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veterinary pathobiologist Carol Maddox
The outbreak of a new, virulent strain of Escherichia coli in Europe has scientists rushing to identify the bacterium and find out how people are becoming exposed. University of Illinois veterinary pathobiologist Carol Maddox tells News Bureau life sciences editor Diana Yates about the challenges ahead for those hoping to discover the origins of this new bug.
Researchers are working to sequence the genome of this new bacterial strain. How useful will the genome be in protecting public health from this new threat?
This outbreak is a striking example of the incredible capacity for genetic rearrangement by E. coli, which is often obtaining new virulence genes. The German outbreak strain has been termed a ”super-bug” based upon the unique combination of virulence genes that code for toxins that can cause hemorrhaging in the gastrointestinal tract, including Shiga-like toxin II, and other virulence genes that enable the bacteria to adhere to host cells. This new strain of E. coli also carries several antibiotic resistance genes and has a relatively uncommon serotype (O104:H4).
Some virulence gene patterns may be associated with a specific animal host or geographic region that might help point to a source. Sequencing this pathogen may provide information that can help determine the means by which its unique genetic package was assembled, and provide leads to the source.
How else can science help combat this bug?
Characterizing the E. coli strain by looking at its DNA fingerprint, antibiotic resistance profiles and serotype (surface markers) is critical to tracking the strain. Identifying genes or traits that have not changed much in the strain’s evolutionary history may help scientists develop more specific and rapid diagnostic tests, proper disinfection protocols, safer treatments or even methods for immunological protection. Epidemiological surveys are being used to trace the pathogen back to a source so that new cases can be prevented, but as time passes, the likelihood of identifying the source by these means decreases. Especially, if as suspected, the source was fresh vegetables with limited shelf life, finding the contaminated produce may not now be possible.
International health officials are calling the new E. coli strain “super-toxic.” What factors make some strains of E. coli beneficial or at least benign, and others dangerous?
Most E. coli inhabiting the gastrointestinal tracts of animals are part of their normal flora. They are seldom pathogenic as they lack the critical factors that enable virulent E. coli to attach and secrete toxins or invade host cells. Certain serotypes of E. coli are more apt to acquire and maintain virulence genes than others. Some of the E. coli toxins have more adverse effects on their hosts than others. Most enterotoxigenic E. coli (ETEC with heat labile or heat stable toxins) cause an undesirable couple of days of watery diarrhea, but seldom lead to long-term or systemic disease. By contrast, the Shiga-like toxin positive strains (STEC) are much more insidious and may take three to four days for diarrhea to develop, three to four more days before it becomes bloody and three to four more days before the onset of hemolytic-uremic syndrome (HUS) with damage to the kidneys. While only a small portion of those that become infected with STEC develop HUS, the most vulnerable individuals are children, and kideney failure or even death can result. Those with compromised immune systems and the elderly are also at greater risk. Thus, individual host health status plays a role as well as the virulence of the bacteria.
This bacterial strain seems to have appeared out of nowhere. How do new dangerous strains emerge?
Co-infections may enable strains of E. coli to exchange genetic information, resulting in “novel” strains. Fecal contaminated irrigation or wash water could contaminate produce. Environmental stresses may result in enhanced survival of strains with some selective advantages. Often advantageous genes for the bacteria are associated with virulence or antimicrobial resistance. Since the bacterium can reproduce in as little as 20 minutes, it is easy to see how a beneficial genetic change in the bacteria can soon result in it becoming dominant.
How easily can a disease like this one cross borders? Is it likely to emerge in the U.S.?
It is difficult to predict the likelihood of this E. coli spreading through Europe or to the United States, as investigators still have not identified a source. It may be difficult to pinpoint if it is a result of contamination of produce pre-harvest or with raw meat or other products in kitchens. International trade involving the shipment of fresh produce, meats, milk, eggs and cheeses makes the possibility of new outbreaks very real. While multiple sites involved in the German outbreak point away from an infected individual as the source, people may have carried the infection and traveled, resulting in new cases in other countries. Indeed several individuals in the U.S. acquired the infection while traveling in Germany. Fecal/oral transmission is unlikely with proper hand washing and avoiding preparation of food by those who are ill, both of which are good recommendations to limit the spread. Washing produce, avoiding contamination with uncooked meat or eggs and proper cooking of meat and eggs are key to safeguarding against any food-borne illness.
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