MARINE NATURAL PRODUCTS AS ANTIMICROBIALS

You're Surrounded

We're never alone.

Though they remain unnoticed and largely ignored until we succumb to infection, or some leftovers in the fridge "go bad," bacteria and other microbial life constantly surround us. They are present in the air we breathe and in the water we swim in at the beach. On every surface we come in contact with they also form invisible biofilms that can cause a wide range of problems from bacterial conditions such as lyme disease and surgery-related infections to gum disease. Surprisingly, the answers to preventing or curing such problems may reside in natural products discovered in an Australian seaweed.

Bacterial biofilms are groups of bacterial cells and the extracellular material they produce that assemble on surfaces. On land, biofilms are of great concern to health professionals because it turns out bacteria are most likely to make people sick when they occur in this form. In the marine environment, where they form on everything from fish to rocks, biofilms are of particular interest to research scientists because their behavior is very different in that form compared to how they behave in a planktonic (free living) state.

In the ocean, the same bacterial species are found both as free-floating individuals and in biofilms. Until recently, marine microbiologists considered biofilms to simply be random agglomerations of whatever overlying planktonic bacteria have settled there. Over the past decade, however, a new picture has emerged in which bacteria are believed to communicate with each other to coordinate a number of activities, including the so-called "swarming" events that lead to biofilm formation. This apparent communication has been termed quorum sensing.

Bacteria Talk

Borrowing a term from the business world, a quorum is the threshold number of people required to do official business. In similar fashion, an uncoordinated free-living bacterial population appears to wait to chemically sense the presence of a critical number of individuals before a coordinated swarming event begins. When there is enough chemical signal concentrated, it triggers the swarming response that creates biofilms.

Centers for Disease Control and Prevention estimates that two-thirds of all human bacterial infections are caused by biofilms. This is mainly because the abundant sugars and starches (polysaccharides) that form the matrix holding biofilms together create a "bulletproof" barrier that protects biofilms against antibiotics as well as human defenses such as antibodies and white blood cells. In fact, killing biofilms can take antibiotic doses 1,000 times higher than that required to get free-living bacteria under control.

Can You Hear Me Now?

Because biofilms can do so much damage and are so hard to kill, it would seem logical that being able to prevent biofilms from forming would be a valuable trick. As it turns out, researchers are pursuing this very tactic as a way of controlling biofilms by focusing on the chemical signals that trigger their formation.

Scientists have already discovered several examples in nature where biofilm production is suppressed. One type of seaweed—a marine red alga called Delisea pulchra—that grows in Australia's famous Botany Bay is almost completely resistant to the surface fouling of biofilms, even those that cover nearly ever other exposed surface in marine systems.

Australian scientists Staffan Kjelleberg and Peter Steinberg of the University of New South Wales' Centre for Marine Biofouling and Bio-Innovation (CMBB) have discovered that the explanation for this strange phenomenon is that the seaweed produces a novel class of chemical compounds called furanones. Over 40 of them have thus far been isolated. These natural products have a structure that is similar to the chemical signals used by bacteria to trigger quorum sensing. Chemical signals often work like a lock and key, fitting into specific locations on an organism's surface to trigger certain responses. So far, based on experimental evidence, it looks like the furanones fit into the "keyholes" needed for quorum sensing, thus blocking the biofilm formation response. In a sense, the bacteria become "deafened" to the chemical call for biofilm formation so they remain free-living and therefore much more vulnerable to antibiotic attack. Further research has shown that the furanones may also block bacteria from producing some of the products that cause illness.

These findings suggest the exciting possibility that furanones or similar signal blockers could soon be used to block biofilm formation, rendering previously resistant bacterial infections vulnerable to antibiotic treatment. An added benefit of such treatments is that, because they do not actually kill free-living forms, their use would theoretically not lead to resistant bacterial strains. By contrast, resistance is a major problem with antibiotic use because antibiotics kill only a portion of the bacteria present, leaving those not susceptible to the treatment to grow and divide into a population of bacteria resistant to that antibiotic.

However, significant challenges remain, not least of which is overcoming the fact that illness-causing biofilms are produced by many different bacterial species that all have their own communication systems. Each of these systems can have its own chemical signals that must be blocked separately, and individual furanones tend to be effective at blocking only one system. Possible solutions include isolating or synthesizing a larger range of furanones to be used in treatments, or discovering other related chemicals that have wider effects.

Also of concern, some research has suggested that furanones may be too toxic for long-term treatment of human illness. However, CMBB has already generated more than 150 synthetic chemical compounds based on the structures of natural furanones isolated from D. pulchra. Among such analogues, biochemists will likely be able to find compounds that are not as toxic to humans as the natural furanones.

Another problem is that the furanones that have shown promise in blocking biofilm formation are not very effective at disrupting already established films. However, a separate chemical signaling system that triggers biofilm detachment for one species of bacteria has already been discovered, and researchers expect that they will be able to isolate furanones or other chemical compounds that can act as a substitute for these signals to trigger detachment. If the bulletproof bioflms could be disrupted in this manner, traditional antibiotics or even a patient's own immune system could deal with the bacterial infection.

Some potential biomedical applications for biofilm blocking as these challenges are overcome are described below. Equally intriguing potential environmental applications in the field of biomaterials engineering, particularly with regard to combatting biofouling, are explored elsewhere within the website.

Potential Biomedical Applications

Effective treatments for disrupting formation of biofilms or allowing elimination of established biofilms could be useful against a host of bacterial diseases including a range of skin disorders, lyme disease, cholera, various infections related to surgical procedures and countless others.

There is also great interest in using furanones or similar compounds to fight serious human diseases such as cystic fibrosis, which involves the clogging of lungs with mucus-rich biofilms produced by the normally benign bacterium Pseudomonas aeruginosa. The disease is fatal and generally claims the lives of victims at early ages. P. aeruginosa happens to be a species that has already proven susceptible to chemical signaling disruption by furanones, and research into the effectiveness of furanones in a mouse model of cystic fibrosis has already yielded promising results.

Gum disease is a common human illness related to bacterial biofilm formation. The condition results when a harmful bacterial species called Fusobacterium takes hold within the otherwise benign plaque biofilms in our mouths. If a signal blocker effective against Fusobacterium were discovered gum disease could be prevented.

Contact lenses and implanted or inserted medical devices, such as artificial heart valves and urinary catheters, are unfortunate sources of biofilm contamination and a significant source of infection. Surface-contamination of such devices is the leading source of nosocomial (originating in hospitals) illness. Work is in progress to examine the possibility of fabricating antibacterial materials or coatings for such devices that incorporate furanone molecules into their structures. Treatments similar to those developed for bacterial diseases could also be used to treat existing biofilms on mechanical heart valves and other implanted devices made from traditional materials.

And in fact, furanone discoverers Kjelleberg and Steinberg are part of a commercial venture, Biosignal Limited, whose mission centers on developing biotch applications of furanone-based technology. Biomedical product applications under development include the production of antibacterial contact lenses, and furanone-impregnated implants and catheters. Environmental applications include the develpment of marine antifouling paints and furanone-incorporated industrial products.

Other MBT-based research efforts aimed at developing safer medical implants are examined in this website's spotlight section on biomimetics spotlight section on biomimetics