By David N. Leff
DNA analysis has profiled a long-sought serial killer known as "the hamburger bug."
It's the highly pathogenic O157:H7 strain of the Escherichia coli bacterium, which caused the recall three years ago of 25 million hamburger patties, at a cost of $15 million. It was first fingered as a cause of human disease in 1983, after two outbreaks in the U.S. of an offbeat and horrendous gastrointestinal ailment. It's now regarded as a major global public health menace.
Today's issue of Nature, dated Jan. 25, 2001, reports cracking of the hypervirulent E. coli's genetic makeup in a paper titled: "Genome sequence of enterohemorrhagic Escherichia coli O157:H7." The announcement's senior author is molecular geneticist Frederick Blattner, director of the Genome Center at the University of Wisconsin in Madison.
O157:H7 tips the genomic scale at 5,440,000 base pairs of DNA in its single chromosome, plus two plasmids. (Only two small gaps remain to be filled in, imminently.) This is a good 20 percent bigger than the bug's nonpathogenic E. coli K-12 cousin - a microbial workhorse in research laboratories throughout the world. Blattner also led the Wisconsin team that completed sequencing K-12 in 1997. That harmless version of the bug came in at 4,639,229 base pairs. It numbered 4,405 genes, compared to the 5,416 in the toxic "O" version. Both were obviously derived from a common ancestor. (See BioWorld Today, Sept. 5, 1997, p. 1.)
"A most striking finding," Blattner observed, "is our discovery of 1,387 new genes in O157:H7 that were not found in the previously sequenced nonpathogenic E. coli K-12 strain. Many of these show similarity to genes from other harmful bacteria, such as Salmonella and Yersinia pestis (plague) that are known to be implicated in pathogenic functions. The precise role of each of these in the disease process can now be studied in laboratories worldwide.
"Exactly how the O157:H7 genome acquired these novel genes," he continued, "is also a major area for future research. It seems clear that they must have been transferred laterally from other organisms in the natural environment through some type of horizontal gene-transfer mechanism - possibly by bacteriophage, conjugation, transformation and plasmids. Clearly," Blattner foresees, "this mechanism could be significant in the emergence of new diseases over time, and may lead to a shift in the concept of the biological species as it applies to bacteria."
Ubiquitous - In Gut, In World
"Each person on the planet," he observed, "carries approximately 10 billion E. coli cells in his or her gut, and similar numbers are present in other animals. A tremendous quantity of E. coli is released into the environment on a daily basis. Coliform counts," he noted, "are in fact routinely used by public health officials as an indicator of fecal contamination of environmental and water supplies." Besides direct contamination from the manure of infected cattle, the infection can be picked up indirectly from fruit and vegetables grown in bovine fertilizer.
"O157:H7 is a very toxic type of E. coli bacterium," Blattner told BioWorld Today, "that's virtually endemic in the world. In the U.S. it causes about 73,000 cases of diarrheal illness, 2,000 hospitalizations and 60 deaths per year. Comparable numbers apply in other developed countries. Death," he explained, "is usually the result of kidney failure - a condition known as hemolytic uremic syndrome. The principal victims are children under 5 or older people over 60. It's a food-borne infection, most often occurring in outbreaks traced to undercooked hamburger.
"Ingestion of 10 to 100 O157:H7 bacteria," Blattner continued, "suffices to produce infection. In contrast, a dose of hundreds of thousands of bacteria is required for the less infective strains, which cause ordinary diarrhea."
The illness starts with severe abdominal cramps and watery diarrhea, which turns bloody in 24 hours. This is usually what brings the patient into the medical care system. In fact, this hemorrhagic colitis is often described as "all blood, no stool." The infection in very young toddlers is often spread - via unpasteurized milk or unhygienic practices - from diaper to diaper in child daycare settings.
In recounting the onset and course of O157:H7 infection, Blattner noted how "effacement and tissue damage of intestinal cells in the colonic area results in ugly lesions, which bleed and cause the bloody diarrhea. This damage is caused by secretion of bacterial toxins, such as hemolysin."
Bacterial Toxins: The Smoking Guns
"Deadly Shiga toxin production begins," he narrated, "when these toxin genes are carried by bacteriophage in the genome. They are activated when E. coli goes on the defensive against its human victim's immune system, and the phage attempts to bail out. This Shiga toxin," Blattner pointed out, "is very similar to Ricin - one of the deadliest toxins known, which figures in chemical and biological warfare. The Shiga toxin directly affects protein synthesis by shutting down the ribosomes. It gets into the bloodstream and attacks many organs, including the brain, and especially the kidneys. Kidney failure and death result in about 5 percent of the hospitalized cases.
"Treatment options," he went on, "are minimal, and generally, supportive care is provided while waiting for the infection to run its course, usually in a week or so. Even though O157:H7 is sensitive to most antibiotics," he observed, "these are not currently recommended because of a possibility that some antibiotics contribute to release of the toxin.
"The human body," he added, "activates antibodies and other immune defenses against the E. coli attack, but the pathogens fight back with antibody-cleaving immunoglobulin-A enzymes, and other mechanisms, to shore up its own threatened genome."
Blattner concluded that the just-announced sequencing of the O157:H7 genome "will help in the areas of vaccines and drugs, diagnostic tests, disease treatment and management, and safeguarding the food and water supply." He is now sequencing an E. coli strain that inflicts urinary tract infection.
The Nature article's lead author, evolutionary geneticist Nicole Perna, commented, "I think what distinguishes this paper from some of the other genome reports is that we were able to make the comparison between O157:H7 and K-12, delineating which factors were unique to each of these E. coli organisms."