By David N. Leff
Mankind measures its progress by the age.
The Stone Age lasted upwards of a million years, followed by the Bronze Age and Iron Age - each of which spanned a good many millennia - and so on, down through the ages. In living memory, what can be called the Age of Antibiotics endured for a scant half-century - from the advent of penicillin during World War II to the rising tide of bacterial resistance, which threatens to engulf us any decade now.
In a sense, this is Mother Nature's backlash against the wanton prescription of antibiotics, by target pathogens that evolved resistant strains under the genetic pressure of the antibiotic overload. The poetic irony here is that the large majority of antibiotics derive from the self-defense secretions of natural fungi and bacteria. A chart of 39 modern antibiotics lists only half a dozen produced by chemical synthesis.
Many of the familiar germ-killing drugs prescribed by your physician or hospital - from adriamycin, chloramphenicol and erythromycin, to streptomycin and tetracycline - all derive from strains of a single soil bacterium, Streptomyces. (Penicillin comes from two strains of fungus, Penicillium notatum and P. chrysogenum.)
Of late, the race against the killer microbes has taken a new turn - away from mining more natural substances to imitating the molecular structures of these peptides and proteins.
"Pretty much all multicellular organisms," observed chemist Samuel Gellman at the University of Wisconsin-Madison, "have to defend themselves against microbial invasion. And a widespread defense mechanism involves administering relatively small peptides with amphiphilic secondary structures that display broad-spectrum activity against microbes."
Amphiphilic means exhibiting both hydrophilic (water-loving) and lipophilic (fat-loving) properties.
"Examples of such small peptides," he added, "are the magainins, in which the secondary structure is a helix. Such helical peptides seem to be widely distributed, particularly among lower organisms such as frogs and insects." (See BioWorld Today, July 8, 1998, p. 1.)
In Zapping Bacteria, Structure Is Key
"With magainins and related peptides," Gellman explained, "it's only when that structure is formed that its amphiphilic nature is revealed. That is, in the helical state you've got the cationic [positively charged ion] groups lined up on one side of the helix and the lipophilic on the other. That conformation is thought to be critical for antibiotic function - presumably disruption of the bacterial membrane.
"Helices," Gellman continued, "are one very common shape in the natural world. The natural peptides we're trying to mimic are made from the natural alpha amino acid building blocks, and their alpha-helical shape are very widely found in peptides and proteins.
"Our beta peptides form a helix that's different in the details," Gellman pointed out, "but similar in their overall structure. My lab has been interested for several years," he recounted, "in the prospect of mimicking the kinds of molecular shapes that one sees in natural proteins, with unnatural types of oligomers [small polymers] - in this case using beta amino acids instead of alpha amino acids.
"Over the last two years we've gotten pretty good results just at the basic structural level. That is, we can make helices, sheets, reverse turns - all the regular types of secondary structures one sees in proteins. This effort was really the first success we had in trying to make use of our ability to control structure, on the assumption that if we could control shape, then we could control function."
Gellman is senior author of a brief communication in the current issue of Nature, dated April 6, 2000. Its title: "Non-hemolytic b-amino-acid oligomers." His co-senior author is Bernard Weisblum, professor of pharmacology at Wisconsin.
"This work," Gellman said, "demonstrates the feasibility of developing a new class of antibiotics that can potentially overcome the growing resistance of disease-causing bacteria to current medicines. And it reveals the potential of using nature as a guide to build medically important synthetic molecules that can outperform their naturally occurring counterparts."
It isn't enough for a new antibiotic to resist resistance by its target microbes. It must also be harmless to human cells. The co-authors used the cellular equivalent of canaries in a coal mine:
"You get a useful antibiotic activity only if you don't harm the host organism's cells," Gellman pointed out. "The standard initial way to test that," he explained, "is just to look at human red blood cells - which are somewhat delicate, and relatively easily to kill - and see whether they are broken apart by the peptide in question. Naturally occurring antibiotics are selective in disrupting bacterial membranes. They don't disrupt blood cells or other host cells."
The team co-cultured its experimental antibiotic peptides with strains of four species of infectious bacteria: Staphylococcus aureus is a major pathogen in hospitals, resistant to penicillin; Enterococcus faecium causes gut, wound and urinary tract infections, and resists vancomycin. The other two, Escherichia coli and Bacillus subtilis, are non-pathogenic bacterial workhorses harnessed to biotech lab research.
Pilot Tests Put Beta Peptides On Par With Controls
"Results of this standard assay, which exposed the bacteria to escalating concentrations of our beta-peptide," Gellman said, "averaged over the four bacteria, proved our beta-peptide antibiotically comparable in all four cases to the artificial magainin we used as a control, and had less hemolytic activity. Moreover, the magainin peptide is readily degraded by proteases - proteolytic enzymes - and beta peptides' backbones aren't recognized by those things."
The co-authors have not yet extended their tests beyond the initial four bacteria, Gellman said, "but it's certainly on the books to do. The thing we're doing right now," he added, "is preparing a whole series of analogues to optimize the antimicrobial function, before we go in vivo. Raising enough of the material to put into mice," he pointed out, "is going to be quite an investment of effort."
The university has filed a patent covering the beta-peptides, and, Gellman allowed, "I'm very interested in talking to people about pursuing their potential commercial ramifications. Particularly since I don't think they're limited to antibacterial agents, though that's probably the first application. I think there's a tremendous potential," he concluded," in the biomedical and pharmaceutical arena for using unnatural oligomers."