By David N. Leff
Compare these two seminal crystal balls, each foretelling the future of antibiotics:
"By the late 20th century, one should anticipate the virtual elimination of infectious disease as a significant factor in social life. The subject is almost something that will pass into history." Thus, Nobelist immunologist and virologist Sir Frank Macfarlane Burnet (1899-1985), who rendered this prediction in 1962.
Just 30 years on, in 1992, molecular biologist Mitchell Cohen, director of bacterial and mycotic diseases at the U.S. Centers for Disease Control and Prevention, wrote in Science: "Unless currently effective antimicrobial agents can be successfully preserved and the transmission of drug-resistant organisms curtailed . . . a post-antimicrobial era may be rapidly approaching in which infectious disease wards housing untreatable infections will again be seen."
Now, a scant decade later, research chemist Shahriar Mobashery, at Wayne State University in Detroit, surveys the current world scene of bacterial resistance to antibiotics:
"It's quite grave," he told BioWorld Today. "This gravity came to light in the past 10 or 15 years. Currently," he pointed out, "there are seven major classes of antibacterials in clinical use, and there are cases of resistance to all seven. There are infective microorganisms that either cannot be treated with antibacterial agents, or can be treated only by one class. So the situation is quite serious."
A professor of chemistry, pharmacology and biochemistry at Wayne State, Mobashery is director of the university's Institute of Drug Design. He is senior author of a paper in the latest Proceedings of the National Academy of Sciences (PNAS), dated Feb, 13, 2001, titled "A 1.2-A snapshot of the final step of bacterial cell wall biosynthesis."
Road To Anti-Resistant Antibiotics
Mobashery made the point, "This X-ray crystallographic analysis has a tangible consequence, because the structural and mechanistic information may be used by drug developers in pharmaceutical companies and research groups - including our own - to think of new ways of exploiting these bacterial target sites to design next-generation antibacterials.
"One trait that all bacteria have in common," Mobashery pointed out, "is that they all have to cross-link their cell walls. These are polymeric [multi-molecular] structures, and the last step in their biosynthesis is the cross-linking, which gives that wall strength and shape." He compares cross-linking to a chain-link fence. "One link wraps on to the other, and then to the next one over, and so on. They're chained to one another, creating a rigid entity such as bacteria require for their survival.
"In bacteria," Mobashery continued, "these two cross-linked molecular entities are referred to as peptidoglycans. They are the building blocks, cross-linked to give that wall the strength the bacterium requires. The cross-linking is carried out by a family of enzymes called transpeptidases. These are known to be the target sites for penicillins and related antibacterials - cephalosporins and imipenems."
Co-author Judith Kelly at the University of Connecticut, Storrs actually carried out the X-ray analysis, which, Mobashery recounted, "was very challenging. Synthesizing the probe - the final product - involved about 15 steps, and took two years. Then when that compound was at hand, we put it in contact with the transpeptidase enzyme, and made the complex, which Kelly then subjected to X-ray analysis. By that we could computationally arrive at the position of every atom in the structure in 3-dimensional space of this very large molecule.
"And then when we made the connectivities," he went on, "it showed in glorious detail what this enzyme looks like, and how our probe has bound there to components of the cell wall. Finally, for the first time we could see how these different components of the peptidoglycan are coming together in the cross-linking. This was a time point just prior to the cross-linking event taking place. It gives us the image of the two strands of peptidoglycan on the verge of attaching to one another."
Mobashery noted two novel findings of the PNAS paper. "First is the fact that we have elucidated how bacteria do this final cross-linking reaction, by designing a probe that complexes to the active site of a transpeptidase. Solving the atomic structure of that complex to a resolution of 1.2 Angstroms is a very high resolution as structural parameters go. Just a handful of proteins of that size have been crystallized to that resolution.
"Second," he added, "it validates the proposals that have been put forth by others years ago- in the absence of any structural data - about how antibiotics such as the penicillins and cephalosporins work. Now we have structural information that supports their concepts. Knowing how the drug works, and how the biochemical step works, we understand how the two are not compatible with one another. In other words, if the antibiotic inhibits the enzyme, the biosynthetic process cannot take place. So cross-linking is not carried out, and that's how bacteria burst open and die.
"The mechanisms by which these microorganisms acquire resistance to antibiotics," he explained, "are quite diverse. It could be genetic mutations that accumulate and give a specific new trait to the organism, or it could be horizontal transmission of whole resistance genes from other organisms. These are the two dogmas on how resistance to antimicrobials comes about."
Star Drug Down For Count In 20 Years?
"Cephalosporins," Mobashery went on, "are the No. 1 class of antimicrobial agents that are clinically used globally today. Penicillins used to be, but cephalosporins are more important now. These are wonderful antibacterials that have been in use for about 50 years, and have very few side effects. You keep on hearing here and there that the cephalosporins are on their way out, but in reality we have nothing to replace them with - at least for the next 20 years, I think."
Meanwhile, little thanks to physicians, their patients and the meat industry, bacteria are winning the resistance race against antibiotics.
"Over-prescribing or misprescribing by physicians," Mobashery pointed out, "is a serious concern. So is lack of compliance by patients in taking the medicine properly or completely. Another is exposure of domestic animals to antibiotics as growth factors for raising meat. These are societal problems of great consequence." n