By David N. Leff
It's now going on 12 years since an Australian hospital resident deliberately swallowed a culture containing 1 billion organisms of a pathogenic bacterium named Helicobacter pylori.
The do-it-to-yourself experimenter, Barry Marshall, woke up feeling hungry and irritable, with a headache, bad breath and gastritis - inflammation of the stomach. Two weeks later, he was asymptomatic.
"Marshall, at Royal Perth Hospital, was co-author of the original research paper that claimed H. pylori to be the true cause of gastric ulcers," recalled microbiologist Trevor Trust. The organism was discovered in 1983-85. He probably didn't drink it until about 1987.
"There was a lot of skepticism about that finding," Trust went on, "and Marshall was getting very frustrated that no one would believe him. A couple of years later, Arthur Morris, a New Zealander, really felt that Marshall's experiment was flawed. So he drank more of the organisms, and infected himself for a much longer period of time."
Trust, senior vice president of research and development at Swedish pharmaceutical giant Astra AB's Boston-based Astra Research Center, is co-senior author of a paper in today's Nature, dated Jan. 14, 1999. It bears the title: "Genomic-sequence comparison of two unrelated isolates of the human gastric pathogen Helicobacter pylori."
His co-senior author is molecular microbiologist Gerald Vovis, senior vice president of pathogen genetics at Genome Therapeutics Corp. (GTC) in Waltham, Mass.
"This is the first time," Vovis told BioWorld Today, "that two different, unrelated strains of the same bacterial species have been compared. Their publication is going to be the beginning of other such comparisons. Rather than looking at only one example of an important pathogen, one can begin to look at several, and ask questions about their commonality. What constitutes the basis of a given species."
What led to this duplicate DNA sequencing exercise was an effort to test a widely held dogma in the infectious-diseases community that the several disorders of which H. pylori is guilty each arises from a different strain of the microbe.
"Fully 50 percent of the world's population is thought to be infected with this bacterium," observed microbiologist Richard Alm, a research scientist at Astra and first author of the Nature paper. "Its diseases range from asymptomatic to gastritis and inflammation, gastric and peptic ulcer, then two forms of cancer, which are epidemiologically linked to infection with Helicobacter pylori."
Trust added: "In parts of the world where there is a lot of crowding and close contact between individuals, you get very high incidence very early in life. Whereas in North America, where people are not so close together in the early stages of their lives, incidence is much lower. It's probably acquired from other humans at some stage early in life, and may be saliva-driven.
"Some people argue," he continued, "that it's spread by situations where kids sleep together. In Third World countries and the like, that may well be one of the environmental factors."
H. Pylori Sins: From Zero To Cancers
Peptic ulcer disease affects 25 million Americans, and 500,000 to 850,000 new cases are diagnosed annually in the U.S. The American Cancer Society estimated 22,600 new cases of stomach cancers in 1998, and 13,700 deaths - 40 to 60 percent of them related to H. pylori infection. "It's the only bacterium," Vovis pointed out, "that the World Health Organization has classified as a Class 1 carcinogen."
H. pylori's total genome was first sequenced by GTC in 1994, but was not made public until today's Nature article. It has also been posted on the World Wide Web by both GTC and Astra. TIGR (The Institute for Genomic Research), in Gaithersburg, Md., announced its sequence of H. pylori's genome in 1997. GTC sold Astra access to its H. pylori genome soon after sequencing it.
The two genomes reported in Nature that GTC sequenced and Astra analyzed side by side came from two infected patients, an American in Tennessee with a duodenal ulcer - whose genome GTC sequenced in 1994 - and a British patient with gastritis, whose genome was mapped prior to 1987 in Britain.
"To obtain DNA starting material," Vovis explained, "one puts a tube down the esophagus into the stomach, and isolates the bacteria directly. For the most part, the stomach is virtually a sterile environment. The only type of bacteria that one sees is pylori, so one takes out this specimen and puts it on a media-rich plate.
"With this strain," he continued, "we minimized the amount of time that we grew it outside in the lab, because our concern was to try to look at the genome of a pathogenic organism as close to its human host as possible. Even while growing this strain, one sees changes occurring. And that becomes a concern when you're trying to come up with new therapeutic products to treat the organism. You want to make sure that what you're looking at is not a lab curiosity but a real-world curiosity."
Seeking Products To Totally Eradicate Infection
Currently the only treatment for H. pylori infection consists of antibiotics, to which the smart organism is becoming resistant, and an acid-suppressing medicine from Astra Pharmaceuticals Inc., of Wayne, PA, called Prilosec. This deep-purple capsule, Vovis observed, "is the highest-selling prescription drug in the world. Even higher than Prozac or Tagamet. Its mode of action is to alleviate symptoms rather than directly attack the bacteria. It's mainly used for acid reflux [heartburn], a very significant condition."
"Astra's had the GTC genome sequence since the end of '95," Trust recalled, "and we use it to identify antigens for vaccine discovery and targets for therapeutic drug discovery. We have identified in that genome potential antibacterial drugs specific for Helicobacter pylori. Now, we have to find chemicals against those targets. Antigens, the same thing."
As for the conclusion reached by the two-genome comparison analysis, largely negating the dogma that each of H. pylori's diseases arises from a separate bacterial strain, Astra's Richard Alm concluded: "In the selection of drug targets and vaccine antigens, the comfort of knowing that most strains are going to be pretty similar gives us a lot more faith that we're going to eradicate all of the strains from people infected with the bacteria, not just a certain percentage of them." n