Searching for self-replicating antimicrobials

Alexander Sulakvelidze, PhD, thinks physicians and veterinarians could eventually combat antimicrobial-resistant bacteria with natural bacterial predators known as bacteriophages, viruses that lyse bacteria.

"Mother Nature gives you a never-ending source of effective antimicrobials that she develops as part of normal evolution," he said. "And the beauty of this approach is that, if you have the right regulatory strategy and right regulatory approach, you should be able to upgrade your phage preparation on a regular basis when it's needed to keep up with the emergence of resistance or emergence of super bugs."

Dr. Sulakvelidze is the chief scientist, vice president of research and development, and a founder of Intralytix Inc., a company formed in 1998 with the goal of developing bacteriophage cocktails to combat disease-causing bacteria. In 2006, the company received approvals from the Food and Drug Administration and the Department of Agriculture to use a bacteriophage cocktail on meat and poultry to eliminate Listeria; the phage product is the first such product approved as a food additive.

Bacteriophages were first used to treat bacterial infections in humans and animals in 1919, Dr. Sulakvelidze said, and early study of the viruses showed promise for future disease-fighting applications. But they were largely abandoned in Western medicine following the discovery of broad-spectrum antimicrobials.

Overshadowed by chemical antimicrobials

David M. Shlaes, MD, PhD, said bacteriophage therapy has faced a set of problems over the past 50 years: antibody formation, allergic reactions, species specificity, and the emergence of resistance. He works with the pharmaceutical industry through his company, Anti-Infectives Consulting LLC.

"Although we understand phage much better today and we might be able to overcome some of this—in my opinion, we still have a long way to go," Dr. Shlaes said.

The potential to use this gift from nature, if you will, is right now seemingly great.

–WILLIAM E. HUFF, PHD, MICROBIOLOGIST, POULTRY PRODUCTION AND PRODUCT SAFETY RESEARCH UNIT OF THE USDA AGRICULTURAL RESEARCH SERVICE

Antimicrobials' broad target range has made them more attractive for combating infections—particularly those involving unknown agents—than phages, which typically target only a specific species or subspecies of bacteria. That specificity not only makes it more likely that a treatment will fail but also that it will encounter additional difficulties in receiving drug approvals.

Ryland Young, PhD, a phage biologist at Texas A&M University and director of the university's newly created Center for Phage Technology, said phage therapies in human and veterinary medicine would require cocktails of phages. For human applications, each phage would be required to receive a drug approval by itself and in combination with the other phages in the cocktail. A company that endured the cost to gain approval for a cocktail capable of combating only a single species of bacteria could still have difficulty protecting that investment, he said.

"A big problem facing phage therapy, not as an alternative but as an addition to our antibacterial armamentarium, is the fact that it's not clear where the intellectual property lies," Dr. Young said. He added that, unlike finding and culturing phages, "If you have an antibiotic chemical molecule that you developed, then you can patent that structure, and it's yours."

However, Dr. Sulakvelidze thinks multiple phages can be evaluated as a single product. His company produced an eight-phage, human-use wound treatment cocktail that targets Staphylococcus aureus, Escherichia coli, and Pseudomonas aeruginosa, and an FDA-approved and physician-initiated double-blind clinical trial evaluated the whole cocktail.

Dr. Young hopes the Center for Phage Therapy will become not only the world center for phage research but also a catalyst for changing the regulatory process governing phage therapy and raising public awareness of the technology's importance.

Bacteriophage research is also getting a new journal dedicated to the topic starting in January. Dr. Sulakvelidze will be editor-in-chief of "Bacteriophage" when it debuts, and he expects the publication will provide a venue for an international community of researchers to share data and publish studies describing practical applications.

Use continued on the other side of the Iron Curtain

Despite Western medicine's shift toward antimicrobials and away from phages, researchers in the Soviet Union continued developing phage applications during the Cold War, Dr. Sulakvelidze said. People can buy bacteriophage cocktails in some countries of the former Soviet Union without prescriptions, as he has done in his native country of Georgia. He moved to the U.S. in 1993.

Dr. Sulakvelidze was inspired to start Intralytix following a conversation in 1996 with a physician friend who was horribly upset after a patient died of a drug-resistant infection. He asked the physician if bacteriophages would have worked.

"He looked at me, and I realized he had never heard about this approach," Dr. Sulakvelidze said. "And it hit me that it's not just him; almost no one in the United States—even as internationally renowned and superb a physician as he was—knew about it."

Intralytix was started in part to combat difficult infections and decrease the pressures leading to development of antimicrobial resistance. Dr. Sulakvelidze said both goals are aided by the differences between the modes of actions of phages and antimicrobials.

Dr. Young expressed concern that chemical antimicrobials are becoming less useful while large pharmaceutical companies are developing fewer new antimicrobials.

"And even when they do, it seems that resistance appears very quickly," he said.

But Dr. Young has hope that phage-based treatments could be easier to find and develop. While choosing the right phage can be difficult, he said "a high school kid can go out in the dirt and find phages that will kill TB."

"You've got phage particles all over you, in your gut and everywhere else," Dr. Young said. "So they can't hurt people, they don't infect anything but bacterial cells, and yet, at least in theory, they're in the same regulatory regimens as chemical molecules."

Field tests show mixed results

William E. Huff, PhD, a microbiologist with the USDA Agricultural Research Service's Poultry Production and Product Safety Research Unit, said his concerns about the risks posed by antimicrobial resistance led him to study phages, and that similar concerns are a common driver among researchers in the field. And because of the European Union's decision to ban the use of antimicrobials in feed to promote livestock growth, he expects an increased push for development of alternative means to improve animal growth and health.

Dr. Huff also thinks phage treatments could be produced in ways that counter resistance, as researchers could identify bacteria resistant to a given phage and isolate additional phages able to combat the resistant strains.

"So you can envision ending up with a bacteriophage product that already has incorporated in it the ability to kill resistant bacteria," Dr. Huff said.

Dr. Huff said it is clear to him that the naturally abundant viruses kill bacteria, and bacteriophage technology has great potential and proven efficacy. While he acknowledges that researchers have had difficulty in developing practical applications, he thinks the technology could broaden treatment options and make money for investors.

"The potential to use this gift from nature, if you will, is right now seemingly great," Dr. Huff said. "But we just need more research and learning on how we can use it in animals and production systems."

Dr. Huff has studied phages for more than 10 years, particularly with applications for poultry. For example, he said he and other researchers have shown that aerosol administration of a phage cocktail targeting E coli can prevent colibacillosis in chickens. However, if the infection becomes systemic, the treatment has to be delivered by injection, a practice that is seen as impractical for facilities containing tens of thousands of birds.

Dr. Huff said his research has shown that the injectable therapies have also led to immune responses similar to those caused by influenza vaccines, and the immune responses can decrease the efficacy of subsequent treatments.

But Dr. Huff said someone may be able to find phages capable of eliminating systemic infections when delivered through feed, water, or aerosol applications. And he noted that producers can try to prevent systemic infections by injecting the phages into eggs or treating chicks through aerosol applications.

A big problem facing phage therapy, not as an alternative but as an addition to our antibacterial armamentarium, is the fact that it's not clear where the intellectual property lies.

–RYLAND YOUNG, PHD, DIRECTOR, CENTER FOR PHAGE TECHNOLOGY AT TEXAS A&M UNIVERSITY

Intralytix is also developing a chicken-use Clostridium perfringens phage preparation. The company's studies indicate that, when administered by gavage or in the feed or water, the preparation reduces the mortality rate connected with the disease by 90 percent and improves the flock's feed conversion ratio, Dr. Sulakvelidze said.

Dr. Huff also said some studies indicate that environmental application of bacteriophages could improve animal health, as seen in a study that associated the application of bacteriophages to cattle and calf bedding with reduced onset and severity of E coli infection.

Dr. Daniel E. Lafontaine, who chaired the recently sunset AVMA Antimicrobial Use Task Force, said the use of bacteriophages to control bacteria has potential, and he has talked with members of the meat industry about their research into use of the technology, particularly in combating E coli O157:H7. But he said those researchers have had difficulty developing effective applications. For example, he described one company's study of a bacteriophage treatment intended to eliminate or reduce the presence of E coli O157:H7 when the treatment was sprayed onto cattle, and the treatment had not been highly effective.

Dr. Huff said companion animal medicine could provide a better market for phages, particularly those requiring injection, because people may be more willing to pay for such treatments for animals such as racehorses and beloved pets. He said his own Cocker Spaniel frequently experienced ear infections, and a bacteriophage cocktail could have helped the dog.

Gaining acceptance

When Dr. Sulakvelidze formed his company in 1998, he wanted to develop phages for food safety and human therapeutics.

"I was laughed at by many, many scientists, financial people, and funding agencies that would simply trash our grant applications," he said.

His company has since received research funding from the National Institutes of Health, U.S. Army, and Environmental Protection Agency. But he expects the use of bacteriophages will also require acceptance from the public, processors, veterinarians, and physicians.

Dr. Sulakvelidze thinks further study could show the potential for phages to help manage infections in an environmentally friendly and targeted manner. He described antimicrobials as hydrogen bombs and phages as laser-guided missiles, stressing that phages could kill targeted bacteria without disturbing beneficial bacteria in the gastrointestinal tract.

Dr. Young said that while bacteriophages are safe for humans and animals, the medicinal use of viruses could scare some people. He described phages as "the enemy of my enemy."

"There's this huge world of phage technology that we could be exploiting in combating bacteria, and we really need to pay attention to it," Dr. Young said.

Phages could be considered for "organic" agriculture

Because bacteriophages are naturally occurring viruses, some researchers think phage-based products could be used in production of food animal products labeled as organic.

Soo Kim, a spokeswoman for the Agricultural Marketing Service of the Department of Agriculture, said bacteriophages could be classified as natural ingredients, provided they are not derived from genetically modified materials, and the USDA could consider allowing their use in organic livestock production. However, the phages would have to comply with existing regulations, as would all ingredients delivered with them.

Kim said regulations governing organic livestock production also allow the use of some synthetic, non-genetically-modified vaccines, which could include bacteriophages if the USDA Animal and Plant Health Inspection Service decides to classify phage-containing products as vaccines.

William E. Huff, PhD, a microbiologist with the USDA Agricultural Research Service's Poultry Production and Product Safety Research Unit, said phage applications could benefit producers of food labeled as organic if the products receive the necessary approvals.

"If they're going to produce animals that are antibiotic free, bacteriophages certainly can help them get those animals to the marketplace, and they can still claim that it is natural," Dr. Huff said.