Viruses Modified to Combat Harmful ‘Biofilms”

In one of the first potential applications of synthetic biology, an emerging field that aims to design and build useful biomolecular systems, researchers are engineering viruses to attack and destroy the surface "biofilms” that harbor harmful bacteria in the body and on industrial and medical devices.

The researchers, from Massachusetts Institute of Technology (MIT: Cambridge, MA, USA) and Boston University (BU; Boston, MA, USA), have already successfully demonstrated one such virus, and due to a "plug and play” library of "parts,” believe that many more could be custom-designed to target different species or strains of bacteria.

The study, reported in the July 3, 2007 issue of the journal Proceedings of the [U.S.] National Academy of Sciences, helps push synthetic biology from a conceptual science
to one that has proven practical applications. "Our results show we can do simple things with synthetic biology that have potentially useful results,” stated first author Dr. Timothy Lu, a doctoral student in the Harvard University (Cambridge, MA, USA)-MIT Division of Health Sciences and Technology.

Bacterial biofilms can form nearly anywhere, even on teeth if they are not brushed for a day or two. When they accumulate in hard-to-reach areas such as the insides of food processing machines or medical catheters, however, they become persistent sources of infection.

These bacteria excrete a variety of proteins, polysaccharides, and nucleic acids that combined with other accumulating materials form an extracellular matrix, or in Dr. Lu's words, a "slimy layer,” that encloses the bacteria. Conventional remedies such as antibiotics are not as effective on these bacterial biofilms as they are on free-floating bacteria. In some instances, antibiotics even encourage bacterial biofilms to form.

Dr. Lu and senior author Dr. James Collins, professor of biomedical engineering at BU, aim to eradicate these biofilms using bacteriophage, tiny viruses that attack bacteria. Phages have long been used in Eastern Europe and Russia to treat infection.

For a phage to be effective against a biofilm, it must both attack the strain of bacteria in the film and degrade the film itself. Recently, a different group of researchers discovered several phages in sewage that meet both criteria because, among other things, they carry enzymes capable of degrading a biofilm's extracellular matrix.

This finding led the researchers to try modfying phages to carry enzymes with similar capabilities. The reason for this is that finding a good naturally occurring combination for a given industrial or medical problem is difficult. In addition, "people don't want to dig through sewage to find these phages,” noted Dr. Lu.

Therefore, Drs. Collins and Lu defined a modular system that allows engineers to devise phages to target specific biofilms. As a proof of concept, they used their strategy to modify T7, an Escherichia coli-specific phage, to express dispersin B (DspB), an enzyme known to disperse a variety of biofilms.

To evaluate the engineered T7 phage, the team cultivated E. coli biofilms on plastic pegs. The investigators discovered that their engineered phage eliminated 99.997% of the bacterial biofilm cells, an improvement by two orders of magnitude over the phage's nonengineered cousin.

The team's modular approach can be thought of as a "plug and play” library, according to Dr. Collins. "The library could contain different phages that target different species or strains of bacteria, each constructed using related design principles to express different enzymes.” Creating such a library may soon be possible with new technologies for synthesizing genes quickly and inexpensively. "We hope in a few years, it will be easy to create libraries of phage that we know have a good chance of working a priori because we know so much about their inner workings,” said Dr. Lu.

Synthetic biology also makes it possible to control the timing of when a gene is expressed in an organism. For instance, Dr. Lu inserted the DspB genes into a precise location in the T7 genome so that the phage would strongly express it during infection rather than before or after. Such control was possible because T7 was very well characterized by other researchers, such as MIT synthetic biologist Dr. Drew Endy, an assistant professor of biological engineering.

Although phages are not approved for use in humans in the United States, recently the U.S. Food and Drug Administration (FDA) approved a phage cocktail to treat Listeria monocytogenes on lunchmeat. This makes certain applications, such as cleaning products that include phages to eradicate slime in food-processing plants, more immediately promising. Another potential application: phage-containing drugs for use in livestock in exchange for or in combination with antibiotics.


Related Links:
Massachusetts Institute of Technology
Boston University