bug is on the point of transferring its high-resistance genes to Staphylococcus ¿ thus putting vancomycin out of the life-saving business.
¿When a bacterium infects its human host,¿ Chiosis ¿ the paper¿s lead author ¿ told BioWorld Today, ¿it starts to grow a thick outer wall. Vancomycin acts by attacking that wall. Obviously, the bacterium is developing resistance. What happens is that to grow its cell wall so it can survive in the body, the pathogen is making dextro-alanine-dextro-alanine amino acid termini.
¿What vancomycin is doing,¿ she went on, ¿is trying to bind to that D-ala-D-ala structure. If it succeeds in binding to it, the bacterium cannot weave its wall. So because of the osmotic pressure in the bacterial cell, the bacterium pretty much blows up. That¿s the way vancomycin works.
¿In response, to protect its cell from this fate, the bacterium starts making a pool of a different cell-wall peptide complex ¿ D-alanine-D-lactate ¿ to accomplish its resistance. Now vancomycin¿s binding affinity for such new termini is 1,000 times less. Faced with that resistance, it¿s not effective as an antibiotic anymore.
¿Usually what people do when confronted with drug resistance is to try modifying the drug, or producing more and more new antibiotics,¿ Chiosis pointed out. ¿Our approach to vancomycin resistance, as we reported in Science, hadn¿t been tried before. Our main finding is trying to disable the resistance by re-sensitizing the bug to the drug. It is unlikely that the bacteria should re-acquire resistance.¿
The co-authors¿ countervailing weapon is a unique small molecule. ¿The way I went about developing it,¿ Chiosis recounted, ¿was that I screened several random combinatorial libraries of peptides to find what would be the requirements for a selective and catalytic cleavage of the ester bond by which the bacterium held its new wall structure together.
¿I put together the findings from the combinatorial libraries,¿ she continued, ¿with different patterns that looked to me like requirements for cleaving that peptide. I assembled those requirements in 200-Dalton small molecules. They didn¿t look like the peptides that came from the combinatorial libraries, but functionally were similar. They restored the resistant bacterium¿s sensitivity to vancomycin.¿
Giving Antibiotic Anti-Resistance A Sidekick
¿Using these peptides as a guide,¿ Boneca added, ¿we designed a small molecule that can be given in combination with vancomycin. This combination, we found, combated vancomycin-resistant infections in vitro, suggesting future therapeutic use for these small orally available molecules in humans.¿
On this score, Chiosis observed, ¿Columbia this week filed a patent application covering our invention. The university is trying to license it to pharmaceutical companies to have it fully developed.¿
A separate but relevant article in today¿s issue of Science bears the title: ¿Genetic basis for activity difference between vancomycin and glycolipid derivatives of vancomycin.¿ Its senior author is chemist Daniel Kahne at Princeton University.
He and his co-authors report that they can target a gene in E. coli bacteria that allows them to quickly kill bacteria that have developed resistance to vancomycin. Whereas the antibiotic merely slows bacterial growth, they find, its derivatives kill off the bacteria with rapidity. They identified the gene, located on the inside of the bacterial cell membrane, by determining that its absence made E. coli resistant to certain vancomycin derivatives. They propose such targeted interventions as future subjects of research to address resistant strains of other bacteria.