Escherichia coli (E. coli) as a biological weapon
Escherichia coli (E. coli) is a Gram-negative, rod-shaped bacterium commonly found in the intestines of humans and other organisms. While most strains are harmless and play a critical role in gut health, certain pathogenic variants, particularly E. coli O157:H7, are associated with severe foodborne illnesses. These pathogenic strains can produce toxins leading to symptoms ranging from mild diarrhea to potentially life-threatening conditions like hemolytic uremic syndrome. The potential for E. coli, especially pathogenic strains, to be repurposed as biological weapons raises concerns. If introduced into water supplies, these bacteria could result in widespread health issues. E. coli O157:H7 is classified by the CDC as a Class B biological agent, indicating it is less lethal and easier to control than more dangerous agents. Additionally, the bacterium’s simple genetic structure allows for modification, which could enhance its virulence. Understanding the dual nature of E. coli—both as a beneficial organism and a potential biological threat—highlights the importance of monitoring and managing its pathogenic strains effectively.
Escherichia coli (E. coli) as a biological weapon
DEFINITION: Gram-negative, rod-shaped bacterium that inhabits the bowels of humans and many other organisms.
SIGNIFICANCE: Because Escherichia coli is an important member of the bacterial flora found in the human colon, the presence of E. coli and related species (known as coliform bacteria) is used as an indication of fecal contamination of water and fresh foods. In addition, pathogenic strains of E. coli, especially O157:H7, are implicated in certain types of food poisoning. These strains carry toxin genes that cause diarrhea and sometimes the more serious disease hemolytic uremic syndrome, which can be fatal. E. coli O157:H7 has some potential to be used as a biological weapon.
Escherichia coli has been used as a model research organism for many years, and scientists know a great deal about the structure and physiology of this organism. E. coli was one of the first bacterial genomes to be sequenced completely and published in 1997. E. coli is normally not pathogenic. It is one of about two hundred different species of bacteria that inhabit the bowels of humans, where it serves a beneficial role by inhibiting growth of pathogenic species such as Shigella and Salmonella. Under normal conditions, it does no harm. However, pathogenic varieties are a threat to be developed into a biological weapon.
E. coli as an Indicator of Contamination
E. coli and many other bacteria make up some 70 percent of fecal matter by weight. Although E. coli itself (other than the O157:H7 strain) is not pathogenic, feces may contain other bacterial pathogens as well as many disease-causing viruses and protozoa. Water supplies my be contaminated by pathogens in several ways, including inadequate sewage treatment and runoff from irrigation of land where fertilizers containing animal excrement have been used. It is essential that any such contamination be identified quickly.
Although it is possible to test for specific disease-causing bacterial species and many viruses, such tests are relatively time-consuming and expensive, and many tests would have to be performed to identify possible pathogens. It is generally more useful to employ a faster, more generic test for the indicator organisms known as coliform bacteria. In the United States, the criteria for the presence of coliform bacteria are as follows: water-borne aerobic or facultative anaerobic, gram-negative, nonsporulating, rod-shaped bacteria that ferment lactose to form gas within forty-eight hours at 35 degrees Celsius. Because some soil and plant bacteria that are not constituents of fecal matter could be identified by this test, a more specific test for “fecal coliform” bacteria, primarily E. coli, requires growth in a special medium that turns yellow if E. coli is present and a separate culturing in another medium that produces a product that fluoresces blue under ultraviolet light. The US Environmental Protection Agency (EPA) is responsible for establishing criteria for the limits of acceptable levels of coliforms or fecal coliforms in water supplies, including the frequency of testing.
Pathogenic E. coli
The hundreds of strains of E. coli are distinguished by the presence of two surface antigens: O, an outer-membrane antigen, and H, an antigen on the flagellum. The strain of E. coli designated O157:H7 is pathogenic in humans. Another species of enteric bacteria is Shigella. Shigella dysenteriae causes dysentery in humans through the production of two Shiga toxins, Stx1 and Stx2, but cattle are relatively resistant to Shigella. At some point in the past, the genes for these two toxins were transferred laterally from Shigella bacteria to the E. coli O157:H7 strain, so that this strain acquired the ability to produce the Shiga toxins.
Cattle serve as a reservoir for both Shigella and E. coli O157:H7, and studies have shown that as many as 50 percent or more of dairy and beef cattle herds carry E. coli O157:H7, which may thus enter the food chain through water contaminated by cattle feces. O157:H7 contamination also sometimes comes from meat-processing facilities, where fecal material may contaminate processed beef. This is a particular concern in the case of ground beef destined for hamburgers at fast-food restaurants because in such processing the meat from hundreds of cattle is combined.
In addition to strain O157:H7, several other strains of E. coli can produce Shiga toxin. These are collectively called Shiga toxin-producing Escherichia coli, or STEC. Only about one-third of testing laboratories routinely test for STEC.
Disease Produced by E. coli
Pathogenic strains of E. coli produce diarrhea. In mild cases, the diarrhea is watery. This is common in newborn infants, and is not uncommon in pediatric wards of hospitals. Travelers who visit developing countries often experience a similar type of diarrhea, commonly referred to as traveler’s diarrhea. Since 1993, a more serious form of diarrhea caused by O157:H7 has been recognized. This bloody diarrhea can lead to hemolytic uremic syndrome (HUS). The US Centers for Disease Control and Prevention (CDC) estimates that some seventy-three thousand cases of HUS occur each year, resulting in approximately sixty deaths. Antibiotics are not always used to treat cases of E. coli, as they may make the situation worse by killing beneficial bacteria in the colon. Instead, the infection is allowed to run its course.
Although E. coli pathogenic strains have low lethality, concerns have been raised about the use of such strains as biological weapons. If introduced into local water supplies, E. coli O157:H7 or other STEC strains could cause widespread illness. The CDC classifies the O157:H7 strain as a Class B biological agent, meaning they are less easy to spread and less deadly than agents listed as Class A. Additionally, the simple structure of the E. coli bacterium that makes it useful for genetic and other biological research also means that it can be altered relatively easily. This means that the bacteria could be modified in a number of ways to produce a more potent biological weapon, such as by adding genes from more harmful substances, such as anthrax.
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