Sheep Provide Potential Link in Control of E. coli O157:H7
by Beth Kyle

Researchers from Evergreen State College in Washington recently announced findings that may be an important step in controlling Escherichia coli (E.coli) O157:H7 in live cattle and sheep. Most strains of E.coli are harmless or even beneficial bacteria that reside in the digestive systems of mammals, including humans. However, a few strains of these bacteria are capable of producing severe illness and occasionally death when ingested by humans. The most notorious of the harmful E.coli is the O157:H7 strain, which was responsible for the Walkerton tragedy and outbreaks of hamburger disease from undercooked meat. Many cases of E.coli O157:H7 can be traced to ruminants, such as cattle, sheep and deer. The bacteria is shed through the manure of infected animals and may be transferred to humans through the environment, meat contaminated during processing from manure on hides, or by direct transfer from live animals to humans such as at petting zoos. Infected animals remain healthy and are not affected by the bacteria. The percentage varies from location to location, but it is estimated that as many as one-third of all cattle and sheep have small amounts of the pathogen in their systems. Although many checks are in place to greatly reduce the risk from O157:H7, such as processing plant sanitation and consumer education regarding proper handling and cooking of meat, even small numbers of the bacteria (reportedly as few as 10) can cause illness in humans. Therefore, finding a practical method of reducing the level of the bacteria in live animals has been an increasingly important area of research.

Research into reducing the levels of O157:H7 in living animals has studied the influence of animal diet and pre-slaughter fasting, the use of probiotics and vaccination to prime the animal’s immune system, and reducing the spread of the bacteria in the animal’s environment. To a greater or lesser extent, these methods have potential in reducing the levels of O157:H7, but each also has certain limitations. According to Dr Andrew Brabban of Evergreen State College a naturally occurring microbe residing in the digestive system of a small flock of sheep in Texas may provide the ‘magic bullet’ in solving the O157:H7 dilemma. The microbe belongs to a group of organisms referred to as bacteriophages (See panel). During their studies into O157:H7, USDA researchers regularly infect sheep with the bacteria. A group of six animals at a Texas research station was notorious for testing negative for the bacteria within a few days of infection. After ruling out potential defects with the bacterial culture, Evergreen College student Peter Varey thought to look for E.coli specific phages. What was isolated from the sheep’s manure was a ‘T4’ type phage dubbed CEV1. In subsequent studies this phage was found to be very effective in attacking and killing various strains of E.coli bacteria. Tragically, the groundbreaking sheep were themselves attacked and killed by a pack of coyotes, which obviously had very little appreciation for scientific breakthroughs. Fortunately, the CEV1 phage had been reproduced and remains available for further studies. With funding from the USDA and the National Institute of Health, Dr Brabban with colleagues Dr Betty Kutter , Dr Todd Callaway and Dr Raul Raya, tested another flock of sheep for E.coli specific phages. Although CEV1 was not found again, 20 of the 39 animals tested were found to have another type of phage (CEV2) specific to E.coli bacteria. Combinations of CEV1 and CEV2, a so-called phage cocktail, appear to be more effective in eliminating O157:H7 than either type alone.

The use of phage to control E.coli is not a new idea and has been successfully attempted in the past. However, it may be found that CEV1 and CEV2 will remain in the animal’s system and eliminate harmful E.coli each time it is introduced. As well, the CEV1 phage is particularly deadly in seeking out and destroying harmful strains of E.coli bacteria.

CEV1 and CEV2 appear to remain specific to all forms of harmful E.coli and are able to adapt to some other types of E.coli to avoid dying off when the harmful strains are eliminated. Although able to adapt to strains within the E.coli ‘family’, the evolutionary leap is too great for the phage to adapt to bacteria outside of this specific group. Therefore, although studies into the effects on animal performance are a long way down the research road, Dr Brabban believes that the microbe balance within the ruminant’s system is left relatively undisturbed and that there is little effect on animal health due to the introduction of E.coli specific phage.

Dr Brabban is enthusiastic about the role phages may have in eliminating O157:H7 from livestock. Next steps in the research include continuing to look for natural phage populations in individual flocks of sheep, studying how phages are spread within a group of animals, and further trials on the effectiveness of the phage in both cattle and sheep.

Sources:A.D.Brabban. May 2003. Personal communication. American Society for Microbiology: Division M Homepage: http://www.asm.org/division/m/M.html Bach, S.J., McAllister, T.A., Veira, D.M., Gannon, V.P.J., and Holley, R.A. 2002. Transmission and control of Escherichia coli O157:H7 – A review. Can .J.Anim.Sci. 82: 475-490. Raya, R., Dyen, M., Callaway, T., Edrington, T.S., Brabban, A.D., and Kutter, E. 2003. Potential eradication of E.coli O157:H7 using bacteriophages isolated from O157-resistant sheep. Press release relating to a poster presented at the 103rd American Society for Microbiology General Meeting, 2003.

What are Bacteriophages?
Bacteriophages are viruses that infect bacteria cells. Numbering up to one billion in a single milliliter of water, it is thought that there are more individual phages in the world than any other life form. Like all viruses, phages require the use of the cell mechanisms of a host in order to reproduce. Looking rather like a lunarlander on the surface of the moon, a phage attaches to the surface of host bacteria cell to transfer its genetic material into the cell contents. Some types of phage (T4-types) will completely destroy the infected bacteria, while other types can co-exist with the bacteria using the cell to reproduce but leaving the cell intact. In either case numerous new phages are produced and bacteria growth will be slowed if not eventually eliminated.

The use of phages in controlling disease is not a new idea. Phages were discovered in the early 1900’s and have been used to control bacterial diseases, such as Salmonella and Staphylococcus, in humans. Phages were used quite extensively for this purpose in some countries. For instance an institute founded in the Soviet Republic of Georgia in 1933 continued to supply phages for therapeutic uses within the Soviet Union until the breakup of the USSR. Research into the use of phages for disease control fell out of favour in the West in the 1940’s with the discovery of penicillin. Although not used for disease control, phages were instrumental in research into gene function until the wide spread use of recombinant DNA in the 1970’s allowed the use of more complex organisms.

Advantages of phages for disease control:
A great deal of research must be done before the use of phages for disease control is an everyday occurrence. However, recent concerns regarding the growing resistance of bacteria to various chemical antibiotics are leading scientists to reconsider the advantages of phages use in both humans and livestock. These advantages include:

• Although bacteria do evolve to avoid predation, phages are able to adapt just as quickly unlike chemical antibiotics
that must be modified by humans.
• Phage populations are self-perpetuating as long as there is a resident population of host bacteria. As a phage infects and
kills a bacteria cell more phages are reproduced, increasing their numbers rather than having to be re-administered as with a chemical antibiotic.
• Much as bacteria will spread from animal to animal, phages may also be passed throughout a population. This would be
particularly cost saving for livestock producers, as every animal would not need to be treated.
• Each type of phage is specific to a particular type of bacteria and therefore should not cause changes in other microbe
populations within the body.
• The phage population may persist for a prolonged time in the body acting somewhat like a vaccine to provide long term
protection against infection.