By David N. Leff
Among the commonest causes of what are famously called "traveler's diarrhea" - food-borne intestinal infections - in the U.S. and Europe is the bacterium, Campylobacter jejuni. You don't have to travel very far to double over with this gut-agonizing ailment.
"A large percentage of the chickens that you buy at the supermarket," observed microbiologist and bacterial pathogenicist John Leong, "are actually contaminated with Campylobacter. When you cook chicken," he added, "usually you kill it, so it doesn't infect you all the time. But it's certainly an important pathogen."
Leong is on the faculty at the University of Massachusetts Medical School in Worcester, Mass. He is the author of a "Perspective" in today's issue of Science, dated Oct. 13, 2000, titled: "Arresting features of bacterial toxins." He characterized bacterial toxins as "the most potent poisons known to humankind."
Leong's editorial comments are on a research article in the same issue of Science, bearing the title, "A bacterial toxin that controls cell cycle progression as a deoxyribonuclease 1-like protein." Its senior author is microbiologist Jorge Galan, who chairs microbial pathogenesis at Yale University School of Medicine, in New Haven, Conn.
"We know the entire nucleotide sequence of this organism, released early this year," Galan told BioWorld Today, "yet literally nothing is known about the mechanism of the disease it causes. That's probably so for two reasons: First, the recognition of C. jejuni as a cause of diarrhea is relatively recent. Second, there haven't been good research tools to manipulate these microorganisms. These are becoming available now."
DNA In Bug's Cellular Crosshairs
Leong observed that "Galan has probably uncovered the mechanism of action of a toxin that acts in a novel way. And the novelty of his finding is that this toxin is targeted at the mammalian host cell's DNA. This is a toxin," Leong told BioWorld Today, "that actually gets into living human cells, into their nuclei, where they presumably are able to cleave DNA.
"As far as intracellular - inside job - targets for bacterial toxins are concerned," Leong said, "that finding of Galan's group, in and of itself, is fairly novel. Most toxins that get inside mammalian cells will cleave or modify their proteins. This one apparently has no protein target, but does target DNA.
"It's not clear," he noted, "what role Campylobacter toxin has in the disease process. Galan's findings open up some avenues to try to understand that role."
Leong added, "This toxin is pretty unusual in that it's expressed by a wide variety of pathogens - including some that aren't very closely related - such as E. coli, Hemophilus, Actinobacillus. They all make this toxin, which, on the face of it, supports the idea that it must be important.
"The other common finding among these pathogens," he said, "is that they all colonize mucosa. So one question is: How does that presumably facilitate infection by these mucosal pathogens?"
Galan rose to that occasion:
"Most intriguing perhaps is the fact that by finding the mechanism of action of this toxin we are facing now the prospects of a microorganism that is widespread in human populations. Probably most of the time it's not causing disease, yet encoding a toxin that by the mechanism of action we have uncovered clearly indicates that the toxin causes genomic instability in rapidly dividing cells - such as those of the intestinal epithelium. And that is something to at least be aware of as an environmental factor that may contribute to intestinal cancer down the line."
To which Leong added: "The possible connection is that by generating DNA damage, you're potentially generatiing genomic instability and mutations. That, of course, is the basis of cancer. I don't think there is any evidence of a link to cancer," he added, "but it's an interesting hypothesis. What you have to say is that this toxin is able to provoke DNA damage.
"Usually in vitro," he pointed out, "the bacterial toxin causes cell cycle arrest. If the cell is arrested, obviously it's not going to be a source of cancer. So what you have to say is that it can cause DNA damage but doesn't invariably lead to cell cycle arrest."
Galan observed that his finding "illuminates a potentially new way that microorganisms utilize to manipulate cellular function - that is, causing limited cellular damage. The cellular response in this case is stopping cell cycle progression for the benefit of the pathogen."
One typical response he reported was the extreme swelling, or distention, of the infected cell nuclei and cytoplasm - the latter often to four or five times its normal size. Campylobacter toxin consists of three subunits, encoded by a cluster of genes labelled cdtA, cdtB and dtC - their acronym standing for "cytolethal distention toxin."
"It wasn't known," the Yale researcher pointed out, "which component was in charge of delivering the toxin, and which had the enzymatic activity. We found that when we injected all three components together into mammalian fibroblasts we could recapitulate all the effects of the whole toxin - mainly, cell cycle arrest and cytoplasmic distention."
Ferreting Out Toxin's Effect On Humans
For the near future, Galan related, "We are setting up in vivo experiments. You cannot use mice or rats, which puts a hamper on the ability to do experiments. Now there is a mode - ferrets - which we are in the process of setting up in the laboratory, to begin to ask this type of questions relating to issues of the activity of the toxin in vivo. That's the only animal apparently that one can infect with this microorganism, and replicate more or less what goes on in humans."
Leong foresees an additional spin: "Here is a toxin that triggers a very specific cell cycle arrest. I suspect that there will be quite a bit of interest in people who are into cell division, using this as a tool to induce cell cycle arrest at a particular phase - and then try to dissect out what human components are involved. You can imagine," he concluded, "cell cycle arrest is central to innumerable therapeutic strategies. So to have a tool here which will maybe allow us to better understand it, is an added bonus of Galan's study."